Another code question- O2 sensors

06-07-2010, 05:21 PM
Getting the code indicating a bad ground in the O2 circuit.

Not looking to defeat emission stuff or hot rod this 99 P71, but would the mods marketed to eliminate O2 sensors for the rodders help me eliminate this fault code on my P71? Or is there a simple fix to fix this according to Hoyle?

Thanks! :cool:

06-07-2010, 05:37 PM
Depends which O2 circuit. If the problem is in one of the front O2 sensors, the engine will no longer run in closed loop mode and would lead to all sorts of problems.

If the problem is in one of the rear O2 sensors, than you can get an O2 simulator, which plugs into the same port and just "simulates" a properly working O2 sensor and catalytic converter. It is illegal in some states/counties that have strict emission laws so be sure you know what you are doing, first.

For the price, you might as well just figure out if the issue is in the wiring harness or the sensor itself is bad. The sensor is about $80. Doing it yourself isn't too difficult, but you may have to heat it to get the sensor out.

06-07-2010, 09:29 PM
thanks.. I have replaced a few O2 sensors in the past, but the code is the one for "Lack Of HO2S11 Switches - Sensor Indicates Lean"

I was told I may be chasing a bad ground in the O2 circuits ? :confused: ?

06-08-2010, 09:12 AM
That's the bank 1 sensor 1 (upstream). That is not an O2 sensor you can run without - it communicates the amount of O2 in the exhaust to help determine the optimal air/fuel mixture.

It may be a bad wire/connection somewhere. It may also be a bad sensor, or it may be an engine issue. If you have a problem with one of the cylinders on bank 1, you can be running lean (i.e. weak fuel injectors).

Use a multimeter to check for continuity in the wiring harness. Swap the upstream sensors from side to side, too. If the code follows it to bank 2, it's the sensor that needs replaced. If it stays on bank 1, than it's an electrical problem or air/fuel mixture problem on that bank.

06-08-2010, 12:38 PM
Thanks for the help! I'll try swapping around the sensors. As far as testing the harness, where would I find continuity to? terminal to terminal in the harness? One side or another to ground?

I suspect with 125,000 miles on a P71 a bad sensor (or 3 or 4 :rolleyes: ) is pretty darn likely. I was just afraid that according to the advice I got that the problem was more likely wring than the sensor and I hate throwing 30-40 bucks a copy at sensors if they aren't the problem. Also, I will usually have sensor faults show up on 1-1, 1-2 and 2-1 when I check codes. I was thinking since so many codes were going off that might indicate a bad wire somewhere messing everything up generally. It isn't really creating any driveability issues...

I appreciate your input. You wouldn't have any insight on my EGR thread by chance, would you? :biggrin:

06-08-2010, 03:16 PM
I don't have any wiring diagrams so I can't really help much when it comes to tracing the EGR voltage issue or this continuity. You'll have to find a wiring diagram and see which lines from the PCM are for the various O2 sensors and make sure the current is flowing from that end to the end of the wiring harness. If it is, than it's isolated to the sensor. And again, if you swap the sensors and it moves from one side to the other, it is definitely a sensor fault.

If you are constantly getting all three of those codes, than I would really put my money on a wiring issue. That is not normal, even at 125k.

Your car will still run okay, but it will not be running with a proper air/fuel mixture. This will waste plenty of fuel and can kill your cats if it goes too long. The PCM automatically stays in open loop mode, meaning it runs on default air/fuel and spark tables that are cautious and dumps extra fuel in the cylinders to prevent lean conditions.

06-08-2010, 03:55 PM
FYI: Save your money on the downstream 02 sensor simulators. They're fine until the ECM runs the monitor on the converter. When it does, the check engine light will come on, guaranteed. When the ECM runs the monitor, it changes fuel mixture from full rich to full lean one time, very rapidly. It then measures the time it takes for the mixture change to be detected by the downstream 02 sensor. If the mixture change isn't detected in the correct amount of time, (too soon or too late) a DTC will be set in memory and the check engine light will come on. As mentioned, NEVER use a simulator in an upstream position, your fuel mileage and performance will be gone.

06-09-2010, 05:45 AM
Good advice, gentlemen... thanks.

I think I need to chase wiring down. It's on me. I realize now there isn't much that can be done over distances to sort this out. If you have any suggestions as to where wiring tends to have problems (notoriously corroding connectors, rub points for harnesses, etc.) I'd appreciate it, otherwise, I'll keep going here...


06-09-2010, 10:35 AM
Before you start looking for bad wiring, I'd swap the sensors and see if it follows. That may shorten the time spent looking for the issue if the sensor is infact the problem.

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