Codes 41 & 91 With New 02 Sensors

Discussion in 'Fox 5.0 Mustang Tech' started by dnbonds, Apr 3, 2014.


  1. dnbonds

    dnbonds Active Member

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    Ok, I am getting code 11 with the engine off, ignition on.

    I currently get codes 21, 41, and 91 (engine on) with brand new 02 sensors. I know 21 has to do with the coolant temperature sensor. Also the test was done while engine was hot, drove about 20 miles before I tested it.

    According to my code book I have it says that codes 41 and 91 also mean the engine is lean. How do I fix all three codes? Thanks in advanced!
    #1
  2. jrichker

    jrichker StangNet's favorite TOOL SN Certified Technician Founding Member

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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 20-Jan-2014 to add code 43 to test collection

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

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

    Backside view of the computer wiring connector:
    [​IMG]

    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/pink 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 (LH O2 with a Gray/Lt blue wire) and 43 (RH 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.


    Testing the O2 sensors 94-95 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. 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 Os 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 Dark Green/Pink Gray/Lt blue wire on the RH Os 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 Os 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

    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.

    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, 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.


    Code 21 – ECT sensor out of range. Broken or damaged wiring, bad ECT sensor.
    Note that that if the outside air temp is below 50 degrees F that the test for the ECT can be in error. Warm the engine up until you get good hot air from the heater and then dump the codes again.

    The ECT sensor has absolutely nothing to do with the temperature gauge. They are different animals. The ECT sensor is normally located it the passenger side front of the engine in the water feed tubes for the heater.

    The ACT & ECT have the same thermistor, so the table values are the same

    ACT & ECT test data:

    Use Pin 46 on the computer for ground for both ECT & ACT to get most accurate readings.

    Pin 7 on the computer - ECT signal in. At 176 degrees F it should be .80 volts

    Pin 25 on the computer - ACT signal in. At 50 degrees F it should be 3.5 volts. It is a good number if the ACT is mounted in the inlet airbox. If it is mounted in the lower intake manifold, the voltage readings will be lower because of the heat transfer.

    [​IMG]

    Voltages may be measured across the ECT/ACT by probing the connector from the rear. A pair of safety pins may be helpful in doing this. Use care in doing it so that you don't damage the wiring or connector.

    Here's the table :

    50 degrees F = 3.52 v
    68 degrees F = 3.02 v
    86 degrees F = 2.62 v
    104 degrees F = 2.16 v
    122 degrees F = 1.72 v
    140 degrees F = 1.35 v
    158 degrees F = 1.04 v
    176 degrees F = .80 v
    194 degrees F = .61
    212 degrees F = .47 v
    230 degrees F = .36 v
    248 degrees F = .28 v

    Ohms measures at the computer with the computer disconnected, or at the sensor with the sensor disconnected.

    50 degrees F = 58.75 K ohms
    68 degrees F = 37.30 K ohms
    86 degrees F = 27.27 K ohms
    104 degrees F = 16.15 K ohms
    122 degrees F = 10.97 K ohms
    140 degrees F = 7.60 K ohms
    158 degrees F = 5.37 K ohms
    176 degrees F = 3.84 K ohms
    194 degrees F = 2.80 K ohms
    212 degrees F = 2.07 K ohms
    230 degrees F = 1.55 K ohms
    248 degrees F = 1.18 k ohms

    Diagram courtesy of Tmoss & Stang&2birds

    [​IMG]

    See the following website for some help from Tmoss (diagram designer) & Stang&2Birds
    (website host) for help on 88-95 wiring http://www.veryuseful.com/mustang/tech/engine/

    Ignition switch wiring
    http://www.veryuseful.com/mustang/tech/engine/images/IgnitionSwitchWiring.gif

    Fuel, alternator, A/C and ignition wiring
    http://www.veryuseful.com/mustang/tech/engine/images/fuel-alt-links-ign-ac.gif

    Complete computer, actuator & sensor wiring diagram for 88-91 Mass Air Mustangs
    http://www.veryuseful.com/mustang/tech/engine/images/88-91_5.0_EEC_Wiring_Diagram.gif

    Vacuum diagram 89-93 Mustangs
    http://www.veryuseful.com/mustang/tech/engine/images/mustangFoxFordVacuumDiagram.jpg
    Last edited: Apr 4, 2014
    #2
  3. madspeed

    madspeed Colonel Mustard Mod Dude Founding Member

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    Ect sensor can cause all sorts of issues when its bad. I would check that thing first
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  4. Mustang5L5

    Mustang5L5 Car used in adult film "Highway Gangbang-InDaButt" SN Certified Technician Founding Member

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    +1 on checking the ECT sensor first.

    I'd replace it (if needed), erase all the codes, and retest.

    Only troubleshoot the 41/91 if they come back without the code 21 as well.
    #4
  5. dnbonds

    dnbonds Active Member

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    Ok, I went to autozone and picked up an ECT sensor. I installed it and ran the codes again but still got 21, 41 , and 91. Temperature outside is about 55 or 60 degrees F. What is the correct way to clear the codes? All I did was hook up my digital code reader and test it again. I am hoping to stay away from pulling out the computer and testing the wires. If I need to do that I'll wait until I start ripping into my interior to replace carpet and seats in a few weeks.

