Code 91

[DougLee25]

I got a code 91 in the KOEO test. I got no codes on the KOER test. I thought a code 91 could only come on during the KOER test? What's the deal?

Doug

 

[naval-avi8or]

I beleive computer not detecting the HEGO switching can be detected on the KOEO test.

 

[BeatDaSHO]

this is douglee25's brother......doug is in bed. doug and i replaced the O2's tonight with basically brand new ones and we got this code. The code came up before these O2's and it stayed after we put the new ones in. We reset the codes and we also pulled the negative terminal off the batery. What can be causing this code to come up?

Greg

 

[naval-avi8or]

Here is a little trouble shootimg guide. It indicates so many other things that can result in O2 problems.BTW- I have a 94 so my codes are a little different in numbers but the same malfunction.

H16 - Check HO2S On Engines With MAF Sensor

NOTE: The purpose of this test is to verify the Heated Oxygen Sensor (HO2S) can generate greater than 0.5 volt during Engine Running Self-Test.

Any vacuum/air leaks in non-Electronic Engine Controls (EEC) areas could also cause Diagnostic Trouble Code (DTC) 136,172 or 176. Check for:

Leaking vacuum actuator (e.g. A/C control motor).
Engine sealing.
Exhaust Gas Recirculation (EGR) system.
Positive Crankcase Ventilation (PCV) system.
Unmetered air leak between Mass Air Flow (MAF) sensor and throttle body.
Lead contaminated HO2S.
Key "OFF".
Disconnect appropriate HO2S from vehicle harness.
Connect Digital Volt/Ohm Meter (DVOM) to HO2S circuit and HO2S SIG RTN or HO2S GND at the HO2S connector.
DVOM on 20 volt scale.
Run engine at approximately 2000 rpm for two minutes.
Rerun Engine Running Self-Test and monitor HO2S voltage. See: Quick Test (Extracting and Clearing Codes)\Key On Engine Running (KOER) Self Test
Does DVOM indicate greater than 0.5 volt at the end of Self-Test?

 

[DougLee25]

So are you saying more than likely it's just a bad O2?

Doug

 

[sdpatt]

The code 91 is defined as 91 (R, M) System running lean and can be detected during hte KOER test or be recalled from the memory codes during the KOEO test. The memory codes are the second batch of codes displayed during the KOEO test - the ones after the separator pulse. If you installed a "basically brand new" O2 sensor, does that mean it had been used before? For the 3.0L SHO, I recommend the Bosch 13942 from AutoZone for $33.99. If you had cleared the codes you should see a different behavior during the tests because there is little chance of only one bank running lean. You clear the codes by installing the test jumper and running the KOEO test. When the codes are being flashed with the CE light, remove the jumper. This will clear the memory codes.

 

[naval-avi8or]

Hey Scott my manual shows what you indicated as KOER and the same code for continuos memory as what I indicated. Since he just replaced the O2's(not to say they still can't be DOA) I was leaning more towards a possible connection or vacuum leak as indicated in the troubleshooting I supplied. Please give me some feed back on your thoughts. Thanks.