If a DTC Code Appears, Check Its Status First
When a scanner code appears, many people immediately think it means a part has failed. But a DTC is not a conclusion; it is a signal to verify. Even for the same `P0420`, the questions to ask before purchase differ depending on whether it is a current code, a past record, or a code that returned after being cleared.
If a DTC Code Appears, Check Its Status First
When a scanner code appears, many people immediately think it means a part has failed. But a DTC is not a conclusion; it is a signal to verify. Even for the same P0420, the questions to ask before purchase differ depending on whether it is a current code, a past record, or a code that returned after being cleared.
Order to Check First
Current / Pending / History / Permanent- MIL ON/OFF
- Freeze Frame
- Readiness Monitor
- Rescan after a test drive
- Relevant service history
Looking at It Like a Real Case
Suppose P0420 and P0140 appear together on a local scanner screen. It can be risky to immediately calculate the catalytic converter replacement cost at this point. P0420 may be a signal related to the catalyst efficiency system, but the oxygen sensor waveform, exhaust leaks, fuel trim, misfire history, and Catalyst Monitor readiness should be reviewed together. P0140 may be a signal related to O2 sensor activity, so wiring harness, sensor power supply, and downstream sensor data are needed.
If both codes are History, the MIL is off, and they do not recur on rescan, the intensity of the purchase questions may be lower. Conversely, if they are Current or Permanent, the MIL is on, and they repeat on rescan, this should be reflected in cost negotiation or a decision to hold off on the purchase.
What LemonInspect Does
Rather than translating codes into part names, LemonInspect organizes the code status and the order of checks. What customers need is not a definitive statement that "this part has failed," but rather "what should be checked further before purchase."