How do you lower the hydrocarbon levels for emission testing on a 1995 gmc sierra?

Answer 1

What the limit is and what the reading was would help. Did it fail the idle section of the test or the running test or both? what was the NOx and CO2 readings like?

Anyway, a generic explaination & answer:

A high hydrocarbon level when tested is the vehicle not burning all the fuel introduced into the engine for whatever reason.

Eg: An engine miss can cause this by not burning the fuel injected into the cylinder at the time of the misfire.

Eg: A defective oxygen sensor can tell the computer to richen the mixture to a point where too much fuel is introduced into the engine

Check the following, start with the basics:

Pull all the plugs and check them, check ignition wires with an ohm meter for resistances. Check any and all ignition pieces. Test O2 sensors, fuel pressure as well as injectors, vacumn hoses, etc, etc, etc...

Unless you have the testing equipment and know how to use it the list goes on & on and you will spend hours upon hours to diagnose this problem. Take it in and have the diagnostic done by a professional is my recomendation. Check the basics first though just in case you find a simple problem to fix. When was the last time you did a tuneup? If it failed by a small enough margin how about an oil change? Dirty oil will also store hydrocarbons. How does the truck run in general, might be related. Is the check engine light on so you can pull some codes for direction?