Buying and selling a used car
Can you hide faults by clearing the codes before selling a car?
Clearing the codes turns the light off for a while, but it fixes nothing and usually leaves traces: the readiness monitors go back to incomplete, many cars count the distance and time since the last clear, and some keep permanent codes that a scanner cannot erase. An attentive buyer or a pre-purchase report often spots it.
Reviewed by Daniel Morales, founder of Reprorace SL, on
What happens when the codes are cleared?
The check engine light goes out and the stored codes, freeze frame data and monitor status are lost. If the fault is still there, the car detects it again when the test conditions are met, sometimes within minutes and sometimes after several days.
Meanwhile, the readiness monitors show as not complete, which is the first thing that gives away a recent clear.
What other traces are left?
Standard diagnostics include data such as the distance traveled and the time elapsed since the codes were cleared, and the number of warm-ups since then. If they are very low, the clear is recent.
Many recent cars also have permanent codes: the control unit keeps them until it confirms on its own that the fault is gone, and a scanner cannot erase them.
Will the buyer or the inspection notice?
A buyer with a scanner sees the incomplete monitors and the counters since the clear. At the inspection, reading the OBD port can show those monitors and, depending on the country and the procedure, the emissions test may not be accepted.
A pre-purchase report from a shop that works with Diaglana flags recent clears.
What is the honest thing to do if I am selling?
Read the codes, fix them or explain them to the buyer. Hiding a known defect can have legal consequences depending on the country and the type of sale; this is not legal advice, so check your own case.
If you have just repaired something and cleared the codes yourself, drive for a few days so the monitors complete, and tell the buyer.
General guidance only: every manufacturer and model has its own details. Always follow your vehicle's manual and, if in doubt, ask a workshop.
See what your car is saying
With the ECO-LITE, a shop that works with Diaglana can read the car and issue a pre-purchase report that flags recent clears and the mileage stored by each control unit.
Presale price: €129 · US$129
Frequently asked questions
How long until the light comes back on?
It depends on the fault: some are detected at start-up and others need several trips under specific conditions.
Does replacing the battery also clear the codes?
On many cars, yes, and it leaves the monitors incomplete just like a clear.
Do all cars keep permanent codes?
Not all. They are common on recent cars, especially those built for the US market.