Reset the Check Engine Light, and When Not To
An amber engine symbol on your dash. A reset can turn it off in seconds. It can also make you fail a smog test the next day.
Story in Brief
- A flashing light is not a reset job. EPA says it means a rather severe misfire, so slow down and get service.
- A loose gas cap can trip the light, and EPA says it should go out within a few days after you tighten it.
- Clearing codes or disconnecting the battery can leave readiness monitors incomplete. CARB says driving normally for about a week usually sets them.
- Permanent codes survive a scan tool and a battery disconnect, per California's Bureau of Automotive Repair.
- Federal test rules let a 2001 or newer car pass with no more than one unset monitor. California sets its own list.
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Tools and Parts for This Job
- OBD II code reader or scan tool
- Your phone, to photograph the code screen
- Owner manual, for the port location
- Pen and paper
Specs for This Job
| Item | Spec |
|---|---|
| Light goes off by itself | After 3 clean checks in a row. EPA 420-F-97-003 |
| Gas cap fix | Within a few days of tightening. EPA 420-F-97-003 |
| Driving to set monitors | About a week of normal use. California Air Resources Board |
| Unset monitors allowed, federal test rule | Two for 1996 to 2000, one for 2001 and newer. 40 CFR 85.2222 |
| Unset monitors allowed, California gas 2000 and newer | Evaporative system only. Bureau of Automotive Repair |
| Permanent code ignored in California when | 15 warm up cycles and 200 miles since the last clear. Bureau of Automotive Repair |
An amber engine shape is glowing on your dash. EPA says a steady light means a problem was detected and service is needed, not that you have to stop the car. You can clear it in under a minute with a cheap reader. The question is what you lose when you do.
Clearing the light does not repair anything. It erases what the car stored about the fault, and it can wipe the self test results an inspection station needs. Done at the wrong time, a reset can turn a car that might pass into one that fails. So this guide tells you how to reset it and when to leave it alone.
Steady or Flashing Decides Everything
| What You See | What to Do |
|---|---|
| Flashing | EPA says it means a rather severe misfire. Reduce speed and get service as soon as you can. CARB says it could be damaging your catalytic converter. Do not clear it. |
| Steady, and the car feels wrong | Read the code and get it looked at soon. CARB says driving a long time with a fault can harm other parts. |
| Steady, and the car feels normal | Check the gas cap, then read the code. This is the only case where a home reset may make sense. |
The Right Way to Clear It
- Tighten the gas cap and wait. EPA says a loose cap can trip the light, and that it should go out within a few days once tightened. You may need nothing else. Risk. Clearing right away can cover up a cap that is cracked or the wrong part.
- Find the port. It is usually under the dash near the driver's knees, and your manual shows where. Plug in the reader with the car off, then turn the key on as the reader directs. Risk. Forcing the plug can bend pins. It only goes in one way.
- Read and save the codes before anything else. Take a photo of the screen. Note any pending codes. Risk. Once you clear, the stored data is gone, and a mechanic has less to work with.
- Find and fix the cause. Look up what the code means, and treat it as a pointer, not a parts order. Risk. A code can name a sensor when the true cause is a leak or wiring. Swapping parts on a guess is how a small job grows.
- Use the erase function on the reader. Confirm when it asks. Risk. This also resets your readiness monitors. Do not do it within days of an inspection.
- Start the car and look at the dash. The light should stay off after the bulb check. Risk. If it comes straight back, the fault is still there. Clearing it again gains nothing.
- Drive normally for about a week. That figure comes from CARB, which says normal driving usually sets the monitors. Then plug in the reader and check readiness. Risk. Many short trips may not run every self test.
Why the Battery Trick Costs You More Than It Saves
You will see advice to pull the battery cable and wait. It may turn the light off. It also erases the same data a reader would, with no record of what the code was. You also lose the chance to save the code first, and it can wipe other memory on the car such as radio presets.
