Engine HEGO stuck on 0.0v

Jack Welch

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My 1988 stock 5.0L manual transmission Mustang has the HEGOs stuck on 0.0- 0.2v at warm idle closed loop with no sensor switching. The sensors are new Bosch and respond to propane enrichment at idle. Above idle around 2200 RPM there is switching. TAB and TAD valves/solenoids check OK. Fuel pressure good and vacuum steady 16 in. at idle. Injector duty cycles are high suggesting ECU command to trim a lean condition, but I can't find the source of any lean conditon (vacuum leaks, PCV, CAP, leaking EGR etc.). ECT and IAT sensors are good. Any suggestions?? KOEO is code 11, system pass. KOER is code for no HEGO switching.
 
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jrichker

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Code 41 or 91. Or 43 Three digit code 172 or 176 - O2 sensor indicates system lean. Look for a vacuum leak or failing O2 sensor.

Revised 24 Aug 2018
1.) To correct the RH & LH mismatch on 91-93 5.0 Mustangs
2.) To add Tmoss’ wiring diagrams for 88-95 Mustangs


Code 41 is the passenger side sensor, as viewed from the driver's seat.
Code 91 is the driver side sensor, as viewed from the driver's seat.

Code 172 is the passenger side sensor as viewed from the driver's seat.
Code 176 is the driver side sensor, as viewed from the driver's seat.

Code 43 is not side specific according to the Probst Ford Fuel injection book.

The computer sees a lean mixture signal coming from the O2 sensors and tries to compensate by adding more fuel. Many times the end result is an engine that runs pig rich and stinks of unburned fuel.

The following is a Quote from Charles O. Probst, Ford fuel Injection & Electronic Engine control:

"When the mixture is lean, the exhaust gas has oxygen, about the same amount as the ambient air. So the sensor will generate less than 400 Millivolts. Remember lean = less voltage.
When the mixture is rich, there's less oxygen in the exhaust than in the ambient air , so voltage is generated between the two sides of the tip. The voltage is greater than 600 millivolts. Remember rich = more voltage.
Here's a tip: the newer the sensor, the more the voltage changes, swinging from as low as 0.1 volt to as much as 0.9 volt. As an oxygen sensor ages, the voltage changes get smaller and slower - the voltage change lags behind the change in exhaust gas oxygen.

Because the oxygen sensor generates its own voltage, never apply voltage and never measure resistance of the sensor circuit. To measure voltage signals, use an analog voltmeter with a high input impedance, at least 10 megohms. Remember, a digital voltmeter will average a changing voltage." End Quote

Testing the O2 sensors 87-93 5.0 Mustangs

Measuring the O2 sensor voltage at the computer will give you a good idea of how well they are working. You'll have to pull the passenger side kick panel off to gain access to the computer connector. Remove the plastic wiring cover to get to the back side of the wiring. Use a safety pin or paper clip to probe the connections from the rear.


Disconnect the O2 sensor from the harness and use the body side O2 sensor harness as the starting point for testing. Do not measure the resistance of the O2 sensor, you may damage it. Resistance measurements for the O2 sensor harness are made with one meter lead on the O2 sensor harness and the other meter lead on the computer wire or pin for the O2 sensor.
Computer wiring harness connector, computer side.
88243.gif


Backside view of the computer wiring connector:
View attachment a9x-series-computer-connector-wire-side-view-gif.gif


87-90 5.0 Mustangs:
Computer pin 43 Dark blue/Lt green – LH O2 sensor
Computer pin 29 Dark Green/Pink – RH O2 sensor

The computer pins are 29 (RH O2 with a dark green/pink wire) and 43 (LH O2 with a dark blue/lt green wire). Use the ground next to the computer to ground the voltmeter. The O2 sensor voltage should switch between .2-.9 volt at idle.


91-93 5.0 Mustangs:
Computer pin 43 Red/Black – LH O2 sensor
Computer pin 29 Gray/Lt blue – RH O2 sensor

The computer pins are 29 (RH O2 with a Gray/Lt blue wire) and 43 (LH O2 with a Red/Black wire). Use the ground next to the computer to ground the voltmeter. The O2 sensor voltage should switch between .2-.9 volt at idle.


