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High Hydrocarbon Emissions - 1991 GMC Sonoma

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Failed emissions


High HCs means un burned fuel ie it runs rich at idle. Could be a egr valve partially stuck open. (it should be 100% closed at idle) could be worn out injector nozzles, could also be a sticky iac (idle air control) motor. But probably not a computer problem.

Nov 05, 2013 | Cars & Trucks

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Check engine lite on with PO421 code


P0421 = Catalyst System Efficiency below Threshold(Bank1) The catalyst's efficiency is demonstrated by its ability to oxidize CO and hydrocarbon emissions. The Powertrain Control Module (PCM) compares the output signals of the front and rear oxygen sensors to determine whether the output of the rear sensor is beginning to match the output of the front oxygen sensor. Air/fuel mixture compensation keeps the frequency of the front oxygen sensor high due to the changes from rich-to-lean combustion. The catalyst causes the rear oxygen sensor to have a lower frequency. As the catalyst wears, the rear oxygen sensor's signal trace begins to match the front oxygen sensor's signal trace. That is because the catalyst becomes saturated with oxygen and cannot use the oxygen to convert hydrocarbon and CO into H???O and CO??? with the same efficiency as when it was new. A completely worn catalyst shows a 100% match between the frequency of the front and rear sensors.

Aug 24, 2011 | 2004 Hyundai Santa Fe

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Mine want pass emissions high hydrocarbon runs slugish. sometimes wont start takes a couple of times to start could that be main relay


Many things can cause this, so I'll just try to hit the high spots.

1. Poor fuel ignition (bad plugs, wires, cap, rotor, etc.
2. Improper Ignition Timing
3. Lean air fuel ratio mixture
4. Defective Catalytic Convertor
5. Defective Air Injection System.
6. Low Compression

These are general things that can cause this condition, but they may not be applicable to your particular vehicle.

Try a good old fashioned tune up: Plugs, wires, cap, rotor and points (if applicable) and a new air filter.

Wouldn't be a bad idea to also change the oil and oil filter.

The final item (#6) is a sign of a well worn engine and if this proves to be the primary problem, I'm afraid a engine rebuild is in order.

Anyway, good luck!

Feb 01, 2011 | 1991 Honda Civic

1 Answer

My son's 1995 Ford Taurus failed the HC-hydrocarbon emissions. How do I fix this?


did the car have an oil change before the test? has the throttle vlave ever been cleaned of accumulated carbon build up, is the engine reaching normal operating temperature? if not these are common reasons for the HC failure.

Sep 22, 2010 | 1995 Ford Taurus

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My 91 305 willnot pass deq has high hydrocarbon level what can i do , does not seem to be as peppi at throttle response


A high hydrocarbon level would most likely be caused by a fuel delivery problem or a vacuum leak in the intake system causing the engine to run too lean, or a problem with the ignition timing not being timed correctly.

Jun 08, 2010 | 1991 Chevrolet Camaro

1 Answer

Hydrocarbon & carbon monoxide levels too high


Cat or secondary air system not doing there job.
Have the vehicle serviced by a repair shop this emissions stuff expensive and legallies be careful.

May 21, 2010 | 1991 Subaru Legacy

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1996 spotage kia p0422


P0422 Main Catalyst Efficlency Below Threshold (Bank 1) The catalyst’s efficiency is demonstrated by its ability to oxidize CO and hydrocarbon emissions. The Engine Control Module (ECM) compares the output signals of the front and rear oxygen sensors to determine whether the output of the rear sensor is beginning to match the output of the front oxygen sensor. Air/fuel mixture compensation keeps the frequency of the front oxygen sensor high due to the changes from rich-to-lean comvustio. The catalyst causes the rear oxygen sensor to have a lower frequency. As the catalyst wears, the rear oxygen sensor’s signal trace begins to match the front oxygen sensor’s signal trace. That is because the catalyst becomes saturated with oxygen and cannot use the oxygen to convert hydrocarbon and CO into H₂O and CO₂ with the same efficiency as when it was new. A completely worn catalyst shows a 100% match between the frequency of the front and rear sensors. The ECM sets P0422 if the average of caculated oscillation size of rear HO2S value is higher than threshold value during predetermined time.

Apr 23, 2010 | 2000 Kia Sportage

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My 2000 Nissan Frontier won't pass emissions for an unknown reason. It isn't coming up with any codes that would explain this. I had the mass air flow sensor replaced about 2 months ago. The check engine...


Well what kind of numbers are they getting from the exhaust on the smog machine? Are the hydrocarbon numbers close to passing level or are they at gross polluter level? There are a lot of factors involved with high emission numbers. Sometimes just doing a basic tune-up can help clean up the numbers. Things like making sure the air filter is clean. Changing the fuel filter,plug wires, and spark plugs can help. Is the catalytic converter ok? Could have a bad batch of gas also. You could try running some fuel injection cleaner in the tank to help clean up the numbers. And the O2 sensor can cause problems with the emissions. It would be hard to pinpoint the exact problem with out a physical inspection of the vehicle.

Nov 26, 2009 | 2000 Nissan Frontier

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