OBD-II readiness guide
NOT READY? START HERE.
A plain-language guide to readiness monitors, why they reset, and the driving conditions that commonly help them complete.
Drive cycle at a glance
Cold soak
Often overnight
City warm-up
Gentle stop-and-go
Steady cruise
55 mph · 8 minutes
Safe coast
55 down to 15 mph
Cruise again
Several more minutes
Scan status
Do not clear codes
WHAT “NOT READY” REALLY MEANS
A readiness monitor is an emissions self-test performed by the vehicle computer. “Not Ready” means that a test has not finished since memory was cleared; it does not automatically mean the part has failed. A monitor may stay incomplete because the enabling conditions have not occurred, a related fault is pending, or the vehicle has a software or component problem.
Why monitors reset
- The battery was disconnected, discharged, or replaced.
- Diagnostic codes were cleared with a scan tool.
- The engine computer was replaced or reprogrammed.
- An emissions repair required memory to be reset.
Continuous and non-continuous monitors
Misfire, fuel-system, and comprehensive-component checks generally run continuously while the engine operates. Catalyst, oxygen-sensor, EVAP, EGR/VVT, secondary-air, and diesel aftertreatment checks run only when their programmed conditions are met.
A safe general readiness route
- Start after a cold soak. Let the vehicle sit for several hours—often overnight—if a cold start is required.
- Warm it normally. Start without pressing the accelerator and allow a short idle.
- Drive in town. Use gentle acceleration, several complete stops, and normal idle periods.
- Cruise steadily. When safe and legal, hold about 55 mph for roughly eight minutes.
- Coast safely. If traffic permits, release the accelerator and coast toward 15 mph without touching the brake or clutch. Brake normally if safety requires it.
- Cruise again. Smoothly return to highway speed and maintain a steady cruise for several more minutes.
- Recheck with a scanner. Do not clear codes again; read which monitor is still incomplete.
EVAP needs the right conditions
Many EVAP tests need roughly ¼ to ¾ of a tank, a sealed fuel system, a cold soak, and a long key-off period. A completely full or nearly empty tank can prevent the monitor from running. EVAP may need several drive, refueling, and overnight cycles before all conditions line up.
Stop repeating the cycle when…
If the check-engine light is on, pending codes return, the vehicle runs poorly, or one monitor refuses to complete after several normal trips, repeating the route is unlikely to solve the cause. Diagnose the incomplete monitor and use the exact manufacturer procedure.
STILL NOT READY?
Bring the year, make, model, engine, fuel type, and a list of the incomplete monitors. The Smog Dawg can help you understand what the vehicle is reporting and what should happen next.