High Carbon Monoxide, P0174

[BeemerBoomer123]

Hi

I've had a 2005 BMW 330CI for a couple of years. The last MOT showed that carbon monoxide emissions were high - very close to the legal limit




The guy doing the MOT pointed out that it 'smelled a bit rich' and I could see what he meant.

So I run some Techron through it and replaced the air filter. I also got myself a Veepeak OBD2 scanner and OBDFusion

Now if I run that I quickly realize I'm way out of my depth in diagnosing modern engines. I got these results. These are all at idle, but I gave the engine time to warm up - the engine temp needle had reached its equilibrium position in the middle of the gauge.




So it falsely thinks it's running too lean and is thus running too rich?

Here are the short term and long term fuel trims



From my limited understanding of things mostly from watching videos like this, these are too high.

And here are the O2 sensor readings, I think B1S1, B2S1 are pre catalytic converter and B1S2 and B2S2 are post catalytic converter.



This is at idle - haven't had a chance to do a highway run yet. However, it looks like the good result in this video. Next time I do a highway run I'll check again.

The reason I want to figure out what is going on is that I've got an appointment with a garage to regas the airconditioner. If I could figure out what's wrong I could get them to do a repair on that too. I don't trust them to do troubleshooting based on past experience but if I could diagnose it I'd trust them to do a part replacement. Especially if I can check something like fuel trims to make sure they've actually done it.

[MarcoZandrini]

How many miles on the car? Are the ore-catalytic converter oxygen sensors original. They are considered to be consumables by car manufacturers. They need to be replaced every 100k miles as they under-report the amount of unburned oxygen and react slowly. How does the car operate? How’s the acceleration?

[whiskychaser]

The needle on the temp gauge is weighted and will get to the 12 o'clock position at about 75C. I doubt that is hot enough for accurate diagnostics - your kit will be more accurate. You have a thermostat fault code and that could affect how efficient your engine is, particularly at part load. So it may contribute to the rich mixture. I'm not saying replace it - just investigate it. Maybe pop the bonnet and make sure the wiring is intact?

P1074 means that the system has reached the point where it cannot adjust the mixture any more. It has probably put your SES light on but you can always turn it off again. Don't assume you have faulty O2 sensors because a more likely cause is a vacuum leak. If the short term fuel trims go down when you increase revs, that could well be the case.

[MarcoZandrini]

A vacuum leak would cause a P0171/0173 code to pop up. The engine is running rich.

[bpde328i]

On my '05 330Xi, was plagued too with P0171/P0173 codes. Replaced all vacuum lines, but still codes would appear occasionally. Ironically, just recently installed NGK spark plugs and Delphi coils. No more codes since! Nice boost in pedal response too! If you're near needing plugs or coils, you may want to make the jump.

[whiskychaser]

Dug out my last MOT certificate for comparison. FWIW, my car has 120k miles on the clock.

Fast idle Your actual My actual
CO 0.197 0.057
HC 75 14
Natural idle
CO ? 0.066

Your trims are all + so the system thinks the mixture is lean and is adding fuel. If it thought the mixture were rich, those would be - numbers and the long term trims would come down. Of course, the MAF could be telling lies. My LTFTs were 12% until I discovered my MAF was up to 40% out. So it is well worth checking the air flow.