The car failed emissions on NOx, everything else within spec. Test is an
acceleration on a dyno from 0 up to 30 mph and then done to 0. The test
runs for 30 seconds. NOx spikes at 8 seconds to about 12 seconds and then
drops off to nothing by about 20 seconds. Normal, since that's the
acceleration part of the test, but it exceeds the limit by about 1/3.
Although the common cause is the EGR, the engine doesn't have one (even
though the garage said they were getting me a price on changing it, after
they told me the car failed).
I've run through the diagnostics and at this point, I'm at a loss. Two
possible causes are low MAF readings and failure of the knock sensor. The
MAF reads about 14 gpm at 2500 RPM (read live from the ECM) and the knock
sensor shows 0 throughout acceleration. The MAF sensor wires have been
cleaned, with no difference. I've also checked its passage. The knock
sensor has been tested and it responds to raps on the block. I've also
checked its connection. The oxygen sensor is fairly new and seems to
Does anyone know where I can find typical operational values for these
sensors? Or have any other suggestions? The engine is a 3.3L, with about
112K on it. It runs well, especially since I had the injectors cleaned. I
put some Guaranteed to Pass in, hoping that the problem is carbon build-up,
but I'd feel better knowing that everything else is in shape. I could just
change the MAF and the knock, but at $210, I don't want to just waste the
Thanks for any help.