Clearing the warning light the day before the roadworthiness test: does it show? “Not ready” checks and distance since clearing

What clearing actually does

Clearing the codes means sending the engine control unit a standard OBD2 command (“mode 04”). The control unit then forgets the confirmed codes, the pending codes and the data at the time of the fault, then turns off the light. It repairs nothing: if the sensor is dead or the leak is still there, the control unit will run its test again and turn the light back on, sometimes on the very first trip, sometimes after several days. For details: clearing a fault code, when it helps.

Clearing also resets several pieces of information that the control unit keeps precisely to show what happened.

The three traces of a recent clearing

1. The emissions checks go back to “not ready”

The control unit continuously or periodically tests the systems that affect emissions: catalytic converter, oxygen sensors and their heaters, EGR, evaporative system on a petrol car, particulate filter, NOx and boost pressure on a recent diesel. Each completed test switches to “ready”. These are what are called readiness monitors.

After clearing, or after disconnecting the battery on most cars, all these tests go back to “not ready”. Only a few continuous checks (misfires, fuel system, components) come back very quickly. A table with almost everything “not ready” on a car driven every day is the typical sign of a very recent clearing.

2. Distance and time since clearing start again from zero

Recent cars count the kilometres driven and the minutes of operation since the codes were last cleared. A counter showing 3 km the day before a test speaks for itself. Not all cars provide these two values, especially the time, but when they exist, any OBD2 reader can read them.

3. Permanent codes remain

On cars that support them, a permanent code is not erased by the clear command: it only disappears when the control unit itself has found that the fault is gone. Clearing the day before therefore leaves the permanent code in memory, with the light off.

What the inspector looks at in France

In France, the roadworthiness test is called the contrôle technique. An engine warning light that is on is a major defect there: re-test required. On the cars concerned (petrol since about 2002, diesel since about 2004), the procedure also involves plugging a reader into the diagnostic port; our article roadworthiness test: check your codes beforehand explains the rules in detail.

Exactly what this reader records depending on the vehicle, and how the “not ready” status is interpreted, we cannot state for every case. The cautious reasoning is simple: anything a standard OBD2 reader sees, the inspector's reader can see too. And even if nothing were recorded at the port, the light can come back on during the engine test at the inspection or on the drive home.

What you see in DiagAuto AI

• Diagnostics screen: “Emissions checks” with the number of completed tests (for example “4 of 7 completed”) and the “Ready” / “Not ready” list.
• “Read fault codes”, “Pending codes” and “Permanent codes”.
• Sensors screen: “Distance since codes cleared” and “Time since codes cleared”, when the car provides them.

So you see exactly what an outside reader would see, before the appointment. More details on the app's features.

Your test is this week: the right method

1. Read the codes today, without clearing anything. Also note the data at the time of the fault and look the code up in the fault code dictionary.
2. Assess the repair: a fuel cap not properly closed, a corroded sensor connector or an accessible sensor can sometimes be sorted in an hour. A catalytic converter or a DPF (diesel particulate filter), no.
3. Repair, then clear once. Clearing after a repair is useful: it resets the counters to check that the fault does not come back. DiagAuto AI asks for confirmation and offers to save the report just before; no reprogramming, ever.
4. Drive in varied conditions: a cold start, some town driving, a few kilometres at a steady speed on the open road, deceleration without braking. Read the emissions checks again every day.
5. If some checks are still “not ready” or the fault cannot be repaired in time, postponing the appointment by a few days often costs less than a re-test, and gives you time to have it repaired properly.

In other countries

Rules differ from one country to another and change over time; always check with the test centre.

• Spain (ITV): recent vehicles may have their OBD port read, and a very recent clearing of codes may be recorded.
• Italy (revisione): OBD reading plays a growing role in the revisione of recent vehicles.
• Germany (HU/AU): the emissions test (AU) for recent vehicles includes an OBD reading, including the status of the emissions checks, in addition to the tailpipe measurement.
• Poland (badanie techniczne): dashboard warning lights are checked; reading the OBD port depends on the station and the vehicle.
• Turkey (TÜVTÜRK): the inspection checks warning lights and emissions; ask about OBD reading for your vehicle.
• India (PUC): the certificate is based on an emissions measurement; the role of OBD reading is evolving for recent vehicles.
• Romania (ITP): warning lights and emissions are checked; an OBD reading may be done on recent vehicles.
• Japan (OBD検査): an OBD inspection is part of the test for some recent vehicles and looks in particular for codes from safety systems.
• China: the periodic emissions inspection includes an OBD reading (warning light, codes, status of the checks) on equipped vehicles.