damonoresky

Here are all the results attached. All the numbers look great except the ones in yellow highlight...

GregBBRD

Yup, that is what was needed.

See the measured results in the CO column (0.00)? Too lean, driving up the NOx. Intake leak, dead MAF, or bad O2 sensor, fuel pressure too low (uncommon).

Start with the intake system first...if your mechanic has a smoke tester, have him do a smoke test. If he doesn't have one...find someone who does.

damonoresky

Super info. I have a line on trying a different MAF from Rich Is it worth just replacing the O2 sensor anyways? They aren't that expensive.

GregBBRD

You can check the output of the O2 sensor with a volt meter. It should fluctuate from .2 to .8 volts (when connected), fairly quickly....never actually settling on a voltage, but constantly changing back and forth.

The injection system responds to the O2 sensor's output and changes the amount of fuel injected....constantly going from rich to lean. When the O2 sensor reads lean, the injection system injects more fuel. This "additional fuel" makes the engine too rich and the O2 sensor reads rich, and the brain then reduces the amount of fuel....starting the process all over. This rich/lean thing goes on multiple times a minute...and is simple to see with a volt meter.

The problem: With an intake leak or a bad MAF, the O2 sensor may correct the fuel mixture as much as it can (it has limits), but not be able to correct the mixture enough. It will then be "stuck" in one spot, which can mean it is bad....or at the limits of its ability to correct.

Like you say, they are cheap. If it is stuck at one spot, replace it. If it is still stuck after you replace it....there is something else wrong.

John Speake

The problem may be the air pump sending air into the cats ? In the UK there is a lambda test for the last GTS cars with cats. They always fail lambda unless the vac line to the air pump diverter valve is unplugged and capped.

V2Rocket

looking at those numbers it does look quite lean.
"ideally" the CO2% should be around 15% and the O2 % should be 0.1% or so.

dr bob

Reading tailpipe CO and HC numbers trying to diagnose high NOx is a crapshoot if the cats were warm and working. They may give you a hint but nothing definitive. Greg's simple voltmeter test from the oxy sensor is a much more reliable indicator of -overall- combustion mixture. One or two weak injectors will trash NOx with "good" CO numbers as the LH compensates.