Symptoms
- Emissions test result shows HC or CO above the limit
- Possibly rough idle or poor fuel economy alongside it
- Exhaust may smell strongly of gasoline
- Check engine light may or may not be on
Likely causes
- CommonWorn spark plugs or ignition componentsWeak spark causes incomplete combustion, raising unburned hydrocarbons.
- CommonDirty or failing oxygen sensorA bad sensor can cause the engine computer to run the mixture too rich.
- PossibleClogged air filterRestricted air can push the fuel mixture rich, raising CO.
- PossibleFailing catalytic converterA converter that's lost efficiency can't clean up HC and CO like it should.
- PossibleVacuum leak or dirty fuel injectorsThese upset the air/fuel ratio and combustion quality.
What to do
- Get the check engine light scanned even if it isn't on, since pending codes can point to the cause.
- Replace the air filter if it's dirty or overdue; it's cheap and easy to rule out.
- Have spark plugs inspected and replaced if worn, especially if it's been a long time since the last change.
- Have a shop test the oxygen sensor and fuel trims to see if the engine is running rich or lean.
- Have the catalytic converter's efficiency checked if the above don't resolve it.
- Retest at an emissions station once repairs are done and the check engine light has been off for the required drive cycle.
Tools and parts
- OBD-II code reader
Spark plugsSearch: “spark plugs for [year make model]”
Engine air filterSearch: “engine air filter for [year make model]”
Oxygen sensorSearch: “oxygen sensor for [year make model]”
When to call a pro
A shop with emissions diagnostic experience can read fuel trims and pinpoint whether it's ignition, sensor, or converter related before you pay for a retest.
Safety
Generally low risk
Generally low risk for a careful person. Stop if anything looks different from what this guide describes.