CARB says a dead, disconnected or replaced battery can leave readiness indicators incomplete. So you trade a light for a car that is not ready to test. On top of that, BAR says permanent codes cannot be erased by a scan tool or by disconnecting the battery. They clear only when the car has verified the defect is gone.
So the trick does not even hide a real fault from a modern test. In California, a permanent code check has been part of Smog Check since July 1, 2019, per BAR. A reader costs little and gives you the code. The battery trick gives you neither.
What This Does to an Emissions Test
Readiness monitors are the car's self tests. After a clear, they read incomplete until the car runs them again. CARB says this does not always mean something is wrong, but too many incomplete indicators will fail a Smog Check.
The limit depends on where you test. The federal OBD test rule allows two unset monitors on 1996 to 2000 models and one on 2001 and newer. It also fails any car whose light is commanded on. States write their own programs, so check yours.
| California Smog Check | Incomplete Monitors Allowed |
|---|---|
| Gas, 1996 to 1999 | Any one |
| Gas, 2000 and newer | Evaporative system only |
| Diesel, 1998 to 2006 | Zero |
| Diesel, 2007 and newer | Particulate filter and NMHC |
That table is from the BAR test reference. California also adopted a rule, effective October 1, 2025, that all readiness monitors must be set. BAR says current pass and fail criteria stay in place while it studies the data, and any change will be announced well ahead of the start date. Check before you test.
A note on permanent codes. BAR says Smog Check ignores one if the car has done at least 15 warm up cycles and 200 miles since its last clear. So the worst timing is a clear right before the test.
When the Light Goes Off by Itself
You may not need a reset at all. EPA says the system turns the light off once it has checked a part 3 times in a row with no problem. That fact sheet dates to May 1997, and newer cars add more checks, but the logic is the same in spirit. A fixed fault can clear with normal driving and leave your monitors set.
That makes patience the cheapest tool for a light that came on after a small fix. If it is still on after a normal week, read the code. For other home checks, see our DIY guides. If the fix turns into a shop visit, the repair price index shows how repair prices have moved.
When to Stop and Call a Mechanic
- The light is flashing. Slow down and get service now, since a severe misfire can damage the catalytic converter.
- The car runs rough, loses power, overheats or has a second warning light on.
- The light returns after you cleared it. The fault is still there, so do not keep clearing it.
- You have a code you cannot trace to a simple cause like the gas cap.
- Your monitors will not set after a normal week of driving, and an inspection is coming up.
- You failed a test on a permanent code or a stuck monitor. In California, BAR points to the Smog Check Referee for help.
Reader Mail
- How do I reset the check engine light?
- Plug in a code reader, save the code, fix the cause, then use the erase function. If the fault is truly gone, EPA says the system can also turn off the light by itself after 3 clean checks in a row.
- How many miles to drive after disconnecting battery for Smog Check?
- No fixed mile number holds for every car. The California Air Resources Board says to drive normally for about a week. Then have a reader show that your monitors are set before you go.
- Will disconnecting the battery reset the check engine light?
- It may turn off the light, but it does not fix the fault. CARB says a disconnected battery can leave readiness incomplete. BAR says permanent codes stay anyway.
- Can I drive with the check engine light on?
- EPA says a steady light means service is needed, not that you must stop the car. Still get service as soon as you can. A flashing light means slow down and get service right away.
- How long does it take for a code like P0420 to reset?
- There is no one number. EPA says the light turns off after the system checks that part 3 times in a row and finds no problem. Some checks run less often than others.
Record and Data Links
- Frequently Asked Questions About On Board Diagnostics, EPA 420-F-97-003, May 1997, U.S. Environmental Protection Agency
- On Board Diagnostic II Systems Fact Sheet, California Air Resources Board
- On Board Diagnostic Test Reference, California Bureau of Automotive Repair
- New OBD Readiness Monitor Regulations Explained, Fall 2025, California Bureau of Automotive Repair
- 40 CFR 85.2222, On Board Diagnostic Test Procedures, U.S. Environmental Protection Agency