failed emissions test


Old 02-26-04, 07:27 PM
Thread Starter
Join Date: Apr 2002
Posts: 52
Unhappy failed emissions test

1988 Sentra. 120K miles. Failed the Hydrocarbons test. I guess the car is passing unburned fuel through the system. Can someone please tell me what I should look at?
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Old 02-27-04, 04:16 AM
Join Date: Apr 2002
Location: Pittsburgh,Pa
Posts: 2,417
Cap,rotor,wires,plugs,injectors,valve adjustment,vacuum leak,overadvanced ignition timing just to name a few.Readings from the test and mileage would help as would past maintenence.
Old 02-27-04, 04:32 PM
Thread Starter
Join Date: Apr 2002
Posts: 52
Hydrocarbons standard= .80 My reading was .99

Maintanance- standard upkeep,oil. tune-ups.
Old 02-27-04, 08:39 PM
Join Date: Feb 2004
Location: So Cal
Posts: 248
failed emissions - HC


You're right about the unburned fuel.

Here in So Cal we usually start with a check of the oxygen (O2) sensor to make sure the vehicle is "in fuel control". I use a graphing multimeter to do this so I can "see" what the waveform looks like and make sure the sensor is switching back and forth like it should.

If you don't have this capability and it's been a while, it's best to replace the O2 sensor. OEM is not that critical and AZone is cheap. Honda wanted $197, but AZone had them for $40. Just be sure to get one you can plug right in as the universal ones you have to crimp or solder don't always do the best job. I recently had my 86 Accord smogged and dropped the NOX emissions from 1800 ppm to 454 ppm just by replacing the O2 sensor. It had failed "lean" and was causing the failure.

A couple of other things to check; cap, rotor, plugs, wires, fuel filter, timing (7 degrees BTDC on that model if I remember) and for safe measure, an oil change (oil gets saturated with gas and drives up the HCs).


Old 02-29-04, 01:11 AM
Join Date: Jun 2002
Location: Long Island, NY
Posts: 1,531
Sometimes the oil being diluted with gas due to short trips possible cold weather operation will cause HC failure. You are not over the limit by much. How old was the oil change when it was tested?

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