    Thanks for the help guys!
    #5
  6. 7991LXnSHO

    7991LXnSHO Active Member

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    My cheap code reader can reset codes. If you lost the manual, unhooking the battery for 20 minutes and turn on the head light switch with the battery disconnected will do it too.

    Your outside temp is at the borderline for setting off a code with a good IAT. Working on the interior until a nice day is not a bad idea. It will give you time to re read, re read and digest all that jrichker posted so it is easy breezy.
    #6
  7. dnbonds

    dnbonds Active Member

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    I should still be able to test it with it being borderline temp if the engine is hot right? My code reader only has Power, Test, and Memory (it stores the codes it pulled) no button to clear the codes though. I'll reset the codes tomorrow and hope for the best. Thanks
    #7
  8. Noobz347

    Noobz347 Stangnet Facilities Maint Tech... Er... Janitor Admin Dude

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    Did you have these codes BEFORE you swapped the o2 sensors?
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  9. dnbonds

    dnbonds Active Member

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    Yes, I had the 41/91 codes along with another one that dealt with the TPS out of range. I replaced the 02 Sensors along with fixing the surging idle about 3 weeks ago. I just recently re-tested it and it came up with code 21 (which was new) and the same 41/91 codes.
    #9
  10. MFE92

    MFE92 Active Member

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    The funny thing about codes is, some of them (like EVP out of range) will trip if you look at it funny, but it almost takes a total failure or an open circuit to trigger the 41/91. If I were you I'd inspect and/or clean the main O2 harness connector under the main air intake tube/next to the passenger frame rail, and the 10-pin connectors at the back of the intake throat, which are famous for poor connections.
    #10
  11. dnbonds

    dnbonds Active Member

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    I guess I'll just get my electrical cleaner and clean every connector. Thanks for the suggestion, it wouldn't hurt to clean more than I need lol. It'll also keep me occupied while I wait for the rest of my interior parts to come in.
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  12. Noobz347

    Noobz347 Stangnet Facilities Maint Tech... Er... Janitor Admin Dude

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    I would hazard to guess that you have a wiring problem.

    Is your car a mass air conversion by chance?
    When you got the new sensors did you purchase them by matching them with what was already in there or did you buy sensors for your year, make, and model?
    #12
  13. dnbonds

    dnbonds Active Member

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    Yeah, I think I have a wiring problem also. It shouldn't be converted to mass air because it's an 89. No clue if they swapped the engine. The only thing I know that has been done to the car by one of the POs was nitrous and aftermarket exhaust (they left the stock mufflers on...). I have hacked-up wires all around my engine bay but nothing seems majorly wrong with the car so I just assumed they were left over when they took the nitrous out.

    I bought the sensors for my year, make, and model from autozone. They are bosch if it makes any difference. And my exhaust has 2 cats instead of 4 if that helps at all.
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  14. Noobz347

    Noobz347 Stangnet Facilities Maint Tech... Er... Janitor Admin Dude

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    @jrichker

    Are the portions highlighted in red correct?


    They appear to be in conflict or am I looking at it wrong?

    My diagram shows:

    Dark Green/Pink as RH sensor and pin 29
    Dark Blue/Light Green as LH sensor and pin 43
    #14
  15. Noobz347

    Noobz347 Stangnet Facilities Maint Tech... Er... Janitor Admin Dude

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    Bosch is what I buy as well. You should have the correct sensors. You'll need to do the troubleshooting of the harnesses outlined in JR's post above. It may look a bit overwhelming but it's really not when you get right down to it.

    Check the fusible link beneath the battery tray as well. Should be a Gray/yellow wire. That link powers the heating element in both sensors.
    #15
  16. jrichker

    jrichker StangNet's favorite TOOL SN Certified Technician Founding Member

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    That's a typo which is my fault, thank you for pointing it out. I have corrected it.
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  17. madspeed

    madspeed Colonel Mustard Mod Dude Founding Member

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  18. Mustang5L5

    Mustang5L5 Car used in adult film "Highway Gangbang-InDaButt" SN Certified Technician Founding Member

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    Are you running the test when the engine is hot? If not, you will always get an ECT code 21. The engine needs to be fully warmed up before running the codes.

    To clear the codes, disconnect your code reader while they are in the processes of displaying the codes. Ungrounding the connector during the test will erase the codes.

    You can also unhook battery for a few mins as well
    #18
    89oem likes this.
  19. dnbonds

    dnbonds Active Member

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    Ok so I cleaned up some connectors that were near the ECT sensor and then I let the car idle for about 15 minutes. I cleared the codes and then started another test. Code 21 is now fixed and I still get Codes 41/91.

    I did find some cut wires, would anyone know what these are for? (see attached picture) They are in between the battery and ignition coil, its the orange and pinkish wires. I looked at the diagram from jrichker and thought that maybe the orange is the HEGO ground? Any feedback would be helpful. Thanks!

    Attached Files:

    #19
  20. jrichker

    jrichker StangNet's favorite TOOL SN Certified Technician Founding Member

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    With the exception of a very few items, Ford wiring uses a solid color and a stripe. That helps to identify the wiring.. What are the wire sold colors and stripe colors?
    #20

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