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93 LS400 emissions problem


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Just had my throttle body and idle air control valve replaced bc when I took my foot off the gas it would die. Now it's working fine. But went to get it inspected failed the emissions test. Here are the results :

High Speed Emission Results

Hc(ppm) 206 Fail

CO (%) 1.70 Fail

CO2 (%) 13.0

02 (%) 0.2

Low Speed Emission Results

HC (ppm) 149 Fail

CO (%) 1.41 Fail

Co2 (%) 13.2

02 (%) 0.2

The Nox (ppm) and Dilution both passed

I was told it is running too rich, that it was using too much gas and not enough oxygen. Could anyone tell me what the problem might be or what to check first to pass inspection? Thank you very much


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The things it can be are. And they are NOT in any special order here.

A leaky fuel injector, leaky cold start valve, ignition system misfire (caused by bad spark plugs, wires, cap, rotor etc. even the ecu or the ignitor) too much fuel pressure, worn out O2 sensors, bad or weak ECT (Engine Coolant Temp sensor)

Do not attempt to clean the airmeter / mass airflow sensor, that will ruin it.


If it was me, the things I would replace, that is if they have not been replaced in many years are one the O2 sensors, and two the ECT. And normal tune up stuff like spark plugs, checking the wires and other ignition components.

And yes before just jumping in on that try to figure out what is happening first. A check of fuel pressure with pump on and off will tell one the pressure and then if the system holds the pressure for a bit, if not then possible injector leak.

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