Failed Emissions for High CO

CO stands for carbon monoxide, a gas that forms when fuel doesn't burn all the way. A high reading means the engine is running "rich": it's getting too much fuel for the air it takes in, as SmogTips explains.

Either side can upset the balance: not enough air getting in, or a sensor telling the engine computer to add fuel it doesn't need.

High CO tells you the engine is running rich, but not why. The cause could be anything from a clogged air filter to a faulty sensor feeding the engine computer.

Wynn's, a UK fuel-additive company, says a high CO reading may simply show the engine is not using fuel well.

The air filter is the cheapest place to start, but a diagnosis can show which part is actually at fault:

1. Engine air filter. SmogTips says it is the emissions part people forget most. When it is dirty, airflow drops and the air and fuel balance is upset. Replace it on your car's recommended schedule, usually about every 15,000 miles, or at least before an emissions test.
2. Oxygen sensor. Found in the exhaust, it measures the oxygen remaining after combustion. The engine computer uses that signal to set fuel delivery. If the sensor is worn, it can send a false signal, and the computer may supply fuel the engine does not need. A new one is common and fairly affordable.
3. Catalytic converter. This exhaust part cleans the gases before they reach the tailpipe. Damage or clogging lets more emissions pass. It is the priciest item here.

These are RepairPal's published averages, so your local shop may quote a little more or less.

Because the test result alone can't name the bad part, a diagnostic check before you buy anything can save you from paying for parts you didn't need.

If repairs don't get your car to pass, some states offer a waiver once you've spent a set amount on emissions repairs. How much you must spend, who qualifies and how long it lasts depend on where you live. Spending that amount by itself is not enough: proper receipts are needed, and some states also ask for a failed retest. These are 2026 limits for several states.

Colorado's DMV lets oxygen-sensor and catalytic-converter work count toward that total. Hoses, gaskets, and clamps tied to the repair count too. Labor counts only when done by a professional mechanic at a licensed business, so save each receipt. A Colorado waiver covers one emissions cycle.

Two of these states draw a line at the catalytic converter: Colorado won't grant a waiver if the converter is missing or tampered with, and Arizona won't grant one for a converter failure.