OBD-II Monitor Readiness Status Explained

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

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:

1. Cold soak: leave the car off for 8+ hours so coolant and intake temps are near ambient.
2. 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.
3. Drive 15 mph to 30 mph: accelerate moderately to 30 mph and hold steady for 2-3 minutes.
4. Highway run: accelerate to 55-60 mph and cruise for 5-10 minutes without hard throttle.
5. 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.
6. 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:

Diesels, hybrids, and EVs often have different rules. Confirm with your local inspection station before the appointment.

Readiness Is Not the Only Thing They Check

Monitors are one half of the OBD inspection. The other half is a functional test of the check engine lamp itself, and it is separate from whether any code is stored. California's inspector manual spells out what a fail looks like, and the rule catches people from both directions.

The lamp must illuminate with the key on and the engine off, then go out once the engine is running. A brief flash during start-up is normal. It is recorded as a fail if the lamp does not come on at all at key-on-engine-off, or if it stays on or flashes with the engine running.

So removing the bulb fails the test. That is worth stating plainly because it is the first thing people try. A dead lamp and a lit lamp are both failures, and the only passing state is a lamp that proves it works and then goes out. The same logic appears in the federal rules for ABS lamps on commercial vehicles and for tyre pressure lamps, which is not a coincidence: an inspection regime that accepted a missing warning lamp would be trivially defeated.

One thing that is not part of the test: a maintenance reminder. The manual explicitly tells inspectors not to confuse a time-or-mileage service light with the MIL. An oil-change reminder has no bearing on your result.

Vehicles without an OBD II system, as determined by the underhood emission control label, do not receive the OBD II functional test at all, and most vehicles over 14,000 lb GVWR are not OBD II equipped.

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