Will Your Car Pass the Emissions Test? Check Readiness Monitors First

You booked the inspection two weeks ago, drove across town on the day, and the car failed. Not for the exhaust readings, which were fine, but because the tester plugged in, saw that half the readiness monitors said Not Ready, and could not pass it. The battery had been changed a few days earlier, which you had completely forgotten about. You paid the fee, you have to rebook, and the only thing wrong with the car was that it had not finished testing itself yet.

That is one of the most common avoidable fails at an emissions test, and you can rule it out at home in under a minute.

What are readiness monitors?

Every car built to the OBD2 standard runs a set of self-tests on its own emissions equipment. These self-tests are the readiness monitors, and there is one for each major system: the catalytic converter, the evaporative emissions (EVAP) system, the oxygen sensors and their heaters, the EGR system, and the basic engine and misfire checks that run continuously. When a monitor has finished its test since the last reset, it reports Ready or Complete. When it has not, it reports Not Ready or Incomplete.

The point of all this is that the car is constantly checking whether its own emissions controls still work. The monitors are the record of that checking. They are part of standard OBD2, which means any car can report them through the same diagnostic port a workshop uses, and any generic adapter can read them. If you want the wider picture of what that port exposes, the beginner's guide to OBD2 covers the basics.

This is different from a stored fault code. A code means a specific test failed and found a problem. A Not Ready monitor means a test simply has not run to completion yet, so the car has nothing to report either way.

Why do readiness monitors fail an emissions test?

Here is the part that catches people out: a car can be in perfect health and still fail purely on readiness.

When you take a modern car for its periodic inspection, part of the test is electronic. The examiner plugs into the OBD2 port and reads three things: whether the check engine light is commanded on, what fault codes are stored, and how many readiness monitors are complete. The logic is simple. If the monitors have not finished their self-tests, the station has no electronic proof that the emissions systems actually work, so it cannot pass that part of the inspection on trust.

Most testing regimes allow a small number of incomplete monitors to account for normal driving. The common standard is one incomplete monitor permitted on petrol cars from 2001 onward, and two on many older vehicles. Cross that threshold and it is an automatic fail, no matter how clean the tailpipe is. This is true across the EU emissions inspection (the AU portion of the German HU, the Spanish ITV, the Lithuanian techninė apžiūra, the Polish przegląd techniczny) and the UK MOT. A separate guide covers which actual OBD2 codes fail an inspection once the monitors are complete and a real fault is present.

Why are my readiness monitors not ready?

Almost always, because the car's memory was recently wiped. Two things do it.

The first is a battery disconnect or a flat battery. The moment the car loses power, the stored self-test results are lost and every monitor drops back to Not Ready. If you have had a new battery fitted, jump-started a flat one, or done any work that involved disconnecting the negative terminal, the monitors are reset and need re-completing.

The second is clearing fault codes with a scan tool. When a mechanic clears a code after a repair, or you clear one yourself, the same thing happens: the monitors reset. This is the trap that catches people who fix a fault, clear the light, and drive straight to the test. The light is off and the fault is gone, but the monitors have not had time to re-run, so the car fails on readiness instead. If you have just dealt with a check engine light, this is the bit that bites.

There is a third, less common reason worth knowing: if a particular monitor will not complete no matter how much you drive, that usually means there is a genuine fault in that system stopping the test from passing. In that case the Not Ready is a symptom, not just a timing issue.

How do you get readiness monitors to complete?

You complete them by driving, because each monitor runs its self-test under particular conditions. There is no button that sets them. A generic OBD2 drive cycle that completes most monitors on most petrol cars looks like this:

1. Start the engine cold, ideally after it has sat overnight, and let it idle for two to three minutes with the air conditioning and rear demister on, then off.
2. Drive at a steady main-road speed of around 50 to 90 km/h for a continuous 15 to 20 minutes, avoiding hard acceleration and sudden stops.
3. Come to a gentle stop and idle for another minute or two.
4. Accelerate smoothly to around 90 km/h, then lift off the throttle and let the car decelerate on its own without braking, for about 10 seconds. This deceleration phase is what the EVAP monitor often needs.
5. Repeat the cycle over a day or two, with the car fully cooled between drives, until the monitors read Ready.

The catalyst and EVAP monitors are the slowest and the fussiest. The EVAP test in particular only runs when the fuel tank is roughly a quarter to three-quarters full and the temperatures are in a specific range, which is why it can lag behind the others. Watching the count climb from a few Ready to all Ready is something you can do live on a phone, which the guide to reading OBD2 live data explains in more detail.

Can you pass the test with the check engine light on?

No. A lit check engine light is its own automatic fail, completely separate from the readiness question.

The malfunction indicator lamp being commanded on tells the examiner that the car has an active emissions-related fault, and the stored code confirms it. The tester reads both electronically, so there is no driving around it. This is why the order of operations matters when you are preparing for a test: first deal with the stored code so the light goes out, then clear it, then drive enough to re-complete the monitors that clearing the code reset. Skip the last step and you trade a light-on fail for a Not Ready fail and still go home rebooking.

If the light is on and you are not sure why, reading the code is the starting point, and the 15-euro approach to finding out what is wrong is the cheapest way to do it before you involve a shop or a test station. Knowing the code also tells you whether you are looking at a quick fix or a real repair, and what a diagnostic actually costs if you do hand it over.

How do you check readiness yourself before the test?

This is the whole point, and it costs about 15 euros plus two minutes. Before you book the inspection, plug a Bluetooth OBD2 adapter into the port, open the app, and look at two things: the readiness monitor list and whether any codes are stored.

If every monitor reads Ready (or you are within the one-incomplete allowance) and no codes are stored with the light off, you are clear to book with confidence. If several monitors say Not Ready, you know the car needs a few days of driving first, and you have just saved yourself a wasted test fee. If a code is stored, you deal with that before anything else. The honest limit is worth stating plainly: checking readiness and codes tells you whether you will pass the electronic OBD part of the test. It does not measure the actual exhaust gases, which the station does with its own analyser. What it does is catch the avoidable electronic fail before you pay for it.

Before you book the emissions test, check the readiness monitors yourself. Skanyx pairs with any 15-euro Bluetooth OBD2 adapter and shows each monitor as Ready or Not Ready in plain language, alongside any stored codes and the check engine light status, so you know whether the car will clear the electronic check before you hand over the test fee. It reads the same monitors the station does; it just does not run the official tailpipe measurement. skanyx.com/download

What to do before your next inspection

Check the monitors a week before, not the morning of, because incomplete ones need driving time you cannot rush. If the battery has been changed or a code cleared recently, assume the monitors are reset and give the car a few days of mixed driving first. Make sure the check engine light is off and no codes are stored, because that is a separate fail; do those three things and the electronic part of the test becomes a formality instead of a coin toss, with only the exhaust reading left to chance.