How Do You Reset the Check Engine Light on Your Car?

The check engine light (CEL) comes on when the on-board diagnostics second generation system (OBD II) detects an emission system problem. And yes, you can legally turn off the CEL yourself. But is that wise?

After almost 50 years in the auto repair industry, I know that nothing panics a driver — including yours truly — more than the CEL coming on, and the fear of unexpected/expensive car repairs. I’ve discussed CEL troubles with my friends, family and customers for years.

OBD II and emission control systems increase fuel efficiency while reducing smog. Unfortunately, too many drivers hope the CEL will magically turn itself off. I’ve explained, more times than I can count, that ignoring a CEL can lead to costly repairs. Here’s what you need to know.

What Are Readiness Monitors?

Your vehicle’s engine control module (ECM) includes readiness monitors, a self-diagnostic software program that verifies the OBD II system is operating as designed. Depending on make, model, year and equipment, a vehicle can have up to 12 readiness monitors.

Using thousands of pieces of sensor data per second, including engine and vehicle speed and distance, the software identifies problems with your emissions system and establishes readiness monitor status: Ready or Not Ready.

• Ready means the self-test determined the emissions system is working properly.
• Not Ready means the self-test cannot determine if it’s working as it should.
  • On 2001 and newer models, if more than one readiness monitor remains in Not Ready status, the CEL illuminates and your vehicle will fail emissions inspection. Not Ready doesn’t mean there’s an actual system or part failure. A CEL can be caused by a dead battery or a loose gas cap.

What Are Trouble Codes?

The OBD II system stores diagnostic trouble codes (DTCs) in the ECM. DTCs help identify which offending part, component or system causes the CEL to illuminate. You’ll need a scan tool to read DTCs.

How To Reset the Check Engine Light

The simplest way to turn off a CEL is to find and fix the problem. However, sometimes the CEL remains on after repairs are made, even for the pros. Here’s what to do:

Turn off the CEL, pre-1995

• Disconnect the battery. Remove the negative (-) cable first, then remove the positive (+) cable, wrapping the cable terminals with electrical tape.
• With the battery cables removed, cycle the ignition switch ON and OFF three to five times, then hold the horn button for 60 seconds to power down the ECM.
• Wait 15 to 30 minutes before reconnecting the battery cables: Positive (+) first, negative (-) last.

Turn off the CEL OBD II, 1996 and newer

• Try the steps listed above.
• Purchase a scan tool that can read readiness monitor status, clear DTCs and turn off the CEL following on-screen menus or prompts. A scan tool cannot reset readiness monitors.
• Reset the readiness monitors by completing a drive-cycle (see below). A drive-cycle helps the ECM confirm and verify the emissions systems are working correctly.

How To Complete a Drive-Cycle To Reset the Check Engine Light

Although it’s tricky and can be time-consuming, doing this yourself will save hundreds of dollars in labor and diagnostic fees.

Follow these steps to complete a generic OBD II drive-cycle. Check your owner’s manual or online to locate the specific steps for your vehicle:

1. Let the vehicle sit overnight. This allows air and coolant temperatures to stabilize.
2. Connect a scan tool to the diagnostic link connector.
3. Check monitor status and DTCs.
4. Start the engine, turn the air conditioning ON (if equipped) until engine temperature reaches at least 160 degrees. Do you know what CC means in an engine?
5. Confirm the fuel tank is between one-quarter and three-fourths full.
6. Turn the air conditioning off.
7. At 25% throttle, accelerate up to 50 to 60 mph (55 mph preferred) for three to five minutes. Better scan tools can read throttle position, or just push the gas pedal down about a quarter of the way. If you are using a scan tool, take a helper with you to monitor any scan tool readings while you focus on driving.
8. Accelerate to and maintain 50 to 60 mph (55 mph preferred) for three to five minutes.
9. Without using the brake or clutch, slow down to 20 mph.
10. Speed up to 55 to 60 mph at 75% throttle by slowly pushing the gas pedal down about three-quarters of the way. Maintain that speed for five minutes.
11. Stop the vehicle and let it idle for 10 seconds.
12. Turn the key off and wait two minutes for the ECM to power down.

Even if the CEL is off, repeat Steps 5 through 10 if readiness monitor status doesn’t reset. The pros don’t always get this right the first time, either.