Check engine light on and your emissions test is due? Do this first

Your registration renewal says the emissions test is due, and the check engine light is on. In Fairfax County that means the car will not pass as it is. The test reads the car's computer, and a lit light is an automatic fail.

Why clearing the code does not work

It is tempting to have the code cleared and go straight to the test. The problem is that clearing the code also resets the car's readiness monitors, the self-checks the computer runs while you drive. The emissions test looks at those too. A car that was just cleared usually shows as not ready, and if the fault is still there the light comes back on its own.

The order that gets you through

1. Find the cause. A code points to a system, not a part, so the part behind it needs to be tested.
2. Fix what actually failed. Common ones are a loose gas cap, an oxygen sensor, an evaporative leak or a misfire.
3. Drive normally for a few days so the readiness monitors complete.
4. Test.

Solid or flashing

A solid light means get it checked soon. A flashing light usually means a misfire that can damage the catalytic converter, so stop driving it hard and call the shop. A catalytic converter costs far more than most of the problems that set the light in the first place.

Leave time before your deadline

Bring the car in a week or two before the test is due, not the day before. That leaves room for parts and for the monitors to finish.

John's Auto Care at 8401 Terminal Rd, Unit G in Lorton diagnoses check engine lights and does Northern Virginia emissions testing. You get the cause and the price before any work starts.

Check engine light diagnostics at John's Auto Care.

Sources

• AirCheck Virginia: Northern Virginia's emissions inspection program.
• EPA: OBD and emissions testing: How onboard diagnostics and readiness monitors are used in emissions tests.