How to clear the check engine light
You clear the check engine light with an OBD2 scanner's clear codes command, which erases stored codes and turns the light off. Clearing does not fix the fault: if the problem is still there, the light comes back, and clearing also resets the readiness monitors an inspection checks.
Key facts
| Key fact | Detail |
|---|---|
| How to clear it | Scan tool clear codes command (OBD service $04) |
| What it erases | Stored codes, freeze frame and readiness monitor results |
| What it cannot erase | Permanent codes (PDTCs); the car clears them itself after verifying the repair |
| If the fault remains | The code and the light return |
| Clears on its own? | The light can go off by itself if the fault does not recur over three consecutive drive cycles |
| Before an inspection | Do not clear codes just before; monitors will show not ready |
Before you clear anything
- Read and write down every stored and pending code.
- Save the freeze frame data. It shows the conditions when the fault happened, and clearing can erase it.
- Look each code up and fix the cause first. EPA inspection guidance notes that codes and their freeze frame data are important for proper diagnosis before repair.
How to clear the light with an OBD2 scanner
- Plug your ELM327 class adapter into the OBD2 port and switch the ignition on.
- Connect from the app and read the codes first.
- Use the clear codes function. In OBD terms this sends service $04, which clears stored trouble codes and turns the malfunction indicator lamp off.
- Start the engine and check whether the light stays off. Then check again after a few drives.
What clearing erases
Clearing removes stored codes and the freeze frame, and resets the readiness monitors to not ready. The monitors then have to run again during normal driving before the car is ready for an OBD inspection.
Disconnecting or replacing the battery has a similar effect on readiness, which is why inspection programs treat many not ready monitors as a sign of a recent reset.
Permanent codes cannot be cleared
On newer vehicles a confirmed code is also kept as a permanent code. It cannot be erased with a scan tool or by disconnecting the battery. The car removes it itself once the related monitor has run and found no fault. California's Smog Check, for example, checks for permanent codes.
Why the light comes back
The light turns on when the OBD system confirms a fault. If you clear it without repairing the cause, the same self test fails again and the code returns, often within a few drives. A pending code after clearing is an early sign that the fault is still there.
Can the light turn off by itself?
Yes. If the fault does not recur over three consecutive drive cycles, the OBD system can turn the light off.
Clearing codes before an emissions test
Clearing codes right before an OBD inspection usually backfires: the readiness monitors show not ready, and inspection programs fail or reject vehicles with too many not ready monitors. US EPA guidance lists clearing codes just before a test as one of the reasons monitors are not ready. Repair the fault, drive normally until the monitors are ready, then test.
Try it on your own car
OBD Master Pro turns your Android phone and a generic ELM327 adapter into a code reader: read and clear codes, view freeze frame and live data, and check emissions readiness.
Keep reading
Related guides: Check engine light on: what to do · How to read an OBD2 code · Stored, pending and permanent codes · OBD2 readiness monitors explained · ELM327 adapter not connecting · What is fuel trim · OBD2 live data and PIDs · OBD2 port pinout · What is a U code? · OBD2 glossary
Common codes: P0171 System Too Lean · P0420 Catalyst Efficiency · P0300 Random Misfire · P0455 EVAP Large Leak
All OBD2 guides · OBD2 by car brand · OBD2 trouble code guides
Sources
- Wikipedia: OBD-II PIDs (SAE J1979 services and standard parameter IDs)
- U.S. EPA: Performing Onboard Diagnostic System Checks as Part of a Vehicle Inspection and Maintenance Program (EPA420-R-01-015)
- U.S. Code of Federal Regulations, 40 CFR 86.1806-05, On-board diagnostics (govinfo.gov)
- California Bureau of Automotive Repair: On-Board Diagnostic Test Reference
- California Bureau of Automotive Repair: Permanent Diagnostic Trouble Codes, workshop presentation (2018)
- Texas Department of Public Safety: Ready or Not (OBD II readiness monitors)
- California Air Resources Board (CARB): On-Board Diagnostic II (OBD II) Systems fact sheet
- U.S. EPA: What to do with the Check Engine Light (EPA420-F-00-041)
- OBD Master Pro on Google Play (app features, supported connections and protocols)
Published . Updated . General information only. Details generally vary by model, year and market, so always follow the owner's manual and service information for your vehicle.