94-95 5.0 Mustangs; note that the 94-95 uses a 4 wire O2 sensor.
The computer pins are 29 (LH O2 with a red/black wire) and 27 (RH O2 with a gray/lt blue wire). Use pin 32 (gray/red wire) to ground the voltmeter. . The O2 sensor voltage should switch between .2-.9 volt at idle.


Note that all resistance tests must be done with power off. Measuring resistance with a circuit powered on will give false readings and possibly damage the meter. Do not attempt to measure the resistance of the O2 sensors, it may damage them.

Testing the O2 sensor wiring harness
Most of the common multimeters have a resistance scale. Be sure the O2 sensors are disconnected and measure the resistance from the O2 sensor body harness to the pins on the computer. Using the Low Ohms range (usually 200 Ohms) you should see less than 1.5 Ohms.



87-90 5.0 Mustangs:
Computer pin 43 Dark blue/Lt green – LH O2 sensor
Computer pin 29 Dark Green/Pink – RH O2 sensor
Disconnect the connector from the O2 sensor and measure the resistance:
From the Dark blue/Lt green wire in the LH O2 sensor harness and the Dark blue/Lt green wire on the computer pin 43
From the Dark Green/Pink wire on the RH O2 sensor harness and the Dark Green/Pink wire on the computer pin 29


91-93 5.0 Mustangs:
Computer pin 43 Red/Black – LH O2 sensor
Computer pin 29 Gray/Lt blue – RH O2 sensor
Disconnect the connector from the O2 sensor and measure the resistance:
From the Red/Black wire in the LH O2 sensor harness and the Red/Black wire on the computer pin 43
From the Gray/Lt blue wire on the RH O2 sensor harness and the Gray/Lt blue wire on the computer pin 29

94-95 5.0 Mustangs:
Computer pin 29 Red/Black – LH O2 sensor
Computer pin 27 Gray/Lt blue – RH O2 sensor
From the Red/Black wire in the LH O2 sensor harness and the Red/Black wire on the computer pin 29
From the Dark Green/Pink Gray/Lt blue wire on the RH O2 sensor harness and the Gray/Lt blue wire on the computer pin 27


There is a connector between the body harness and the O2 sensor harness. Make sure the connectors are mated together, the contacts and wiring are not damaged, and the contacts are clean and not coated with oil.

The O2 sensor ground (orange wire with a ring terminal on it) is in the wiring harness for the fuel injection wiring. I grounded mine to one of the intake manifold bolts

Check the fuel pressure – the fuel pressure is 37-41 PSI with the vacuum disconnected and the engine idling. Fuel pressure out of range can cause the 41 & 91 codes together. It will not cause a single code, only both codes together.

Make sure you have the proper 3 wire O2 sensors. Only the 4 cylinder cars used a 4 wire sensor, which is not compatible with the V8 wiring harness. The exception is that the 94-95 uses a 4 wire O2 sensor.

Replace the O2 sensors in pairs if replacement is indicated. If one is weak or bad, the other one probably isn't far behind.

Code 41 can also be due to carbon plugging the driver’s side Thermactor air crossover tube on the back of the engine. The tube fills up with carbon and does not pass air to the driver’s side head ports. This puts an excess amount of air in the passenger side exhaust and can set the code 41. Remove the tube and clean it out so that both sides get good airflow: this may be more difficult than it sounds. You need something like a mini rotor-rooter to do the job because of the curves in the tube. Something like the outer spiral jacket of a flexible push-pull cable may be the thing that does the trick.

If you get only code 41 and have changed the sensor, look for vacuum leaks. This is especially true if you are having idle problems. The small plastic tubing is very brittle after many years of the heating it receives. Replace the tubing and check the PVC and the hoses connected to it.

