1996 740iL O2 sensor circuit problems

I recently replaced my catalytic converters with some Flow Maxs cats, due to fact that I was receiving a 430 code indicating that Bank 2 cat was operating inefficiently. However, after replacing the cats the 430 cleared, but I started getting the codes 130, 136, 150 and 156 indicating that all my O2 sensors were degraded or that the O2 sensor circuits were malfuntcioning. Sounded pretty strange that all of them would degrade all of a sudden. Nevertheless, since the car had about 74k org miles on it, I went ahead and replaced them all. I cleared the codes and drove it ~ 5 miles or so and they all reappeared. The car is also running like it has a slight miss at idle. I replaced the spark plugs also, but there was hardly any difference in performance...the engine light is still 'On' with codes indicated above. Has anybody experienced this problem? Could my engine control unit be going bad?:banghead:

It's possible your ECU is bad, but I doubt it.

Usually a Cat code is O2 sensor related...so you replaced your cats and now O2 sensor codes are coming up...not really surprised personally.

Unlike most Ford vehicles, BMW cats were designed for the car. The engines was tuned with every component connected to the engine, so the factory cats work best, however expensive they are.

There's two possibilities: The aftermarket cats are working fine. But because it isn't factory spec, the O2 sensors are reading different and the engine's computers are picking up readings outside of factory spec.

Or the O2 sensors are truly naffed, but it's not all too common for all four to go at once.





If I were you, I'd check the voltage going to the O2 sensors and make sure they are still in good running order. If those check out, I'd be looking at the aftermarket cats as the problem and maybe invest in a set of factory cats, however expensive they may be, because believe it or not, most of these parts that are connected to the engine are considered 'high performance' by third party companys, such as auto parts stores. It even says in my BMW manual for my 1990 750iL that the ignition parts are high performance parts.

First of all, thanks for your response. Unfortunately, I believe you are correct about the cats. After looking at the OEM prices compared to the after market ones that I have, I suspect that they are not properly tuned for this engine. Also, I miss quoting the name of the cats that I had installed…they are Magna Flows (~$150 each). I thought that they were a pretty reputable brand for high performance, but maybe not for BMWs. I’ll go ahead and check the voltages like you suggested and see what they are reading. Not sure if you read in my first thread that I had already replaced the O2 sensors (all four), but got the same performance.

I also got some O2 readings earlier, but they were taken with the O2 out of the circuit…
The heater circuit is reading ~12VDC as expected and the O2 control circuit was reading around 0.18, I probably need to make this measurement again, with O2 in the circuit.

Nevertheless, my engine diagnostic tool, O2 Test indicates that the new O2 sensors are not functioning. It indicates that they are flat dead at 0.0VDC. The mechanic at the European shop read the same. I not sure if the tools are testing them properly either. However, when I reviewed the snapshot data from the tool, it shows a reasonable voltage for the sensors...they changed as expected when the engine RPMs changed. I’ve just about run out of ideas.

Oh, the mechanic says that he don't believe the ECM is degraded either. He said that he has never replaced one on a BMW before.

I will definitely try and manually read the voltages again as you suggested, but it’s starting to look like I going to have to take it to the BMW dealer :shakehead…Ouch!!! I’ll let you know the new measurements indicate soon.

Sorry, I didn't see that you replaced O2's... my appologies.

If the O2 harness is getting 12V with the O2 disconnected, then it's most definately something wrong with the sensor itself. Since it's pretty uncommon for all four to go at once and the diag is saying there is no voltage to the O2, then this to me sounds like the cause of your problem...however, unfortunately for us, I don't know why the diag would be reading 0V. But, for whatever reason, 0V's is coming up which the ECU is picking up and can't compensate.

Like you say, test the O2 sensors with the sensor connected and see what comes up. You may have to remove the O2 sensor from the exhaust piping.

One thing I'd be interested in finding out is if the car will run fine with factory cats...perhaps the O2 sensors are perfectly good...







For references and more info for everyone, here's more about O2 Sensors ('Lambda' or 'Oxygen' sensors)
http://www.picotech.com/auto/lambda_sensor.html