KOER Code = 41?

Discussion in 'Fox 5.0 Mustang Tech' started by lbperry4227, Aug 19, 2005.


  1. lbperry4227

    lbperry4227 New Member

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    Can anyone tell me what a KOER Code of 41 means?
    I've got some kind of engine problem and ran the KOEO and KOER codes and one of the codes I got on the KOER (running) test is a 41.
    The code tables on fordfuelinjection.com show a KOEO and CM code of 41 related to the right O2 sensor but don't show anything for a 41 on the KOER.

    I'm a little confused on the code sequences.
    I thought the codes you got with the key on but engine not running were the KOEO codes followed by the CM (continuous) codes. I got a KOEO of 67 (because I don't have the transmission harness) and a CM of 11 which indicates a System Passed. After I cranked the engine and it went thru it's testing procedure I got a code 41 and then a 33 which I took to be the KOER codes.
    My confusion is that the code table I shows that a KOEO of 41 is No HEGO switching detected always lean (right side) and a CM of 41 is HEGO sensor circuit indicates system lean (right side); but no definition of a KOER of 41.
    I think that a KOER of 41 would probably relate to the right side O2 sensor, but hoped it would tell be if it was just seeing a bad reading or didn't see the O2 sensor at all.
    I guess the next diagnostic steps would be to switch sides with the O2 sensor to try to narrow it down to the sensor or the wiring? And then either replace the sensor or troubleshoot the wiring?
    Thanks for your help. I'm new at trying to interpret EFI computer codes.
    Lawrence Perry
  2. ss93cobra

    ss93cobra Founding Member

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    Certainly sounds as if you're on the right track. Perhaps verify power to the hego. Regarding switching the two sides, good luck, they're more inclined to break on you vs. coming out..
  3. ram360

    ram360 Founding Member

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    41 : System lean - Fuel control

    I have the same code on my car and can't get rid of it. It went away for like 100 miles and now it's back again. I have brand new 02's and still have it. My whole fuel system is also new and I run 42 psi of fp w/o the vacuum. Try check the ground for the 02's it's the orange wire w/ the ring terminal from the injector harness. I check out all that stuff on mine but still no luck.
  4. jrichker

    jrichker StangNet's favorite TOOL SN Certified Technician Founding Member

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    Code 41 or 91 - O2 indicates system lean. Look for a vacuum leak or failing O2 sensor.
    The computer sees a lean mixture signal coming from the O2 sensors and tries to compensate by adding more 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

    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 dark green/pink wire) and 43 (RH O2 with a dark blue/pink wire). Use the ground next to the computer to ground the voltmeter.

    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

    Code 33 - Insufficient EGR flow detected.
    Look for vacuum leaks, cracked vacuum lines, failed EGR vacuum regulator. Check to see if you have 10” of vacuum at the EGR vacuum connection coming from the intake manifold. Look for electrical signal at the vacuum regulator solenoid valves located on the rear of the passenger side wheel well. Using a test light across the electrical connector, it should flicker as the electrical signal flickers. Remember that the computer does not source any power, but provides the ground necessary to complete the circuit. That means one side of the circuit will always be hot, and the other side will go to ground or below 1 volt as the computer switches on that circuit.
    Check for resistance between the brown/lt green wire on the EGR sensor and pin 27 on the computer: you should have less than 1 ohm.

    See the following website for some help from Tmoss (diagram designer) & Stang&2Birds (website host)

    http://www.veryuseful.com/mustang/tech/engine/images/fuel-alt-links-ign-ac.gif

    http://www.veryuseful.com/mustang/tech/engine/images/88-91eecPinout.gif

    EGR test procedure courtesy of cjones

    to check the EGR valve:
    bring the engine to normal temp.
    connect a vacuum pump to the EGR Valve
    apply 5in vacuum to the valve.
    if engine stumbled or died then EGR Valve and passage(there is a passageway through the heads and intake) are good.
    if engine did NOT stumble or die then either the EGR Valve is bad and/or the passage is blocked.
    if engine stumbled, connect vacuum gauge to the hose coming off of the EGR Valve
    snap throttle to 2500 RPM (remember snap the throttle don't hold it there).
    did the vacuum gauge show about 5in vacuum?

    if not, check for manifold vacuum at the EGR vacuum valve.
    if you have manifold vacuum then connect vacuum gauge to the EGR valve side of the vacuum valve and snap throttle to 2500 RPM.
    should read about 5in vacuum
  5. rockslinger

    rockslinger New Member

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    code 41 possible fix, first post , be gentle

    after reading here and several other sites for months, my attempts at tracking down the cause of a code 41 on my '86 gt were getting me nowhere. my mustang was running way rich (enough to smoke) even though it appeared to be lean on the right bank. at first i thought it was an injector problem, so i went through that whole ordeal replacing injectors with known good ones until it consistently passed the cylinder balance tests. on to the o2 sensors. after tracing/testing every wire from cpu to battery i found no problems , i switched o2 harnesses from right to left and test resulted in code 91 (koer) left side no switching detected. i should have swapped these first and i would've seen that the wiring was ok. after much swearing and wrestling with the now frozen o2 sensor to switch the sensors themselves from left to right i found it easier to buy another sensor (the one that appeared good was the frozen one). here is a good example of what not to do, throwing parts at the problem that is. the new sensor was installed in place of the "non switching" one and the problem remained. now i'm really confused- no bad wires ,new sensor (even swapped cpu), still no switching. being completely fed up i started pulling vacuum lines and plugging them. turns out the thermactor system that reroutes air to the heads/exhaust is controlled by vacuum at a valve on the passenger side which feeds via a crossover tube to both heads. the tube design does not deliver air uniformly to both sides, it enters the passenger side head first via a t connection before continuing to the drivers side. if your engine was neglected (like the way i bought mine) odds are the crossover tube, head passages, or both on the drivers side were clogged with recycled oily carbon deposits from the exhaust i'm guessing. so all the fresh air or exhaust diverted through the thermactor system into the heads flows directly into the passenger side cylinder bank making the o2 sensor appear non-responsive. pull either vacuum line for the thermactor system and observe the voltage on the o2 sensor, it should be switching after unhooked and code 41 should now be gone. instead of cleaning and overhauling the head i opted for freshly rebuilt heads with all new parts, i am installing them in a couple of days and i will post my results. hope this helps someone.
  6. jrichker

    jrichker StangNet's favorite TOOL SN Certified Technician Founding Member

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    Very good info. I will add it to the code 41/91 test path.
  7. sjhm9102

    sjhm9102 New Member

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    does the 1991 mustang also have this crossover tube never mind mine doesnt have it
  8. G302A

    G302A New Member

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    I'm staring at a 91 code as well, motor runs like crap. it is bizarre that you can get a 91 but not 41 at the same time. I swapped HEGOs right to left and then the wiring and proved to myself that it was indeed telling the truth, not just my lousy wiring.

    I guess air is getting in somewhere.

    BTW, I have no air crossover, it is all removed and the heads plugged.

    Carb cleaner tomorrow.....

    Gordon
  9. DragTriper

    DragTriper New Member

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    I'm wondering if anyone here managed to clear the code 41 on their mustang and how they did it. I got an 89 5.0 liter and I have codes 41 & 91 (No HEGO switching detected always lean (right side).& HEGO sensor circuit indicates system lean (left side)). I changed both O2 sensors and the car is running much better with more power but I got both codes on KOER test again after clearing the codes. The car seems to run fine tho (Idles a little erratic for a few seconds on first start up but idles pretty smooth after) . I'm very confused. What do I need to check and why are my sensors detecting system lean?

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