Hello,

I recently purchased a 2001 S500 from my father. Prior to purchasing it the check engine light was on. It was taken to the dealership in Pennsylvania and the code read as the mass airflow sensor. This was replaced and the computer was reflashed. I had it shipped to me in Arizona, where I need to pass emissions. I initially took it for emissions, but was denied because it had not completed the drive cycle. Shortly after the check engine light returned showing codes P0422 and P0432 for both catalytic converters. From what I've been told, it is not typical that both cats go bad at the same time. The car has 82,000 miles on it and I have not heard any rattling from the exhaust, which I have read is typical when the cats go bad. I replaced both o-rings in the housing for the mass airflow sensor and reset the light, hoping this would help because the old o-rings were bad. The check engine light has returned with the same codes. Prior to jumping to replacing both cats, I want to confirm there is not another underlying issue and the cats really aren't bad. There is also a slight misfire at idle, but there is no indication of a misfire when the cylinders are scanned with the computer. Can anyone please help steer me in the right direction?

Thanks,

Marty

 

no codes for the 02 sensors? can you here the start pump? if you have to change cats you don't need to put oem, just go to autozone buy them and have a muffler shop install them. the mis fire could be plugs and coil packs also.

 

Thank you. No codes for O2 sensors. Pulled the plugs and wires. Some corrosion. Hopefully the culprit for the miss. Will know more tomorrow when the wires come in.

 

Fix the misfire before spending money on MAF or cats. A misfire can and will cause codes that point to other sensors/parts, but are actually due to the misfire.

90% of misfires are plugs, wires or coils.

Try a smaller gap (.030") on the plugs.

 

I agree that almost all misfires at higher engine speeds and larger throttle opening are due to secondary ignition problems, with the few exceptions often being injector problems.


But...


Misfires at idle might well be vacuum leaks that only affect one or two cylinders. An unmeasured air leak to one cylinder can easily lean the mixture in that cylinder enough to cause a misfire.


One way to find such leaks is to use the earpieces of a stethoscope and a long (4'/1.5m) hose to locate the hiss.

 

A vacuum leak can affect one cylinder if it is a vehicle with isolated manifolds like the Weber velocity stack carbs of the 60s and 70s. In that type of setup, each cylinder had it's own manifold and you are truly tuning each cylinder independently.
With a shared intake manifold, a vacuum leak can not affect only one cylinder unless it is between the head and the injector, which is usually not the case.