Used-car check

When looking at a used car you want to know whether someone cleared the trouble codes shortly before, so that the check engine light is off. The vehicle tells you, read-only: when codes are cleared (mode 04), every emission control unit resets a few counters to zero, and they only count up again with driving.

obd-diag used-car --port /dev/ttyUSB0          # just the check
obd-diag used-car --port /dev/ttyUSB0 --json   # machine-readable
obd-diag diagnose --port /dev/ttyUSB0 --pdf report.pdf    # full diagnosis, with the check

The full diagnosis (obd-diag diagnose, “Connect & Scan” in the user interface) always includes the check. In the user interface it is on the “Used car” tab, in the PDF report after the readiness. Best with the ignition on and the engine off, as for the diagnosis (First test at the car).

What is read

Request	Meaning	when cleared
0130	warm-ups since codes cleared (0 to 255)	to 0
0131	distance since codes cleared in km	to 0
014E	time since codes cleared in minutes	to 0
0121	distance with the MIL on in km	to 0
014D	time with the MIL on in minutes	to 0
0A	permanent trouble codes	stay
0101	check engine light and readiness tests	tests incomplete again

A warm-up as defined by SAE J1979: the coolant has risen by at least 22 °C since the start and reached at least 70 °C. Not every vehicle reports all counters; the Mercedes A 180 d from the tests on the car knows 0121, 0130 and 0131, but no times. If several control units answer, the smallest value counts for the counters since clearing and the largest for the MIL counters.

How the verdict is reached

A vehicle is suspicious if at least one of these findings applies:

• cleared recently: fewer than 10 warm-ups, 200 km or 300 minutes since clearing

• the check engine light is on, or it was on since the codes were last cleared (distance or time with the MIL on above 0)

• permanent trouble codes: they cannot be cleared; the control unit removes them only once it no longer detects the fault itself

Inconspicuous means: the counters can be read and none of these findings applies. Cannot be assessed means the vehicle reports none of the counters since clearing and nothing else stands out. Incomplete readiness tests are listed as a note: they are incomplete again after clearing, but can also be due to driving conditions.

Note

The limits are a rule of thumb, not a standard. A workshop, a battery change or a new control unit also reset the counters. A finding is a reason to ask the seller when and why the codes were cleared, not proof of a hidden fault.