How to Check Readiness Monitors Before an MOT

Readiness monitors are the self-tests your car runs on its own emissions systems as you drive. They matter less in the UK than most articles claim — the MOT does not read them — but they are still the single best piece of evidence that a repair has actually held, and if you have recently cleared a fault code they are the reason a warning light can reappear on the way to the test.

What a readiness monitor actually is

The engine control unit does not just react to faults as they happen. It also runs deliberate diagnostic routines on the systems it is responsible for, each with its own set of conditions that have to be met before the test can run at all. The catalyst monitor, for example, cannot run until the engine is at operating temperature and has been in closed-loop fuel control for a while at a reasonably steady load.

Each of those routines reports one of two states: complete, meaning the test has run to a conclusion since the last reset, or not ready, meaning it has not yet had the conditions it needs. That is all a readiness monitor is — a flag saying “I have checked this” or “I have not checked this yet”.

The typical set on a petrol car includes misfire, fuel system and comprehensive components, which run more or less continuously, plus catalyst, heated catalyst, evaporative system, secondary air system, oxygen sensor, oxygen sensor heater and EGR system, which run only under specific conditions. A diesel has a different set, covering things like the particulate filter, NOx aftertreatment and boost pressure. Not every monitor applies to every car; a car with no secondary air injection will simply report that monitor as not supported, which is normal and not a problem.

Does the UK MOT check them? No.

It is worth being straight about this, because a lot of advice written for a US audience gets copied over without the disclaimer.

The MOT emissions inspection is a tailpipe test. A petrol car has CO, HC and lambda measured at fast idle and CO again at natural idle. A diesel has smoke opacity measured under acceleration. Alongside that, the tester checks that the engine malfunction indicator lamp comes on with the ignition and then goes out, and looks for emissions control equipment that is missing, obviously modified or obviously defective. Nowhere in that process does anyone plug in a scan tool and read your monitor status.

In the United States it is a different story. Most state emissions and smog check programmes read the monitors directly and will reject a vehicle with too many incomplete, precisely to stop people clearing codes on the way to the test. If you are reading this from the US, your monitors are not advisory — they are the test.

So why bother checking them in the UK?

Three reasons, and they are all practical rather than regulatory.

They prove the repair worked

This is the big one. When you fix a fault and clear the code, the light goes out immediately — whether or not you fixed anything. The monitors do not. A catalyst monitor that has run to completion and reported no fault is the car telling you, on its own authority, that it has re-tested that system and is satisfied. That is a genuinely stronger signal than an unlit lamp.

They warn you the light is about to come back

If you cleared a code a few days ago and the relevant monitor is still showing not ready, the car has not yet re-run the test. The fault may well still be there, waiting to be found. Booking an MOT in that state is booking a coin toss.

They catch the car that has just been to a garage

If you are buying a used car and every monitor reads not ready, someone has cleared the codes very recently — or disconnected the battery. That is worth knowing before you hand over the money. A car that has been driven normally for a week will have most of its monitors complete.

How to read them

Monitor status is part of standard OBD2 and does not require anything manufacturer-specific. Plug an ELM327 adapter into the OBD2 socket — it is almost always within reach of the driver’s seat, under the dashboard or behind a small trim panel near the steering column — turn the ignition on, and connect diagnostic software on a Windows laptop. The software reads the monitor list and shows each one as complete, incomplete or not supported.

It takes about the same amount of time as reading a fault code, because it comes from the same conversation with the engine control unit.

Getting a monitor to complete

This is where people get frustrated, because there is no button for it. The engine control unit will only run a test when its enabling conditions are met, and those conditions are set by the manufacturer.

The general shape of a drive cycle that gets most monitors to run looks like this. Start with the car genuinely cold — parked overnight is ideal, because several monitors require a cold start with the coolant and intake air temperatures within a few degrees of each other. Idle for a couple of minutes. Drive gently up to a steady 40 to 50 mph and hold it there for several minutes without touching the accelerator much, because steady cruise is what the catalyst and oxygen sensor monitors want. Include a few gentle accelerations and a period of coasting with your foot completely off the throttle, which is what the EGR and evaporative monitors tend to need. Then a period of town driving with some stops.

Do not do this on a motorway slip road with your hazards on. It is ordinary driving, done deliberately, and the honest advice is usually simpler: just use the car normally for a week, including at least one journey long enough to get properly warm, and check again.

How many incomplete monitors is too many?

For a UK MOT, strictly speaking, none of them matter. For your own confidence, the question is different: is the monitor that covers the system you just repaired complete, and is it reporting no fault?

If you replaced a lambda sensor, you want the oxygen sensor and oxygen sensor heater monitors complete. If you fixed a small evaporative leak, you want the EVAP monitor complete, and that may take several days and a couple of fuel level changes. If you cleaned a throttle body or fixed a vacuum leak, the fuel system and comprehensive components monitors run more or less continuously and will tell you quickly.

US readers working to a state programme should check their own state’s rules, since the number of incomplete monitors allowed varies and is usually lower for newer vehicles.

What resets them

Anything that resets the engine control unit’s diagnostic memory resets the monitors along with it.

• Clearing fault codes with a scan tool.
• Disconnecting the battery, on many cars.
• A flat battery or a jump start that dropped the voltage far enough.
• Certain engine control unit software updates.

That last point about the battery is worth remembering if you have had one changed recently and cannot work out why every monitor is blank.

The sequence that actually works

Put together, the honest order of operations before an MOT is: read the codes, understand what they point at, carry out the actual repair, clear the codes, drive the car normally for at least a few days, then read it again. You are looking for two things — no returned code, and the relevant monitor showing complete. If you get both, you have a repair. If you get an unlit lamp and a monitor that has not run, you have a hope.

Reading monitor status and live values needs an edition with live data support; the entry-level Standard edition reads and clears codes only. Either way, the useful habit is to check before you book the test rather than after you have failed it.