OBD-II Monitor Readiness Status Explained
Failed an emissions test because of "not ready" monitors? Here's what they are, why they reset, how to run a drive cycle, and how many your state allows.
What Is an OBD-II Monitor?
OBD-II monitors (also called "I/M readiness monitors") are self-tests the PCM runs continuously or during specific driving conditions to verify emissions-related systems are working. Each monitor has one of three states:
- Ready / Complete: the test has run and passed since the last code clear.
- Not Ready / Incomplete: the test hasn't run yet, usually because the car hasn't seen the right conditions.
- Not Supported: this vehicle doesn't have that monitor (most modern cars have no secondary air pump, so that monitor reads Not Supported).
Most modern vehicles have up to 11 monitors. Your state inspection uses monitor status to confirm your emissions systems work, without running a tailpipe test.
The 11 OBD-II Monitors Explained
| Monitor | Type | What It Tests |
|---|---|---|
| Misfire | Continuous | Detects engine misfires via crank sensor pulses. Always ready on a running engine. |
| Fuel System | Continuous | Watches short-term and long-term fuel trim to confirm closed-loop fueling is within range. |
| Comprehensive Components (CCM) | Continuous | Tests all input and output sensors that affect emissions (MAF, MAP, throttle, coolant, etc.). |
| Catalyst | Non-continuous | Compares upstream and downstream O2 sensors to confirm the catalytic converter is storing oxygen properly. |
| Heated Catalyst | Non-continuous | Verifies catalyst warms to operating temp. Most modern cars don't support this. |
| Evaporative System (EVAP) | Non-continuous | Pressure or vacuum test of the fuel vapor recovery system. Very picky about fuel level and temperature. |
| Secondary Air | Non-continuous | Tests the air injection pump on vehicles equipped with one (mostly older and some European models). |
| A/C System | Non-continuous | Refrigerant leak check. Rare on modern cars. |
| Oxygen Sensor | Non-continuous | Confirms O2 sensors switch between rich and lean within spec. |
| Oxygen Sensor Heater | Non-continuous | Confirms the heater element in each O2 sensor works. See P0135. |
| EGR System | Non-continuous | Verifies EGR flow by watching MAP or intake temp drop when the valve opens. |
Why Are Monitors "Not Ready"?
Any of these will reset one or more monitors to "not ready":
- Battery disconnected or died
- PCM reflashed or replaced
- Codes cleared with a scan tool
- Car driven with a pending code that never completed
- Keep-alive memory lost (corroded battery terminals, low voltage)
After any reset, you need to drive the car through a drive cycle so the PCM has time to run each self-test.
Generic OBD-II Drive Cycle
Every manufacturer's drive cycle is slightly different, but this sequence sets most monitors on most vehicles:
- Cold soak: leave the car off for 8+ hours so coolant and intake temps are near ambient.
- Cold start and idle: start the car, let it idle 2-3 minutes in park or neutral. A/C and rear defrost on for the first minute helps.
- Drive 15 mph to 30 mph: accelerate moderately to 30 mph and hold steady for 2-3 minutes.
- Highway run: accelerate to 55-60 mph and cruise for 5-10 minutes without hard throttle.
- Decelerate without braking: let off the gas from 55 mph down to 20 mph without touching the brake. This runs the EVAP and fuel cut tests.
- Idle again: come back to a stop, idle 2-3 minutes, shut off.
Check monitor status with any OBD-II scanner that shows I/M readiness. Some monitors (especially EVAP and catalyst) may need several drive cycles to complete.
EVAP Monitor Is the Pickiest
The EVAP monitor is famous for not setting. Requirements on most vehicles:
- Fuel tank between 15% and 85% full
- Ambient temperature above 40°F and below 90°F
- No large or small leak codes stored
- Cold start with coolant temp under 90°F
- Fuel cap fully tightened
If EVAP won't set, check the fuel cap seal first. See P0440, P0442, P0455 for common EVAP codes.
State-by-State "Not Ready" Allowances
Most states allow 1-2 monitors to be "not ready" and still pass inspection. 1996-2000 model years typically get more leeway (2 allowed) than 2001+ models (1 allowed). General guidelines — check your state's DMV or vehicle inspection site for the current rule:
| Approach | States (typical) | Allowed "Not Ready" |
|---|---|---|
| EPA default (most testing states) | AZ, CO, CT, GA, IL, MA, MD, NV, NJ, NY, NC, OH, PA, RI, TX, UT, VA, WI | 2 for 1996-2000, 1 for 2001+ |
| California (stricter than EPA) | CA | 2000+ models: only the EVAP monitor could be incomplete; as of Oct 2025 ALL monitors must be complete |
| No emissions testing | AL, AK, AR, HI, IA, KS, KY, MI, MS, MT, NE, ND, OK, SD, TN (ended 2022), WA (ended 2020), WV, WY | N/A — no OBD inspection required |
Diesels, hybrids, and EVs often have different rules. Confirm with your local inspection station before the appointment.
How to Check Monitor Status
Any generic OBD-II scanner with "I/M readiness" support will show monitor status. Menu paths vary:
- Cheap readers (Ancel, Autel AL319, Innova 3100): usually a dedicated "I/M Readiness" button.
- Bluetooth dongles + apps (Torque, OBD Fusion, Car Scanner): look for "readiness" or "Mode $01 PID $01" in diagnostics.
- Factory scan tools: under emissions or OBD status menu.
The display will show each monitor as Ready, Not Ready, or Not Supported. Count only the "not ready" items — "not supported" monitors don't count against you.
Related Quick References
Sources
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