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Failed Smog Test

1994 GMC Sonoma high HC, above 2000 ppm.

Emission check failed. My GMC passed the 2500 rpm portion, but the idle test failed. HC should be less than 220 and the test shows 2012 "2012". I think the the MAP or O2 sensor needs replace?. The truck runs fine and currently is getting 24-25 mpg (and always has). Am I on the right track?


Yes and no. Your GMC Sonoma's MAP and O2 sensor might be defective, but we'd recommend going over the basics first. HC amounts above 2000 ppm usually indicate one or more cylinder/s igniting less then 50% off their fuel content, indicating either no a spark condition, no compression condition, or defective ignition timing. It is recommended to begin your vehicle's diagnosis by inspecting the ignition system first. Make sure your spark plug wires are all good, the sparkplugs are not worn out, the distributor cap is not cracked, and the distributor rotor does not have carbon build up. Once these things are verified, you can move on to checking the electronic components (sensors). Definitely don't simply replace the MAP and oxygen sensors without verifying they are defective using a multimeter or scan tool first.

posted by SmogTips Support

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