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Perhaps this may help you in your search for causes. A high hydrocarbon reading is usually unburnt fuel. If the CO reading and the NOx readings are below the limit then look towards an ignition problem as being the most likely culprit. As the prior answer suggested, start checking items and begin with the basics. Year, make model, engine size, options, symptoms etc will help you get a more accurate answer next time.

Check the plugs first, all of them! One bad plug will give you a miss and everytime the car misfires unburnt gas goes into the cylinder and out the exhaust which raises the HC. Same goes for the wires (check resistance with a multimeter) check the distributor cap & rotor (if applicable) for cracks, corrosion or burnt areas. Might want to do an injector service (if applicable), a sticky injector might put fuel in at times when its not supposed to and up goes the HC. If the check engine light is on 1st thing to do is pull some codes and this will give you some direction. Codes can be pulled manually by using the diagnostic connector or a scanner.


High hydrocarbon levels are also seen with a much too lean mixture. If you are looking at an older carburated car, check this first. A lean mixture will not burn completely, and unburned mix goed out the tailpipe. An overly rich mixture burns fine, but to carbon monoxide, not dioxide. So a rich mixture will fail on CO, not HC. This is the reason for the vacuum hose fix - it cuts down the amount of air entering the engine, while leaving the fuel the same ( or increasing it a little if the leaks are really bad ). This said, the ignition system is also suspect, as any misfire will increase HC. So, check your vacuum hoses (lightly spray connections w/ carb cleaner, an RPM increase means a leak), change your plugs and wires if they're more than a year old, and richen the mixture a hair if this adjustment is available.

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I had to replace a lot of vacume lines to get mine to pass. It also could be o2 sensor, bad spark plugs, rich idle(too much gas), bad spark plug wires, bad injectors(or carburetor), bad ignition coil/cap/rotor(weak spark), bad catalytic converter/clogged muffler, exhaust leak, etc.

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โˆ™ 2014-05-09 19:55:15
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