Q: High Nox on 2001 Chevrolet Express 1500

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I'm having problems to pass emissions. I already took my van to a three trained technicians and they couldn't find anything wrong. The level of NOx is a 2.5 were it should be a 2. I really would apreciate any help on this matter. There is no codes stores at the computer. Replace a spark plugs, did a good tune up still failing.
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These are the usual causes for high NOx:
Misfire condition
Malfunctioning or improperly adjusted EGR valve
Failed oxygen sensor
Leak in exhaust tubing upstream of converter
Excessive carbon deposits in combustion chamber
Improper spark advance
Blocked coolant passage
Overly lean air-fuel mixture
Damaged cold air duct
Failed or malfunctioning catalytic converter

In my experience it's mostly a failing cat. converter and a bit higher than normal operating temperature.
I'd check the entire cooling system, make sure the radiator fins are clean, the fan clutch works good, thermostat opens ( you can try a lower temp. thermostat ).
If the cooling system is OK, then I'd go for the catalytic converter. On a 10 years old truck it could be the problem also.

The thermostat wasn't working I replaced it and NOx went a little bit higher when tested again, the van has two catalytic converters they look good visually but you are right there are 10 years old probably a good idea just to replace them. Thanks


Previus owner changed a wrong catalytic converter, I replaced with an obd II converter an it Pass. Thanks