Which warning lights talk to OBD2 and which do not: engine, glow plugs, DPF, ABS, airbag, battery, oil
What you notice
A symbol lights up on the dashboard and you wonder whether your OBD2 reader will be able to tell you why. Lists of warning lights explain what they mean, but rarely which ones an adapter can read. Yet that is what decides whether you will get an answer in two minutes or need a garage.
The colour rule
- Red: danger or risk of damage. Stop as soon as possible, safely, and switch off the engine if it is the oil or temperature light.
- Amber: a fault to have checked, generally with no immediate urgency, unless it is flashing (flashing check engine light).
- Green, blue, white: simple information (lights, cruise control).
The lights one by one
| Light | Colour | Readable in standard OBD2? | Via the control unit scan | Good to know |
|---|---|---|---|---|
| Check engine (engine outline) | Amber | Yes: confirmed, pending, permanent P codes | Yes | This is the OBD2 light par excellence |
| Glow plugs (coil, diesel) | Amber | Most often: when it flashes, it signals a fault stored by the engine control unit | Sometimes a separate unit | See glow plug light flashing |
| DPF (diesel particulate filter) | Amber | The codes, yes (P2002, P2463…) | Not applicable | Soot load and differential pressure: not read |
| AdBlue / emissions control | Amber | Often engine codes | Not applicable | The tank level is not standard OBD2 data |
| Automatic gearbox | Amber | Often (emissions-related control unit) | Yes | P07xx codes |
| ABS | Amber | No | Depending on the make and the adapter | C codes |
| ESP / traction control | Amber | No | Depending on the make and the adapter | Often the same control unit as the ABS |
| Airbag | Red or amber | No | Depending on the make and the adapter | B codes; never work on the module yourself |
| Power steering | Amber or red | No | Depending on the make | Heavier steering: be careful |
| Tyre pressure | Amber | No | Depending on the make | First check the pressures with a gauge |
| Engine temperature | Red | The coolant temperature can be read in real time | Not applicable | The light is an alert: stop immediately |
| Battery / charging | Red | The voltage can be read in real time; sometimes an engine code | Not applicable | Alternator or belt: do not drive far |
| Oil pressure | Red | Generally no: it is a switch | Not applicable | Stop immediately, engine off |
| Oil level | Amber | No | Not applicable | Check with the dipstick |
| Brakes (fluid, parking brake) | Red | Generally no | Electric parking brake: depending on the make | Fluid level and pads to be checked |
| Service (spanner, oil change) | Amber | No: it is not a fault | Not applicable | Reset by a manual procedure |
“Depending on the make and the adapter”: some cars let you read these control units with a good adapter, others reserve them for the manufacturer's tools. The details are in How many control units does your car let you read?
What each app function covers, honestly
- “Read fault codes”: the engine control unit and the emissions-related control units. Confirmed codes, “Pending codes”, “Permanent codes”, “Data at the time of the fault”.
- “Sensors”: readings, including the check engine light status and the distance driven since it came on. The coolant temperature and the control unit voltage help to understand a temperature or charging light. No oil pressure: it is not standard OBD2 data.
- “Control units”: the scan looks for the other control units (ABS, airbag, air conditioning…) and shows those it was able to read, and which ones have faults. It reads their codes on compatible control units; if no additional control unit answers on your car, the app says so.
- The AI: “Explain with AI” details a code that has been read; diagnosis from a described symptom works even when the light has left no code, and the AI can explain the procedure for resetting the service light. It suggests leads, not a certain diagnosis.
The app can clear the engine codes after confirmation, using the standard OBD2 command. It does not reprogram anything, and clearing only turns a light off for good if the cause has been repaired.
What you can do right now
- Identify the colour. Red: stop safely before any reading.
- Check engine, glow plug, DPF lights: plug in the adapter, connect it and read the codes. For a steady check engine light, see check engine light on but the car drives fine.
- ABS, ESP, airbag: standard reading will show nothing, which is normal. Run the control unit scan.
- Oil, brakes, tyres: check the levels and pressures, no adapter will do it for you.
When to go to the garage right away
- Any red light that stays on after restarting.
- Airbag light on: protection may be disabled.
- ABS and brake lights on together: brake force distribution may be affected.
- Check engine light flashing, or accompanied by a clear loss of power.
For the full list of what the app does, see the features.
Frequently asked questions
Is the service light (spanner, oil change) a fault?
No. It is a reminder programmed by the instrument cluster, not a fault code: there is nothing to read via OBD2. It is reset by a procedure specific to each model, which the app's AI can explain to you.
Which of these lights cause problems at the roadworthiness test?
In France, at the contrôle technique (the French roadworthiness test), an engine, ABS, ESP or airbag light that signals a fault is recorded during the test, generally as a major defect, so a re-test. Have the cause repaired rather than clearing the code the day before.
Can a red light come on without any fault code?
Yes, it is even common for oil pressure, brake fluid or battery charging: these are often simple switches or measurements, not codes. The red light is reason enough to stop.