Complete computer, actuator & sensor wiring diagram for 94-95 Mustangs


Complete computer, actuator & sensor wiring diagram for 91-93 Mass Air Mustangs


Complete computer, actuator & sensor wiring diagram for 88-90 Mass Air Mustangs
 

Jack Welch

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Thank you for the reply to my stated problem. I very much appreciate your interest and effort to help. I've checked the HEGO harness resistance with the HEGOs disconnected from the body harness and found them to be within spec. I also load tested the HEGO harness after disconnecting it at both the ECU and the HEGOs using a standard stoplight bulb and an applied 12v battery current. Good current through the harness fully lit the bulb. (The harness could ohm OK with even only one strand of wire remaining. This way we know it is passing adequate current.) The HEGOs are still fixed lean at 0.0v in both open and closed loop at warm idle, but they will respond to artificial propane enrichment when I introduce propane through the vacuum tree in the intake manifold or snap the throttle. Even when the engine is fully warmed to operating temperature, the HEGOs are still fixed at 0.0v in closed loop at idle but will begin to switch above 2300 RPM. I checked the injector duty cycles and they are very high suggesting the ECU is trying to conpensate for a baseline lean condition, at idle, but I cannot find a reason for this. There is a random miss/ spit through the exhaust at idle suggesting a lean misfire. The IAC is new and functioning within spec. Base idle speed setting and timing are correct. I substituted another known good ECU and the problem persists. The injectors were cleaned, tested and found to be well within spec for flow and spray pattern.

The HEGOs are reporting a lean condition that I cannot find a reason for. I thought the Thermactor might be pumping air upstream when it should not, but this is not the case. There are no exhaust leaks upstream of the HEGOs that I can detect.
 

Jack Welch

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Thanks again for your interest. I have a rock steady vacuum gauge at idle, 16"' at the upper intake hose tree.That seems a little low to me but I see it stated as within spec in several places. I get a good PCV suck at the air filter intake hose. PCV valve is new Motorcraft. Brake booster, a/c controls and cruise control are all disconnected and plugged to eliminate them as source of a vacuum leak. MAP sensor checks OK. Cannister purge valve is good. Vacuum gauge responds normally to throttle snaps and returns nicely to 16". Pulls over 21" at higher RPMs. We are talking a totally stock 5.0L here. For reason(s) I just can't find it is running lean according to the HEGO output voltages and has a very low to nonexistent switch rate at idle and low RPMs. The HEGOs are new Bosch.
 

jrichker

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@Jack Welch
Are they 3 wire or 4 wire O2 sensors?

Does your meter autosense the voltage range or does it allow you to select a range?
For manual selection, use the 2 volt range if it has one.

Is there 12 volts present on the O2 heater wiring with the ignition switch in the Run position?
 

stangmike86gt

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I was going to say massive exhaust leak in that side, but you've checked that.

Maybe you have a intake leak on that side, but into the oil galley. Have you tried reseating your lower intake? It wouldn't be the first time I've heard of it happening.
 

Jack Welch

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@Jack Welch
Are they 3 wire or 4 wire O2 sensors?

Does your meter autosense the voltage range or does it allow you to select a range?
For manual selection, use the 2 volt range if it has one.

Is there 12 volts present on the O2 heater wiring with the ignition switch in the Run position?
@Jack Welch
Are they 3 wire or 4 wire O2 sensors?

Does your meter autosense the voltage range or does it allow you to select a range?
For manual selection, use the 2 volt range if it has one.

Is there 12 volts present on the O2 heater wiring with the ignition switch in the Run position?
Thanks again for your continuing interest. They are 3 wire HEGOs with the signal ground through the body of the sensor to the bung threads in the H pipe. I checked the grounds and they are good. I use a Fluke 88v automotive DVM. It does have a range selection and is autoranging for safety backup.
I use the 600mv DC range. There is 12v at the HEGO heater wiring at the sensor connector with the ignition switch in the Run position, I load tested the HEGO heater circuit with a stoplight bulb and there is good current in the circuit that illuminates both elements of the bulb.
 

