A drive cycle is a set of driving conditions that lets the engine computer run all of its self-tests, called readiness monitors. After codes are cleared or the battery is disconnected, the monitors reset to not ready, and they only complete after the vehicle has been driven through the right mix of cold starts, idling, steady cruising and deceleration. A scan tool shows which monitors are ready.
What are readiness monitors?
| Monitor | Runs |
|---|---|
| Misfire, fuel system, comprehensive components | Continuously while driving |
| Catalyst | Once the engine is warm, usually at steady cruise |
| Oxygen sensor and oxygen sensor heater | After warm-up, at steady speeds |
| Evaporative system | Often needs a cold start and a fuel level between about a quarter and three quarters |
| EGR or variable valve timing, secondary air | On engines that have them, under specific conditions |
A typical generic drive cycle
- Start the engine cold, ideally after it has sat overnight, and let it idle for two to three minutes.
- Drive gently in town for several minutes with a few stops and starts.
- Cruise at a steady highway speed, around 55 mph, for at least five to ten minutes.
- Release the throttle and coast down without braking for a stretch.
- Finish with a few more minutes of stop-and-go driving, then idle briefly.
Manufacturers publish their own exact drive cycles, and some monitors need more than one day of driving. Normal mixed driving over several days completes most monitors.
Why monitors stay not ready
- A stored or pending code that stops a monitor from running; read codes first with an OBD2 scanner
- Short trips that never fully warm the engine
- A thermostat stuck open, so the engine never reaches temperature (P0128)
- A very full or nearly empty fuel tank, which delays the evaporative monitor
- A weak battery or charging fault that resets the computer; see bad alternator symptoms
Readiness and inspections
States that check the OBD system during inspection look at readiness as well as codes. Many programs allow one or two incomplete monitors depending on the model year, but the rules vary, so check your state program. Avoid clearing codes or disconnecting the battery shortly before an inspection.
After a repair
Clear the codes once the repair is done, drive normally for a few days, then check readiness and confirm no codes have returned. If a misfire, timing or fuel code keeps coming back, see our OBD2 codes list and check engine light guide.
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