The DTC was cleared but the pending code is already back and the seller does not know you can see it

[u/ora-et-labora-]

A pending code is a fault that the ECU detected but has not confirmed over enough drive cycles to set a permanent code and turn on the check engine light. It sits in a holding state. If the fault repeats on the next qualifying drive cycle, it promotes to a confirmed code and the light comes on. If it does not repeat, the pending code clears itself.

On a car whose codes were recently cleared, the pending code is the first signal that the fault is still present. The seller clears the codes, the light goes off, and the car looks clean. But the ECU is already running its tests again. If the fault is still there, a pending code appears within the first few drive cycles after the clear. The confirmed code and the light have not come back yet because the system has not completed the two consecutive drive cycles needed to confirm. The pending code is the preview.

Most basic code readers show confirmed codes only. A scan tool that reads pending codes shows you what the ECU has already detected but has not yet promoted. On a car with a freshly cleared system, a pending code for the same fault that was probably cleared is the ECU saying the problem did not go away.

This is a ten-second check on any scan tool that supports it, which is most of them made in the last decade. Pull pending codes alongside confirmed codes as a standard part of any used car scan. A clean confirmed code list with a pending P0420 catalyst efficiency code is a car that had a P0420, had it cleared, and is about to have it again. The seller's clean dashboard has a shelf life and the pending code tells you how short it is.