Emission tests

Our 2001 Hyundai Accent failed the emission test by only .03% for the CO% here in Ottawa, Ontario. Our mechanic did a tune-up and he also recommended that we put the highest test gasoline in the car and take a long trip just before having it tested again so the engine is hot too. Does anyone have any suggestions to add to this?

Thank you.


I believe that high CO is a fundtion of an underactive catalytic converter. I’d doubt your cat is bad in such a new car. Getting the car very warm (a 20 minute spin on the highway for instance) will help ensure the cat is operating at maximum efficiency.

I’d stick with regular. Higher octane will not reduce your CO emissions.

Higher octane fuel could make the emissions worse. Higher octane just means it’s a slower burning fuel. It has no other “special” properties. I would recommend a tankful of Exxon(?) gasoline with Techron fuel injector cleaner. Were the spark plugs changed? Fresh spark plugs are one of the techniques of lowering emissions. And change the air filter. Before the test, drive the car on the freeway until it gets to normal temperature. At the test station, DON’T shut off the engine while awaiting your turn. I think you’ll make it.

Agree; the key item here is thorough warmup. I failed a test in the past becaue I did not warm up the car suffuciently. The high tets gas advice is hogwash.

Another thing I’ve been told is to fill the tank with at least some %age blend of ethanol.

“Tune-up” is a bit of a misnomer anymore. It should almost be considered a maintenance service.
Since the spark plugs were obviously replaced what about the plug wires?
How many miles on the car and is there any oil consumption at all (which can cause this)?
Does the engine idle smooth at idle or is there any roughness to it, even a slight roughness? If so, that could be a sign of a couple of things; either of which could cause your problem. One is a vacuum leak and the other is a fuel injector that is developing a bad spray pattern.

If the engine has been tuned up and the oil changed, just run regular fuel and get the engine as hot as you can and take it in. Premium fuel will not improve things on a car that cannot benefit from it, and possibly make things worse.

How long did you drive it before going in for the test? Maryland reccomends that you drive for at least a half hour before going to the emmisions test station to thoroughly warm the car up.

CO, carbon monoxide, is a result of incomplete combustion and/or rich mixture. While the post-platinum palladium portion of the cat converter allows some of the freed up oxygen to bond with some CO to become carbon dioxide, the cat converter is not the source of the CO.

The tune up is a great place to start. A well tuned up engine produces fewer incompletely combusted byproiducts. And yes, getting the engine warmed up is also important. A fully heated up engine produces lower CO for a few reasons. First, a hot chamber more completely combusts the fuel droplets. Second, a cold engine runs richer. Since combustion is incomplete, it needs more gas to compensate. Third, in a modern car the upstream oxygen sensor sends a signal to the ECU to tweak the mix, and on a cold engine that sensor loop is bypassed in the computer to allow the engine to run rich.

Your mechanic has given you a very good start. If you still have readings too high post back and we’ll go deeper.

You indicate that you are very close to passing. What is the max figure allowable for CO? How was the HC figure, on the high side as well? If so, your problem might be a sluggish O2 sensor. Your engine must be cycling between lean and rich rapidly to make the three-way catcons work. When lean, more NOx are produced and reduction of NOx provides oxygen to oxidize the CO and HCs produced when it is rich.

Change the cat with an aftermarket one from an exhaust shop and the CO will be much better.

Thank you all. The spark plugs were changed and we will run it before taking it in again. It failed by such a tiny percentage it should pass this time.
Thanks again.