Readiness monitors explained

Readiness monitors are the self-tests the engine computer runs on its emission systems while you drive. Each one is either complete or not complete, and inspections that read OBD check them.

Updated 16 October 2026

What the monitors are

The engine computer can’t test every emission system all the time. Some tests run continuously while the engine runs. Others only run under particular conditions, such as a fully warm engine, a steady speed or a cold start after the car has stood for hours, and often only once per drive.

Each monitor reports whether its test has run since the fault codes were last cleared. Complete doesn’t mean passed: if a test finds a fault, the car stores a fault code for it. Read the codes as well as the monitors.

Read My Car shows the monitors under Vehicle, emissions readiness. Monitors your car doesn’t have are counted as not fitted, and don’t count against it.

Monitors that run all the time

Misfire

Watches for misfires while the engine runs.

Fuel system

Watches the fuel mixture correction (fuel trim).

Components

Checks sensors and other parts for broken circuits and implausible readings.

Petrol engines

Catalyst

Compares the oxygen sensors in front of and behind the catalytic converter.

Heated catalyst

The same, for a catalytic converter with its own heater. Few cars have one.

Evaporative system

Checks the fuel vapour (EVAP) system for leaks, on cars that run this test. It’s often one of the last to complete.

Secondary air

Checks the pump that blows air into the exhaust after a cold start, on cars that have one.

Particulate filter (petrol)

Checks the petrol particulate filter, on newer petrol engines that have one.

Oxygen sensor

Checks how the oxygen sensors respond.

Oxygen sensor heater

Checks the heaters that bring the oxygen sensors up to temperature.

EGR / VVT

Checks exhaust gas recirculation and, on some cars, the variable valve timing.

Diesel engines

NMHC catalyst

Checks the oxidation catalyst, which burns hydrocarbons in the exhaust.

NOx / SCR

Checks the NOx after-treatment, such as the AdBlue (SCR) system.

Boost pressure

Checks the turbo’s boost control.

Exhaust gas sensor

Checks exhaust sensors such as the NOx sensors.

PM filter

Checks the diesel particulate filter.

EGR / VVT

Checks exhaust gas recirculation, as on petrol engines.

• Diesel particulate filter (DPF) explained

Why monitors show “not ready”

• The fault codes were cleared, by you, a garage or a previous owner. That resets every monitor.
• The battery was disconnected or ran flat, on many cars.
• The engine computer was replaced or reprogrammed.
• The car is mostly used for short trips, so tests that need a warm engine or a longer drive don’t get to run.
• A fault stops a test from running. A faulty oxygen sensor, for example, can keep the catalyst test from running.

Getting them ready before an inspection

1. Have any fault repaired first. A test that keeps failing doesn’t become complete by driving.
2. Drive normally for a few days, often 50–200 km. A mix of drives helps: starts from cold, town driving and some steady driving at higher speed.
3. Check emissions readiness in Read My Car. If one monitor stays not complete, check for pending codes.
4. If it still won’t complete, ask a garage with information for your make: the exact conditions each test needs differ between makes and engines.

There’s no single drive that sets every monitor on every car. Be wary of “drive cycle” recipes that promise to: they’re written for particular makes.

Monitors and inspections

Inspections that read OBD, such as the Dutch APK and the German emissions test (AU), check these self-tests. In the Dutch APK, cars first used after 31 December 2005 that run on petrol only (not LPG or CNG) or on diesel pass the emissions check on OBD when the check engine light works, no fault codes are stored and the self-tests are complete. Otherwise the usual exhaust gas test follows. Since 1 January 2023, diesels with a particulate filter get a particle count instead.

Other countries’ inspections differ in what they test. Check the rules where your car is inspected.

• Clearing fault codes before an emissions test
• Checking a used car over OBD before buying

Questions