How much a garage diagnosis costs, and what an OBD2 app can (or can't) do instead
The three levels of diagnosis and what they cost
| Level | What you get | Indicative price, France 2026 |
|---|---|---|
| Code reading | List of codes, sometimes a generic explanation | €0 – €40 (sometimes complimentary at some auto centres, especially if you buy the part there) |
| Diagnosis with fault finding | A technician reads, interprets and tests (measurements, road test) | €50 – €120 per hour, often a one-hour flat fee |
| Dealer scan tool | Access to every control unit, detailed parameters, special functions | €80 – €150 per session; included in the diagnosis at a dealership |
What an app does just as well
- Read engine codes (confirmed, pending, permanent) and the data at the time of the fault (the “freeze frame”).
- Explain them in plain language: that is exactly the job of the AI in DiagAuto AI, code by code, with the likely causes and the checks to do first.
- Show live sensor data to confirm or rule out a lead (the 10 useful values).
- Clear engine codes after a repair (after confirmation, without reprogramming anything).
- See which other control units report a fault (ABS, airbag, air conditioning…); with DiagAuto AI, read their codes on compatible control units, without purchase.
- Produce a PDF report to show your mechanic (a Premium feature of DiagAuto AI; otherwise, screenshots).
What it doesn't do
- Physical tests: compression, EVAP smoke test, injector leak-off, fuel pressure check with a gauge, oscilloscope.
- Manufacturer functions: forced DPF (diesel particulate filter) regeneration, injector coding, gearbox adaptation, ABS bleeding, service resets. DiagAuto AI never reprograms control units, by design.
- Read certain control units (airbag, ABS, comfort) on makes that don't expose them over standard OBD.
Doing the maths
A common scenario: engine warning light, you go to the garage, €60 for the diagnosis, and the result is “downstream lambda sensor”. With a €25 adapter and the app, you would have read the P0420, understood what it hides, compared the two sensors live, and been able to tell the garage “I have a P0420, the downstream sensor follows the upstream one” or “the downstream sensor is flat, I suspect the sensor”. The garage then spends less time, and you know what you're talking about. How to talk to them about it.
For a complex fault (intermittent, electrical, mechanical), the garage remains essential, and the app will at least have saved you from going in blind.
Frequently asked questions
Will the garage charge for the diagnosis if I have the repair done there?
The diagnosis fee is often deducted or waived if the repair follows. Ask beforehand: it is common practice, but not systematic.
Why did the garage find more codes than my app?
A dealer scan tool reads every control unit (ABS, airbag, comfort, gearbox) using the manufacturer's proprietary protocols. A standard OBD2 app mainly reads the engine control unit and whatever the other units agree to expose. For OBD2 engine codes, both see the same thing. With DiagAuto AI, scanning the control units shows which ones report faults and reads their codes on compatible control units.