After the repair
How to prepare for an OBD2 emissions inspection: monitors and codes
What the inspection equipment checks through the OBD2 port, why you can fail with the light off, and how to get the monitors to ready before your appointment.
Guide contents
In short
- Inspections check the light, stored and permanent codes, and the readiness monitors.
- Many programs allow one or two incomplete monitors depending on the model year; others require all of them.
- Repair at least a week before the appointment and drive a mixed cycle.
- You can fail with the light off if codes were just cleared or the bulb does not work.
In many countries, periodic emissions inspections (smog checks, state inspections, MOT-style tests and similar programs) no longer just measure tailpipe gases. On vehicles with OBD2 they also plug into the diagnostic port. The exact requirements depend on your country, and sometimes your state or city, so always check the local rules. What almost all of them check is the same.
What the inspection equipment looks at
- The check engine light status: it must light up when you switch the ignition on and go off with the engine running.
- Confirmed and permanent codes related to emissions.
- The readiness monitors: internal tests the computer runs on the emissions systems. If they have not completed, the system cannot state that everything is fine.
The most common monitors
| Monitor | What it checks |
|---|---|
| Misfire, fuel system and components | Evaluated continuously while you drive. |
| Catalyst | The converter's ability to store oxygen. |
| Oxygen sensors and heater | How fast the sensors respond and warm up. |
| EVAP system | That there are no fuel vapor leaks. This one takes the longest. |
| EGR | Exhaust gas recirculation flow. |
| Secondary air | Only on engines that have it. |
Many programs accept one or two incomplete monitors, depending on the vehicle's model year. Others require all of them. Check your local rule before you go.
How to get the monitors to ready
- Fix faults in good time, at least a week before the appointment.
- Cold start. Let the vehicle sit for several hours and start with a cold engine and the tank between a quarter and three quarters full; the EVAP monitor usually does not run with the tank nearly full or nearly empty.
- Idle for two to three minutes with the headlights, heater and rear defroster on.
- Drive in town with gentle acceleration for about ten minutes.
- Hold a steady speed of 45 to 55 mph (70 to 90 km/h) for several minutes on the highway.
- Slow down without braking from that speed to about 20 mph (30 km/h).
- Check the monitors with the scan tool. If any is still not ready, repeat the cycle another day. Some need several days of normal driving.
The exact cycle varies between manufacturers. If a monitor will not complete, the OBD2 drive cycle guide explains each one in more detail.
Reasons for failing even with the light off
- Permanent codes still stored after a recent repair.
- Monitors not ready because the codes were cleared or the battery was disconnected.
- A check engine light that does not come on with the ignition, because of a burnt-out bulb or tampering.
- No communication with the computer because of a blown fuse or a damaged connector.