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Q: My "Service engine soon" warning light appeared? on 1998 GMC 1500 Pickup

The service tech scanned it, said it was the transmission due to a possible slip? He checked the transmission, it was ok and he cleared the code. Now for some reason, it wont pass California smog test and he says its because of the previous transmission warning light that he cleared. He said to drive it 50 miles, bring it back and it should clear smog? IT WON'T, why?
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problem has nothing to do with transmission. it is a prob. with exhaust system, more specifically your cat. conv. if your cat.s fail, than you fail smog test. it is possible that your cat.s are done and no way to save them, however there are other things that contribute to failing emissions.
your fuel system could be clogged and due for a clean. ethanol treatment to 1 tank of premium. ethanol in fuel causes build in fuel system, combustion chambers, exhaust system and also causes increased wear to spark plugs. a clean fuel system will allow for cleaner burn and reduced emissions.
a strong ignition system will allow for cleaner burn. perform tune up and upgrade spark plugs to premium type. examine ignition cables. if u need to replace them, rec. performance grade type such as accel's. this will allow more conductivity to the spark plugs giving stronger spark and a better burn. this will reduce emissions.
using premium type oils(synthetics) for engine will help also. recommend using oil stabilizer with next oil change. this will recondition seals, cylinders, and reduce oil consumption. all engines consume some of the oil supply. consumed oil has to go somewhere(out the exhaust). rec. treatment every 3rd oil change.
strong, clean air flow will allow engine to breath properly. engine breaths easier, it burns cleaner. rec. upgrade from conv. air filter to high air flow design such as K&N.
these are all things you can do for yourself(save money on shop labour).
above all, bring ride in to a shop and have a diagnostic run to find any other issues you should be aware of. good luck.
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