How long to drive after clearing codes before an inspection

Why clearing codes sets you back

The engine computer runs a set of self tests called readiness monitors: catalyst, oxygen sensor, oxygen sensor heater, EVAP, EGR or VVT, and others depending on the vehicle. Misfire, fuel system and comprehensive component monitors run all the time. The others only run when certain conditions are met.

When you clear codes with a scanner, or on many cars when the battery is disconnected, all monitor results are erased and set to not ready or incomplete. The inspection station reads these flags. If too many are incomplete, the car is rejected even if the check engine light is off. The readiness monitors explainer covers each one.

Note.Clearing codes right before an inspection does not hide a fault. The station sees the incomplete monitors and the car is turned away until they finish.

Drive cycles vary by make, model and year

A drive cycle is the set of driving conditions that lets the monitors run. Each manufacturer defines its own, and they can differ between engines in the same brand. Some monitors need a cold start where coolant and air temperatures are close to each other. The EVAP monitor on many vehicles needs a fuel level within a certain range and may need the car to sit for hours with the engine off. Some catalyst monitors need steady highway speed for several minutes.

Because of this, any fixed rule such as drive a set number of miles is a guess. Some cars finish all monitors in a day of normal driving. Others take a week or more, and the EVAP monitor is often the last one. For the exact procedure, check the service information for your vehicle or a make specific repair database.

A generic drive pattern that helps

If you do not have your vehicle's exact procedure, this common pattern covers the conditions many monitors need. It is a starting point, not a guarantee. Always drive safely and legally.

1. 1

   Let the car sit overnight so the engine is fully cold.

2. 2

   Make sure the fuel tank is between roughly one quarter and three quarters full, which many EVAP tests require.

3. 3

   Start the engine and let it idle for a couple of minutes with accessories off.

4. 4

   Drive in town with gentle stops and starts until the engine reaches normal temperature.

5. 5

   Drive at a steady highway speed for several minutes without hard acceleration.

6. 6

   Let off the throttle and coast down to a lower speed without braking, where safe.

7. 7

   Come to a stop and let the engine idle for a short time before shutting off.

8. 8

   Repeat normal daily driving for a few days, then check monitors.

Check monitor status with a scanner

You do not need to guess. Almost every OBD2 scanner has an I/M readiness screen that shows each monitor as complete, incomplete or not supported. Check it every day or two. When the monitors your state requires are complete, you are ready.

Many scanners also show monitor status for this drive cycle, which tells you what ran on the current trip. A scanner that shows live data helps if a monitor never completes, because you can see whether conditions like coolant temperature or fuel level are being met. Our best OBD2 scanners for DIY mechanics guide covers tools with clear readiness screens.

Scanners with an I/M readiness screen →

How many incomplete monitors are allowed

Under EPA guidance used by many states, 2001 and newer gasoline vehicles may usually have one incomplete monitor, and 1996 to 2000 vehicles may usually have two. Rules differ by state, and some programs set their own limits. Check your state's emissions program website before you go.

Not supported monitors are not counted against you. Those are tests your car does not have.

When a monitor will not complete

If a monitor stays incomplete after a couple of weeks of mixed driving, check for pending codes and look up your vehicle's procedure. Some advanced scan tools can run certain monitor tests on demand, which is useful but varies by make. See what is a bidirectional scan tool.

• A pending code is present. A fault can stop related monitors from running. Read pending codes and fix the cause.
• Fuel level is too low or too high. EVAP tests often need a mid range fuel level.
• No true cold start. Short trips without overnight rests may never trigger cold start monitors.
• Only short trips or only highway driving. Mix city and highway driving.
• Battery was disconnected again. Some cars reset monitors when power is lost.
• A known software issue. Some vehicles have technical service bulletins for monitors that will not set. A dealer can check for updates.

Frequently asked questions

How long should I drive after clearing codes? Long enough for the required readiness monitors to show complete. That differs by vehicle, so check with a scanner instead of relying on a set distance.

Does disconnecting the battery reset monitors? On many vehicles, yes. Monitors go back to incomplete, just like clearing codes with a scanner. See battery registration after replacement for related steps on newer cars.

Why is the EVAP monitor always last? On many vehicles it needs a cold soak, a mid range fuel level and specific temperatures, so it runs less often than others.

Can I pass inspection with the check engine light off but monitors incomplete? Usually not if more monitors are incomplete than your state allows. The station checks both the light and the readiness flags.