The seller cleared the codes just before your visit: the three signs that show it

What clearing does, and what it leaves behind

Clearing fault codes takes ten seconds with any adapter. The check engine light goes off, the list of codes is empty, and the car looks healthy during the visit. If the fault is real, the light will come back… after a few trips, so often after the sale.

But clearing is not discreet. The engine control unit keeps counters that the clear command resets to zero, and emissions tests that it forces to run again. That is where you need to look. This topic complements the pre-purchase scan checklist.

What the OBD2 port will not tell you

Let us be clear before starting:

• The real mileage: standard OBD2 does not provide for reading the odometer. Some cars provide a recorded mileage, others do not, and a mileage reading can be altered in several control units at once. DiagAuto AI does not detect mileage tampering.
• Wear: clutch, gearbox, shock absorbers, timing belt or chain, rust leave no code.
• The history: accidents, servicing, owners. Invoices, the registration document and the official history are what tell you about it.

Sign no. 1: very low distance and time since clearing

Most recent cars provide two counters that are reset at each clearing: the distance since codes cleared and the time since codes cleared (in minutes of engine operation). DiagAuto AI's check-up also reads, when the car provides it, the number of engine warm-ups since that moment.

How to read them:

• A few kilometres, a few minutes, one or two warm-ups: the codes were cleared just before you arrived, or the battery was disconnected (on many cars, this also resets these counters).
• A few hundred kilometres: recent clearing, perhaps during a repair. Ask for the invoice.
• Several thousand kilometres: no recent clearing. Beyond about 65,000 km, the counter maxes out and the app displays “more than 65,000 km”.

Example: a Peugeot 308 HDi advertised as “used every day to commute to work” that shows 3 km and 6 minutes since clearing. The seller may have a good reason; they should be able to give it.

Sign no. 2: emissions checks “not ready”

The control unit regularly tests the catalytic converter, the O2 sensors, the EGR, the evaporative system… Each completed test switches to “Ready”. Clearing resets them all to “Not ready”, and it takes several varied trips (town, open road, cold start) for them to go back to “Ready”.

• One or two tests not ready: often normal, some (evaporative system, catalytic converter on some models) only complete rarely.
• Most tests not ready: recent clearing or battery disconnection. The check-up displays it as “Several emissions checks not finished”.

This sign is the hardest to hide: only real driving makes it go away. That is also why it counts at the roadworthiness test on some vehicles.

Sign no. 3: an empty memory on an old car

A fifteen-year-old Renault Mégane or Citroën C4 with 200,000 km often has nothing in confirmed codes in the engine control unit: this is possible, old codes clear themselves after about forty warm-ups without recurrence. This sign alone therefore proves nothing.

It becomes telling when it adds to the other two, or when other traces are missing too:

• Pending codes: a pending code on a freshly cleared car shows a fault that is already coming back.
• Permanent codes: they cannot be cleared with a tool. A permanent code present while the main list is empty is a real fault, cleared but not repaired. See the code statuses.
• The other control units (ABS, airbag, body): on an old car, they often keep a few old faults (low voltage, communication). DiagAuto AI's control unit scan shows which ones have faults and how many, and reads the details of their codes on compatible control units. Everything at zero everywhere, just after the engine codes were cleared, deserves a question.

Example: a 2012 Dacia Sandero with no confirmed code, most emissions tests not ready, 2 km since clearing, and already a pending P0420 code. The catalytic converter fault is coming back: allow €300 to €1,500 for the repair (indicative, France), to have quoted before negotiating.

The right way to go about the visit

1. Ask the seller for permission, then plug in the adapter, ignition on, engine off (how to connect it).
2. In DiagAuto AI, open “Buying a used car” and run the check-up: codes (confirmed, pending, permanent), emissions checks, distance and time since clearing, battery voltage, VIN, mileage when the car provides it.
3. Take the test drive, then read the codes again: a pending code that appeared during the drive is an active fault.
4. Ask the questions the check-up suggests, and ask for the invoices.

A recent clearing is not a death sentence. It is a reason to ask, to have the car checked by a garage, or to come back in a week for a second scan.