nc emissions test failed 1997 chev w/t

Firstly, I work with computers and not computers on trucks - I am not an automotive mechanic or have ever claimed to be mechanically inclined.
The check engine light is on steady. We have replaced the gas cap and changed the fuel in the tank with new high grade fuel. It's a 1997 Chev W/T 1500 2wd. It was purchased in Ontario Canada, and is now failing a NC emissions test. A mechanic has checked the truck and has indicated the problem is not the oxygen sensor - but probably some type of problem in the electrical wiring harness. Might that be a ground fault - a bad cable going to ground on the engine or frame? An intermittent fault of some type?
The truck only has ~45000 miles.
What can I check re the wiring - is there anything easy - ie. volts, ohms or current and if so where do I check?
Any other easy checks. I have a very mechanical father in law - so we can probably follow more advanced suggestions.
All advice to resolve the problem is greatly appreciated.
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On my 91 its been the EGR, the Cat and the O2.

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If the check engine light is on, there will be one or more stored trouble codes. You need to have someone read the trouble codes with a scan tool or code retrieval tool. The things that cause the check engine light to come on number in the multiple hundreds, it would be a fools errand to guess on what problems your truck has at this point.
Post back with the trouble codes.
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Greetings,
First, take your truck to another mechanic. If your SES light is on continuously then it's not an intermittent problem. Find out exactly what the codes are in the truck's ECM. There's really no use taking any corrective measures (like a new gas cap and fuel) if you don't know what the problem is. Heck, you could end up spending quite a lot of money on guesses when the first step is to just run down to a local auto parts store and have them hook up a code reader and tell you what the ECM is seeing and start from there.
Any suggestions other that reading the codes first would only be a WAG based on what info you provide.
Cheers - Jonathan

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