Driver guide
Will your car pass its MOT or inspection with the engine light on?
Usually not: the inspection checks the engine warning light and, on many cars, reads the OBD socket. But it depends on the country, the light and the fault, and the test center always has the last word.
Reviewed by Daniel Morales, founder of Reprorace SL, on
What they check on the light and the OBD socket
When you switch the ignition on, the engine light comes on for a moment and goes out once the engine starts: that shows it works. If it stays on with the engine running, the control unit has a fault stored.
On cars with EOBD, the emissions test may include reading the diagnostic socket: the stored codes and the readiness monitors, which say whether the control unit has finished checking the catalytic converter, the oxygen sensors or the vapor system.
Other lights count too: an airbag, ABS, stability control or brake warning that stays on is usually a defect in itself.
In the United Kingdom: the MOT
Since May 2018, the MOT checks the engine malfunction indicator lamp on most cars: if it stays on, or does not work, the car usually fails with a major defect.
The rules and their interpretation are the DVSA's and they may change: check the current MOT inspection manual or ask your test center.
In the rest of Europe
Directive 2014/45/EU sets the minimum requirements for roadworthiness testing in the EU, and each country applies it with its own rules. Warning lights and the reading of on-board diagnostics are part of those minimums, but how they are graded varies from country to country.
Why clearing the codes just before does not help
Clearing switches the light off, but the readiness monitors go back to «not complete». The inspection sees it and may not accept the emissions test until the car completes its drive cycles.
Those cycles have no fixed mileage: they depend on the make and the monitor. You usually need to drive for several days, with cold starts, town and open road.
How to arrive ready
Read the fault codes one or two weeks before, not the day before: you will have time to repair and for the car to complete its monitors.
Fix the cause and check that the code does not come back. Then read again and make sure the readiness monitors are complete.
If the light is the airbag, ABS or brakes, do not leave it to the last day: they are defects in themselves and the repair may take time.
How Diaglana helps
With an ECO-LITE in the OBD socket, you see the fault codes and the inspection readiness in your account, and your workshop can check them remotely, always with your permission.
Guidance only, with no binding legal or technical value: always follow your vehicle manufacturer's information and your country's rules.
Frequently asked questions
Can I clear the code before the inspection?
You can, but it rarely helps: the readiness monitors are left incomplete and the inspection sees it. And if the fault is still there, the light comes back on.
What if the light comes on on the day of the inspection?
If the fault is stored when you are inspected, it counts. Book the workshop with the code and ask the test center how to proceed.
How far do I have to drive after the repair?
There is no single figure: it depends on each monitor and the make. Drive normally for several days and check with a reader that the readiness monitors are complete.
Read your car's fault codes on your phone
See the ECO-LITE page or buy it in the shop at the final price.