Getting ready for an emissions test

Every car and light truck from the 1996 model year on has an on-board diagnostics computer that watches the emissions controls. When it finds a fault, it stores a trouble code and turns on the check engine light. The most common faults are in the catalytic converter or an oxygen sensor. An emissions test on these cars reads that computer instead of measuring the exhaust at the tailpipe.

What you need

• The registration renewal notice, or its due date
• The car's owner's manual
• A testing station near you

Steps

1. 1

   Look at the dashboard

   A check engine light that stays on means the computer has stored a fault, and the test reads it. Get the repair done before the test. If the light is flashing, the problem is severe, so drive as little as possible and get service quickly. See What to do when the check engine light comes on.

   1 min

2. 2

   Wait after a repair

   Clearing the codes after a repair restarts the computer's self-checks, and the test looks at whether they've finished. A gasoline car usually finishes them within two weeks of normal driving. Drive it for about a week after the repair before the test, and a diesel for two to three weeks.

   1 min

3. 3

   Leave the codes alone

   Clearing the codes with a scanner or by disconnecting the battery doesn't fix the problem behind them, and it leaves the self-checks unfinished. On cars from the 2012 model year on, the computer also keeps a permanent record of the fault that neither a scanner nor the battery can erase.

   1 min

4. 4

   Test early

   Book the test well before registration is due. A car that fails has to be repaired and tested again, and testing early leaves time for both.

   5 min

5. 5

   At the station

   The technician connects a scan tool to the car's diagnostic port. It reads the trouble codes, whether the check engine light is on and whether the self-checks have finished.

   10 min

6. 6

   Keep the report

   You get a report either way. Check that the one you keep shows a pass, or the reason for a fail.

   2 min

When the result is "not ready"

A not-ready result means the car's computer hasn't finished its self-checks since the codes were last cleared, such as after a repair. Drive the car for about a week in town and on the highway, then test again.

An annotated example