OBD-II readiness monitors explained

That one fact explains most of the confusion around not-ready monitors: the car is not broken, and the scan tool is not lying. The tests simply have not been run again yet, and an emissions test reads those test results rather than doing the testing itself.

Which monitors does your car run?

A monitor is software, not a part. It is a routine in the engine computer that watches one emissions system and decides whether it is working. Every petrol car with OBD-II has three continuous monitors, which run whenever the engine is running:

On top of those there are up to eight non-continuous monitors, which only run when their conditions are met: catalyst, heated catalyst, oxygen sensor, oxygen sensor heater, evaporative emissions (EVAP), exhaust gas recirculation (EGR), secondary air injection and air conditioning. The set is defined by the emission system design each manufacturer uses, and the standard fact sheets describe the same three plus eight.¹

Not every car has all eleven. A monitor for equipment the car does not have reports as not supported or N/A, and unsupported monitors are ignored at the test; they cannot make a car fail. What matters is the monitors the car does support, and how many of them have completed.

What completes a drive cycle?

A monitor only runs when a set of preconditions, sometimes called enabling criteria, is satisfied. Typical examples: the coolant has reached and held its operating temperature, the ambient temperature is inside a window, a certain time has passed since the start, the engine is under a particular load, the car is holding a particular speed, and — for the EVAP test — the tank is partly filled rather than nearly empty or just filled.

A drive cycle is a driving pattern that walks the car through those conditions. In practice that means a genuine cold start after an overnight soak, an idle period to warm up, a steady cruise at moderate speed, some gentle acceleration under load, and a deceleration with the throttle closed, because the catalyst and oxygen sensor monitors use the fuel cut-off on overrun to take their readings.

Two things surprise people:

• One drive is often not enough. Several monitors need two consecutive trips with a cool-down between them before they will set, which is why state guidance talks about driving normally for about a week rather than one long lap of the ring road.²
• A fault blocks its own monitor. If a system is still failing, or the check engine light is on, the computer will not complete that monitor, and on many cars it will not run the others either. A monitor that refuses to set is sometimes a diagnosis in itself, not a nuisance. California’s test reference makes the same point: monitor completion depends on the components that enable the monitor actually working.³

The exact drive cycle is manufacturer-specific and is published in service information, sometimes in the owner’s manual. The generic cycles passed around forums are approximations of it, useful for a first attempt and unreliable as a promise.

Why does clearing codes reset every monitor?

The OBD-II standard has a service that means “clear diagnostic information”. It erases four things at once: the stored trouble codes, the pending codes, the freeze frame, and the readiness status of every monitor. A scan tool’s clear button, a battery disconnect, a battery replacement or a computer reflash all end in the same place — a car with no codes and no completed tests.

That is why “I cleared the code and the light stayed off” is not evidence of a repair. The light stays off until the monitor that watches that system runs again and fails, and until then the car has nothing to report in either direction. The freeze frame that would have shown the conditions of the original fault is gone too, which is why it is worth saving it before clearing anything.

Some manufacturers turn this into a trouble code of its own. P1000 on Ford — also used by Lincoln, Mercury and Jaguar — and P1000 on Mazda mean exactly this: monitor testing is not complete. Finding P1000 is not a fault diagnosis. It is the car telling you the readiness check will fail before anyone has looked at a component.

Why can’t you pass the test the same day?

An emissions test reads the readiness status before it does anything else, and it can refuse to test a car with too many incomplete monitors. Under US EPA guidance, a 1996 to 2000 model year vehicle may present two non-continuous monitors not ready, and a 2001 or newer vehicle may present one.²¹ The three continuous monitors are expected to be ready and are not counted in that allowance.

Those numbers are guidance, not a universal law, and the state programmes implement them differently:

• California allows any one incomplete monitor on 1996–1999 petrol cars, but on 2000 and newer petrol cars only the evaporative monitor may be incomplete. Diesels have their own limits, including zero incomplete monitors for 1998–2006.³
• Texas applies the EPA pattern directly: one not-ready non-continuous monitor passes on 2001 and newer, two on 1996–2000.⁴
• Arizona rejects rather than fails a car with too many not-ready systems, and a second consecutive readiness rejection can lock the vehicle out of testing until a drive cycle is completed and a station manager releases it.¹

So the practical answer is the boring one. After a repair and a clear, drive the car normally for about a week — a few cold starts, mixed speeds, one steady highway run — then read the monitor status with a scan tool before booking the retest. Do not clear anything on the way to the test: that resets exactly the monitors you just spent a week setting. And if the check engine light is on, readiness is not your problem; a stored code fails the test on its own.

None of this is a substitute for the repair. A monitor that will not complete after a week of sensible driving usually means something is still wrong — the thermostat never lets the engine warm up (P0128), the catalyst is marginal (P0420 or P0430), or the EVAP system still leaks (P0442, P0455, P0456).

Where to go next

• The not-ready codes in the database: P1000 on Ford and P1000 on Mazda.
• The monitors that most often refuse to set: P0420 and P0430 (catalyst), P0442 and P0455 (EVAP), P0133 (slow oxygen sensor), P0128 (thermostat stuck open).
• New to codes altogether? How to read OBD2 codes covers the letter, the digits and the port.
• Want to check monitor status yourself: the Fault Ferret app reads codes, freeze frame and readiness monitors over a Bluetooth or Wi-Fi adapter.
• Where the definitions on this site come from, and how far to trust them: how the data is checked.