Jack Welch

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Did we verify fuel pressure?
Hi and thanks for joining in to help me solve this problem. Fuel rail charges to 44-48 psi at initial activation of the fuel pump and remains steady at 32-35 while the engine runs. Pump and stock Motorcraft inline filter are almost new and deliver fuel volume to spec. . The pressure regulator on the fuel rail holds good vacuum when tested with a hand held vacuum source, and there is no fuel in the vacuum hose from the intake manifold.
 

jrichker

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We are running out of options...

The two grounds for the O2 heater/sensor circuit ae next...

The secondary power ground is between the back of the intake manifold and the driver's side firewall. It is often missing or loose. It supplies ground for the alternator, A/C compressor clutch and other electrical accessories such as the gauges. The clue to a bad ground here is that the temp gauge goes up as you add electrical load such as heater, lights and A/C.

The O2 sensor heaters have their own ground (HEGO ground) coming from the computer. This is different and separate from the O2 sensor ground. It is an orange wire with a ring terminal on it. It is located in the fuel injector wiring harness and comes out under the throttle body. It gets connected to a manifold or bolt on back of the cylinder head.
 

Jack Welch

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I was going to say massive exhaust leak in that side, but you've checked that.

Maybe you have a intake leak on that side, but into the oil galley. Have you tried reseating your lower intake? It wouldn't be the first time I've heard of it happening.
Hi and thanks for your suggestion. I have thought about that and there is a test for just that possibility. It involves disconnecting the PCV valve from the back of the upper intake manifold and plugging the fitting there. Also plug the PCV valve itself and the PCV line from the oil fill on the valve cover to the air filter intake box. Attach a vacuum gauge to the oil dipstick tube and start the engine. If there is vacuum on the gauge you have a lower intake manifold leak into the galley/crankcase. Did this, and no vacuum. The lower intake manifold gaskets are new. You are correct in suspecting a masive exhaust leak somewhere allowing air into the system upstream of the HEGOs and driving them lean. The exhaust system is tight and I cannot hear or feel any exhaust leak in the pipes or manifolds.
 

Jack Welch

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We are running out of options...

The two grounds for the O2 heater/sensor circuit ae next...

The secondary power ground is between the back of the intake manifold and the driver's side firewall. It is often missing or loose. It supplies ground for the alternator, A/C compressor clutch and other electrical accessories such as the gauges. The clue to a bad ground here is that the temp gauge goes up as you add electrical load such as heater, lights and A/C.

The O2 sensor heaters have their own ground (HEGO ground) coming from the computer. This is different and separate from the O2 sensor ground. It is an orange wire with a ring terminal on it. It is located in the fuel injector wiring harness and comes out under the throttle body. It gets connected to a manifold or bolt on back of the cylinder head.
Thanks again for your good insights. As I understand it from the factory wiring diagrams, the HEGO heater circuit is grounded outside of the ECU by the black wire, and current is supplied to this heater circuit directly by the ignition switch. I load tested this circuit and it is intact. 12v current is supplied to the HEGO heaters when the ignition is in the run position. According to the diagrams, the ECU plays no part in the supply of 12v current from the ignition switdh to a heater coil in the HEGO.

As i read the factory wiring diagrams, the wire at pin#49 (orange wire with a loop connector to engine bolt) is designated as "HEGO" ground. Is the the signal wire voltage generated by the HEGO sent to and read by the ECU then grounded by a path through the ECU to a connection with the orange wire at pin#49, then to the looped connection on the intake manifold or block?? In other words, would a voltage variance in the orange wire at pin #49 correlate with voltage fluctuations of a functioning HEGO since the orange wire is occupied with grounding a variable switching voltage from the HEGO during engine operation?
 

jrichker

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Basic electronic theory: Signal grounds are always separate from power grounds.

The reason why is signals from a sensor are typically low voltage and small changes in voltage seen by the computer can have a large effect on the computer operation. Any time a ground wire carries power, there is always a voltage drop between the point of origin and the termination point at the battery post or ground point on the vehicle. If you use a power ground as ground for a sensor, the voltage drop across the ground wire will alter the voltage the sensor delivers to the computer by the amount of the voltage drop.
 

