Car Diagnostics

Resetting DTC Codes

Clearing (resetting) fault codes — Mode 04 in the OBD-II standard — is a command telling the ECU to forget all stored DTCs and reset readiness monitors. It’s an irreversible action on stored data, but not a repair. If the root cause isn’t fixed, the code will return.

When it’s OK to clear DTCs

✅ After a confirmed repair. You replaced the component, fixed the root cause — clear codes to verify the problem is gone. This is standard procedure.

✅ After diagnostic testing. Some procedures (like EVAP or Catalyst Monitor checks) require a clean slate with no active codes.

✅ Before handing a car to a new owner after repair. Logical and appropriate.

When you should NOT clear DTCs

❌ Before selling the car to hide a problem. No codes ≠ no faults. An informed buyer will check readiness monitors — all Not Ready = recent code clear.

❌ Before an emissions inspection. With monitors in Not Ready state — the car will fail the test. You need to complete a Drive Cycle to restore monitors.

❌ Without understanding the fault. “I’ll clear it and see if it comes back” is dangerous for codes like P0217 (overheating), P0520 (oil pressure), P0562 (low voltage). A drive before the code returns can cause real engine damage.

What happens after clearing

  1. All stored DTCs are erased
  2. All pending DTCs are erased
  3. Freeze Frame data is cleared
  4. All readiness monitors reset to “Not Ready”
  5. LTFT (Long Term Fuel Trim) resets to zero on some ECUs
  6. Warm-up cycle counter resets

⚠️ On some vehicles (Subaru, BMW, some GM) clearing DTCs also erases the transmission’s adaptive shift tables — the automatic may shift roughly for the first 30-60 miles until it relearns.

How to clear codes in the app

  1. Open the DTC Codes section
  2. Tap Clear Codes (or the trash icon)
  3. Confirm in the warning dialog (always shown — protection against accidental clearing)
  4. Wait for the ECU confirmation

The process takes 2-5 seconds. The app then automatically re-reads the code list — it should be empty.

Alternative: disconnecting the battery

Disconnecting the negative battery terminal for 15-30 minutes gives the same result as Mode 04 — all codes and monitors reset. But there are side effects:

  • Radio presets, clock, window positions reset
  • On some cars the EPS (electric power steering) resets and may pull temporarily
  • Some immobilizers require a PIN code after battery reconnection

Mode 04 via OBD-II is cleaner and safer.

Verification after clearing

After 1-2 complete drive cycles check:

  1. No DTCs returned
  2. Readiness monitors moved to Ready (especially Catalyst and EVAP)
  3. LTFT readings are within normal range (±5% after 30+ miles)

If a code returns immediately or after a few starts — the problem is not fixed.