What Are OBD-II Readiness Monitors?
Last reviewed May 2026 · Find This Code Editorial Team
OBD-II readiness monitors — sometimes called I/M readiness monitors — are internal self-tests your car's computer runs to verify that key emission control systems are working correctly. Understanding them is essential if you've recently cleared codes or repaired a fault and want to confirm your car will pass an emissions inspection.
What readiness monitors actually do
Your car's ECM (Engine Control Module) continuously monitors dozens of sensors and systems. But some of those tests — particularly the ones related to emissions — can only run under specific operating conditions. For example, the EVAP monitor typically requires the fuel tank to be between 15% and 85% full, the engine to be fully warmed up, and the car to have been sitting long enough for the engine and fuel temperature to equalize.
Each readiness monitor has two possible states: Complete (the test ran and passed) or Incomplete (the test hasn't run yet, or it failed). When a monitor is incomplete, the ECM doesn't know whether that system is working — it simply hasn't had the chance to test it yet.
Why they matter for emissions inspections
State emissions inspections (smog checks) connect to your car's OBD-II port and check two things: whether any diagnostic trouble codes are stored, and whether the readiness monitors are complete.
If too many monitors are showing "incomplete," the inspection will fail — even if no check engine light is on and no codes are stored. Most states allow one or two incomplete monitors to pass (this varies by state and model year), but a car with several incomplete monitors will not pass.
Common readiness monitors
Catalyst Monitor
Tests whether the catalytic converter is reducing emissions to the required level. The ECM compares the oxygen sensor activity upstream and downstream of the catalyst. A failed catalyst monitor sets codes like P0420 or P0430.
EVAP Monitor
Tests the evaporative emission control system for leaks. The ECM pressurizes or pulls a vacuum on the fuel system and measures whether it holds. A failed EVAP monitor sets codes like P0440, P0442, P0455, or P0456.
Oxygen Sensor Monitor
Tests the upstream (pre-catalyst) oxygen sensors to verify they are switching correctly between rich and lean. A slow or stuck upstream O2 sensor sets codes like P0135 or P0141.
Oxygen Sensor Heater Monitor
Tests the heater circuit inside the oxygen sensors. The heater allows sensors to reach operating temperature quickly after a cold start.
EGR Monitor
Tests the Exhaust Gas Recirculation system, which reduces NOx emissions by routing some exhaust gases back into the intake.
Misfire Monitor
Continuously monitors each cylinder for misfires. A misfire monitor fault can set codes from P0300 through P0306.
Fuel System Monitor
Tests the fuel delivery system by monitoring long-term and short-term fuel trim. Lean or rich conditions that persist set codes like P0171 or P0174.
What happens after clearing codes
Both methods of clearing codes — using an OBD-II scanner and disconnecting the battery — reset all readiness monitors to "incomplete." The ECM treats this as a fresh start; it has no memory of whether those systems passed their last test.
After clearing codes, you need to drive the vehicle through a drive cycle — a specific sequence of driving conditions that allows each monitor to run and complete. A basic drive cycle typically includes:
- A cold start (engine hasn't run for 6–8 hours)
- Idle for 2–3 minutes to warm up
- Steady highway driving at 55–60 mph for 5–10 minutes
- City driving with moderate acceleration and deceleration
- Returning to idle and shutting off
The EVAP monitor is particularly demanding — it often requires the car to sit overnight before it will run. Catalyst and oxygen sensor monitors tend to complete after one or two normal drive cycles.
How to check readiness monitor status
Any OBD-II scanner — even a basic $25 code reader — can display readiness monitor status. Look for a "I/M Readiness" or "Readiness Monitors" menu option. You'll see a list of monitors with their status: ready/complete or not ready/incomplete.
If you're preparing for an emissions test, use your scanner to confirm all relevant monitors show "complete" before taking the car in. This will save you the time and cost of a re-inspection.