What causes a car to fail an emissions test?
A vehicle may fail an emissions test for a number of reasons, including:
- Your car battery has been disconnected recently: If your car battery has been disconnected within the last couple of weeks — during repairs, for example — the on-board diagnostics (OBD) system may not have enough information stored in its memory for inspection purposes. You may need to return in a week or so for a test.
- Data Link Connector (DLC) issues: Also known as an on-board diagnostics port (OBD2 port), this is how your car connects to the inspector's testing system. Problems with the port or the connection can cause a test failure.
- Your "check engine light" came on: It mostly appears due to exhaust system problems, but not always. Issues with the battery or its cables can trigger it, as well as third-party alarm systems.
- Actual test failure: In this case, your vehicle inspection report will contain a list of one or more diagnostic trouble codes (DTCs). You'll need to take the report to a qualified mechanic who can tell you which services or repairs are needed.
There are a number of issues that could cause an actual emissions test failure, such as:
- Exhaust system issues: The exhaust system takes the burnt gases from the engine and routes them out through your exhaust pipe. A catalytic converter, which is part of this system, transforms some of those gases into water and carbon dioxide. Issues with a catalytic converter can cause an emissions test failure.
- Other system issues: Your ignition system, fuel injection system, and air injection system all play a part in reducing the number of pollutants and greenhouse gases that wind up in your exhaust system. There are multiple potential points of failure in each.
- Faulty sensors: A failing oxygen sensor or bad mass airflow sensor can trigger your "check engine light."
- Other issues: A gas cap that's loose, cracked, or otherwise ill-fitting can cause an emissions test failure. Waiting too long between oil changes or replacing your engine air filter can also be the culprit.
A common reason that cars fail emissions tests is a faulty oxygen sensor, which can cause issues with the check engine light. A simple (though somewhat pricey) part replacement often solves the issue.
If my car fails an emissions test, what can I do?
Ideally, you'll get your car repaired or serviced so it can pass a retest. You'll typically have a certain amount of time to make repairs, but states, counties, and municipalities may differ on the exact grace period. Check with your local department or bureau of motor vehicles (DMV or BMV) for specific details.
What happens if you fail an emissions test twice?
If your car fails an emissions test more than once, you may have a few options outside of continually getting it repaired:
- Economic hardship: In some cases, you may apply for an emissions testing extension or waiver if you're unable to afford the necessary repairs.
- General repair cost limits: If you've already spent a certain amount of money on repairs that were intended to address a failed emissions test and the issue isn’t fully resolved, you may be able to apply for a waiver.
- Limits on specific cars: Some DMVs or BMVs may grant you a waiver if the repair cost would be more than the maximum amount allowed for your car's age, make, and model.