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The OBD2 readiness monitors

Monitors are the tests the car runs on itself while you drive to keep an eye on emissions. When an app says ready or not ready, this is what it’s talking about.

What they are

The engine control unit can’t watch everything at once or at any moment. Some things it checks all the time; others only when the conditions are just right: engine warm, steady speed, fuel tank neither full nor empty. Each of those tests is a monitor, and OBD reports for each one whether your car has it and whether it has completed since the last clear.

The continuous ones

  • Misfires.
  • Fuel system, meaning the fuel trims.
  • Components: that every engine sensor and actuator gives sensible signals.

They complete almost as soon as you start the engine.

The ones that need driving, on petrol

  • Catalytic converter.
  • Heated catalyst, on few cars.
  • Fuel tank vapours (EVAP). It’s usually the slowest one.
  • Secondary air.
  • Oxygen sensors.
  • Oxygen sensor heaters.
  • EGR or variable valve timing.

And on diesel

  • Oxidation catalyst.
  • NOx reduction: the SCR catalyst with AdBlue or the NOx trap.
  • Boost pressure.
  • Exhaust gas sensors.
  • Particulate filter.
  • EGR or variable valve timing.

Why they show not ready

When you clear fault codes, disconnect the battery or replace the control unit, all the monitors go back to zero. Over a few days of normal driving they become ready one by one. That’s why lots of incomplete monitors give away a recent clear.

How they complete

There’s no trick that works for every car, but it’s nearly always enough to drive normally for a few days, combining:

  • A cold start, with the car left parked since the night before.
  • A few minutes idling.
  • Town driving with gentle acceleration.
  • A stretch of road at a steady speed, between 80 and 100 km/h, for ten minutes or more.
  • Overrun: lifting off the accelerator with a gear engaged.
  • The fuel tank between a quarter and three quarters full, for the vapour monitor.

If a monitor never completes, there’s usually a pending fault behind it: check the pending faults, mode 07, in the OBD modes.

Monitors and the ITV

In the United States, a car with several incomplete monitors fails the inspection. In Spain, the ITV test centre manual doesn’t use them to decide: it looks at the light, the confirmed and permanent faults, and how far the car has driven with the light on. Even so, Final OBD shows them to you, because they give away a recent clear. It’s all in the ITV and OBD.