Truck has high co% concentration during emissions test. What trouble shooting can be done to diagnose cause?

Did the tester not tell you why it failed? I would think no muffler would be sufficient cause.

CO? That means Carbon Monoxide - it's a rather toxic and poisonous gas emitted from the exhaust system.

When they check emissions on cars CO is one of the gases they measure. If the level is too high .. your car fails the emissions test. I believe in the USA the emissions test is known as the 'smog test'?

If you have a problem with excess CO emissions on a car the cause is often an incorrect fuel/air mixture or a faulty sensor called the lambda/oxygen sensor in the exhaust system

sounds like you have a vacuum leak and your running lean.

MAP sensor needs cleaning or is bad ?

Take it to autozone or another place like it and have them read the codes and tell us what they are.

Sounds like you lost O2 Control

Check fuel trim & data with professional
scanner & such

You done as far as working at home, you did all
you can

You have a rich condition (CO)

Seems as though the coolant sensor is not working &
your in open loop

Did you install new spark plugs? Definitely replace the O2 sensor.

NOx emissions are the result of high combustion chamber temps pure and simple. NOx readings go up whenever there are hot spots that exceed about 2400 degrees. Things like lean mixtures contribute to high chamber temps, as does over-advanced ignition timing.Lean mixtures are usually caused by some failed components or by air (vacuum) leaks in the intake.
Poor fuel quality contributes to detonation, another cause of high chamber temps.

1. Check to make sure the vacuum system is tight.
2. Try a full tank of high octane fuel(run at least half a tank through before testing)
3. Have the tester do the rolling tests with the trans locked in first gear.
4. Make sure the oxygen sensors is working.

If you have done all of this and still fail, You will need a new catalyst

theres 3 things ur emisans fail on 1 spark plugs 2 air filters need changing 3fuil filter so yes a full service should do it change ur oil fuil filter air filter oil filter and oil

Do an oil change with 20W50 oil (or one grade higher) to prevent blow by during the test.

You current oil has HC from blow by though the rings.

Do a fresh oil change will lower the HC.

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Change spark plugs,wires,cap and rotor if they are over 24,000 miles
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Make the smog check appointment for the EARLY MORNING. Colder air will help the fuel burn better.

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Clean or change the air filter and pcv valve.

Clean Mass Air Flow sensor with CRC Mass Air flow cleaner or electronic part cleaner.

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Do a quick wipe down on the wires and cap to take the moisture off.

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Check spark plug wire contacts for corrosion.

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Fill the tank to around 3/4.......just in case your fuel pump is weak.

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Ran the truck for 20+ minutes on highway before the test.

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Do not carry excessive cargo on the truck bed.


These are the little things I would do to get the HC and CO down.


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