Roadworthiness test: check your OBD2 codes beforehand to avoid a re-test

OBD2 guides · Updated on · 4 min read · Saïd, creator of DiagAuto AI

What the inspector checks on the engine side

  1. The warning lights on the dashboard at start-up: an engine, ABS, airbag or glow plug light that stays on is a major defect.
  2. On the vehicles concerned (petrol first registered since 2002, diesel since 2004, or 2007 for vans and heavier vehicles), they plug a reader into the diagnostic socket: ignition on, then engine started, they follow the reader's instructions. A stored engine fault code (P0, P1 or P2) is a minor defect, recorded without a re-test; a light that is on or flashing, or a non-compliant result from the reader, is a major defect (source: UTAC-OTC technical instruction “Nuisances”, 2025).
  3. The emissions measurement: smoke opacity (diesel), CO/lambda (petrol). A dead catalytic converter or a holed DPF shows up here.

The 4-step method

1. Three weeks before: scan.
Plug in your adapter (which one), read the confirmed codes, the “Pending codes” and the “Permanent codes” with DiagAuto AI, and look at the “Emissions checks”. Look up each code in the code dictionary.

2. Repair what matters.
Emissions codes come first: catalytic converter (P0420), EGR (P0401), evaporative system (P0455), DPF (P2002), mass air flow sensor, O2 sensors. Even a “small” code turns the light on.

3. Clear, then drive.
Only after the repair. Do several varied drive cycles so the emissions checks go back to “ready”. Allow 100 to 300 km. Why it matters.

4. The day before: read again.
Zero codes, light off, emissions checks complete. And while the adapter is plugged in, look at the engine temperature and the O2 sensors: a fully warm engine at test time does better on the emissions measurements. Arrive after 20 minutes of driving, not with a cold engine.

The classic traps

  • Removing the warning light bulb: the inspector checks that the light comes on with the ignition, and their OBD reader queries the control unit directly. A missing light is itself a major defect, and either way it is hiding a fault.
  • Clearing the day before: emissions checks “not ready”, a light that may come back: you risk a re-test.
  • A “cleaning” additive the day before: it will not fix a dead sensor.
  • Ignoring a flashing glow plug light on a diesel: it is an engine fault (explanation).

What it costs you

A €25 adapter and 10 minutes (indicative, France 2026), versus a re-test (no charge or €20 – 40 depending on the centre) plus a rushed repair, at full price, at the first garage available.

Frequently asked questions

Can the inspector see that I cleared the codes?

Very likely, yes. On the vehicles concerned (petrol since 2002, diesel since 2004), the inspector plugs a reader into the diagnostic socket. After clearing, the emissions checks go back to “not ready” and the light comes back on if the fault is still there. Clearing at the last minute fixes nothing.

Is a stored code without the warning light on a failure?

No, but it is recorded: a stored engine fault code (P0, P1 or P2) found by the OBD reader is a minor defect, with no re-test. A light that is on, or a non-compliant result from the reader, does mean a re-test. And a pending code today can turn the light on tomorrow.

Read also