Jack Welch

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OK, and thanks for that helpful information. I understand why signal and power grounds are separated. I have the secondary power ground you mentioned above at the rear of the engine connected securely between a rear exhaust manifold bolt and the vehicle firewall.

I am not sure what you are meaning when you state that "The O2 sensor heaters have their own ground (HEGO ground) coming from the computer." My diagrams show no ground connection between the HEGO heater circuit and the ECU. They show batt ground through the block and 12v batt voltage delivered directly to the HEGO from the ignition switch. I verified this by load testing this circuit grounding at the block and batt and i am confident in the current supply to the HEGO heaters. My factory diagram (as the one above) does show a HEGO ground through a looped orange wire that comes from pin#49 at the ECU harness connector, and you state that this loop should connect at either a bolt on a cyl head or and intake manifold bolt, and this is the ground for the HEGO signal return to the ECU. This might be a dumb question, but it would seem to me that both of these grounds end up being batt grounds, just connected at different points on the block and having taken different and separate paths to get there? Am I missing something??
 

jrichker

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OK, and thanks for that helpful information. I understand why signal and power grounds are separated. I have the secondary power ground you mentioned above at the rear of the engine connected securely between a rear exhaust manifold bolt and the vehicle firewall.

I am not sure what you are meaning when you state that "The O2 sensor heaters have their own ground (HEGO ground) coming from the computer." My diagrams show no ground connection between the HEGO heater circuit and the ECU. They show batt ground through the block and 12v batt voltage delivered directly to the HEGO from the ignition switch. I verified this by load testing this circuit grounding at the block and batt and i am confident in the current supply to the HEGO heaters. My factory diagram (as the one above) does show a HEGO ground through a looped orange wire that comes from pin#49 at the ECU harness connector, and you state that this loop should connect at either a bolt on a cyl head or and intake manifold bolt, and this is the ground for the HEGO signal return to the ECU. This might be a dumb question, but it would seem to me that both of these grounds end up being batt grounds, just connected at different points on the block and having taken different and separate paths to get there? Am I missing something??
All the grounds end up at the battery negative post. However, the signal grounds carry no power and therefore no voltage drop due to changing electrical loads.
 

General karthief

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I noticed you stated that one of the grounds is connected to an exhaust manifold bolt, I believe that would be a poor place to ground something do to the heat in that area. I would move that to the back of the head or an intake bolt. JMO and not based on any real mechanical experience.
 

Jack Welch

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I noticed you stated that one of the grounds is connected to an exhaust manifold bolt, I believe that would be a poor place to ground something do to the heat in that area. I would move that to the back of the head or an intake bolt. JMO and not based on any real mechanical experience.
Thanks for your suggestion.
 

Jack Welch

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All the grounds end up at the battery negative post. However, the signal grounds carry no power and therefore no voltage drop due to changing electrical loads.
Exactly. That's why the signal ground is routed through the ECU on its path to eventual batt negative via the pin#49 orange wire with its looped connector attached to an intake manifold bolt. The batt negative cable is attached to the engine block on a dedicated boss. Both the power grounds (HEGO heaters) and the signal ground end up being ultimately grounded at batt negative via their connection to the block, it's just the significantly different paths they take to get there. I appreciate you making this clear distinction in helping to solve my problem. I will recheck the integrity of the HEGO signal ground circuit by load testing it after disconnecting it from its connection to the ECU at pin#49 and report back. Thanks again for your help.
 

Jack Welch

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Re -checked the integrity of the HEGO signal wires to the ECU 60 pin connector and HEGO ground wire (orange wire with loop connector from pin#49) from the ECU connector to its attacment at the intake maniflod bolt/stud. Connections and continuity test OK for both. From this I conclude that the HEGO signal and ground circuits are OK, just that I don't get a voltage reading above 0.0-0.1v and no switching at closed loop idle. I'm concluding the exhaust gasses are very lean for some reason(s) I cannot discover, and the HEGOs are driven to reflects this.
 

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