Should You Clear Check Engine Codes Yourself?

Yes, you can clear check engine codes yourself, and it is fine to do so after you have fixed the cause or confirmed a one-off event like a loose fuel cap. Don't clear a code just to make the light go away: you lose diagnostic data, reset your emissions readiness, and the fault will come back anyway.

What actually happens when you clear codes

When you send a "clear codes" command through the OBD-II port (or, on many cars, disconnect the battery), the PCM does more than turn off the light. It typically:

• Erases stored and pending trouble codes.
• Erases freeze frame data, the snapshot of engine speed, load, temperature, and other conditions recorded at the moment the fault set. That snapshot is often the most useful clue a technician has.
• Resets the readiness monitors to "not ready." These are the self-tests for the catalytic converter, EVAP system, oxygen sensors, and more. They only complete again after the car is driven under the right conditions.
• Resets some learned values such as fuel trims on many vehicles, so the car may idle or shift slightly differently for a while as it relearns.

What it does not do is fix anything. If the fault is still present, the PCM will detect it again and the light will return, sometimes on the next drive, sometimes days later.

Permanent codes are a special case

Vehicles from roughly the 2010 model year onward support "permanent" diagnostic trouble codes. A permanent code can't be erased with a scan tool or by disconnecting the battery. It clears itself only after the PCM runs the relevant test and sees that the problem is gone. This exists specifically so a car cannot pass an emissions test just because someone cleared the codes in the parking lot.

When clearing codes is a good idea

The right way to clear a code

1. Read every code first, including pending ones. A single code is a clue; a group of codes often tells a different story. Our guide to reading Mustang trouble codes with your iPhone walks through the process.
2. Record the details. Write down the code, its description, the date, mileage, and what the car was doing when the light came on. In the My Mustang app, you can read the code, then save a service log entry with notes and a photo so the information is still there after you clear it.
3. Fix or address the cause, or at least confirm it is a known one-off event.
4. Clear the code with the engine off and ignition on, or as your scan tool instructs.
5. Drive normally for several days, with a mix of city and highway driving and at least one cold start each day if you can.
6. Check again. If the code returns, the repair didn't address the root cause. Your notes from step 2 now become very useful.

Why "clear and see if it comes back" isn't a diagnosis

Clearing a code to see whether it returns can be a reasonable step once you have recorded the data. The problem is when it becomes a habit. Some faults are intermittent and take a week or more to reappear, so a few quiet days don't prove anything. Meanwhile, a slowly worsening problem, like a lean condition or a weak ignition coil, can do more damage the longer it is ignored. Repeated clearing also means every technician who looks at the car later starts from scratch.

Clearing codes and emissions inspections

In states with OBD-II-based inspections, the station plugs into the same port your scan tool uses. It checks whether the light is commanded on and whether the readiness monitors have completed. If you clear codes or disconnect the battery shortly before a test, you will likely see several monitors marked incomplete. Many states allow a small number of incomplete monitors depending on the model year, but that varies, so check your state's rules. The practical advice is simple: repair the fault, clear the code, then drive normally for a week or so before testing.

Some monitors, especially EVAP, need specific conditions such as a cold soak and a fuel level in a middle range. If you have been chasing an EVAP leak code, expect that monitor to take the longest to complete.

Is disconnecting the battery the same thing?

On many vehicles, disconnecting the battery long enough clears codes and resets learned values, with the same downsides as a scan tool clear, plus a few more: you may lose radio presets and other settings, and some systems may need a relearn after the battery is reconnected. A scan tool is the cleaner method, and it lets you read the code before you erase it.

What you need

All you need is an OBD-II scan tool that supports reading and clearing codes. On an iPhone, that means a Bluetooth LE adapter, since iPhones don't connect to classic Bluetooth OBD adapters the way Android phones can. See choosing a Bluetooth OBD-II adapter that works with iPhone before you buy one.

Frequently asked questions

Is it illegal to clear check engine codes?

Clearing codes on your own car is not illegal in itself. What inspection programs prohibit is tampering with emissions equipment or misrepresenting a vehicle's condition, which is why readiness monitors and permanent codes exist.

Will clearing codes void my warranty?

Clearing a code does not void a warranty. It can make diagnosis harder for the dealer, though, so if your Mustang is under warranty and the cause is unknown, it is usually better to leave the code stored until they have seen it.

How long until readiness monitors are ready again?

It depends on the monitor and your driving. Some complete in a single drive, while others can take several days of mixed driving with cold starts. There is no single number that applies to every car.

Why did the light come back right after I cleared it?

The fault is still present and the PCM detected it again on its next test. That is a sign the cause needs repair, not another clear.