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Have you ever wondered if there is a way to know if a seller cleared the check engine light on a car? Or maybe the mechanic cleared the check engine light without fixing the problem? This article will show you how to use an OBD2 scanner, just like the YOUCANIC Full System Scanner, to determine if the check engine light was recently reset.
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The check engine light (CEL) is an important indicator of potential problems with your vehicle’s engine or emission control system. If the light is illuminated, it is important to have your vehicle inspected as soon as possible. However, in some cases, a mechanic may erase the CEL to clear the code and turn off the light, making it difficult to know if the problem has been resolved. To determine if the check engine light has been erased, there are a few ways you can check. One method is to retrieve the vehicle’s diagnostic trouble codes (DTCs) using an OBD-II scan tool. Another way is to pay attention to the CEL cycle – the light should illuminate briefly when the ignition is turned on, then turn off if no codes are stored in the system. If the light stays on after the initial cycle, it indicates the presence of a trouble code.
How do people reset the check engine light?
First, it is important to understand how the check engine light is reset. Check engine light can be reset by clearing the codes with an OBD2 scanner or disconnecting the battery. Both methods temporarily clear all the Diagnostic Trouble Codes (DTCs) from the On-Board Diangntoic computer. This procedure also reset the various systems in the car to the Not Ready state. Depending on the problem, the check engine light could return immediately or take up to two weeks to turn on.
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You can not pass a state emissions test by clearing the codes this way.
How to tell if the Check Engine Light was reset
To find out if the check engine light was reset, you will need an OBD2 code reader. The UCAN-I Scanner can read check engine lights, and the YOUCANIC Full System Scanner is a professional scanner that can run through all the control modules of your vehicle.
Make sure the scanner you buy can display I/M Monitor Readiness Status; the YOUCANIC Full System Scanner can show your vehicle’s emission readiness status and every control module.
- Turn the key to the ON position. Do not start the car.
- Locate the OBD-II port under the dashboard and plug in the OBD2 scanner.
- The scanner should power on once connected to the OBDII port. Press Read or Enter on the scanner, depending on your scanner model.
- Please scroll down to the I/M Readiness menu and select it.
- Look at all the systems. If most say NOT READY, the check engine light was recently reset. They will say one of the following.
- Ready / OK-On-Board Diagnostic has completed monitoring this system and passed the test.
- Not Ready / Pending – This system’s monitoring is not yet completed. It would be best if you continued driving the vehicle.
- Fail –OBDII monitored this system, but it failed to pass. Typically, a Diagnostic Trouble Code (DTC) is set, and the check engine light has or will turn on soon.
- A car that has not had the ECU codes reset will have all the systems as Completed / Pass / OK and maybe a few Not Applicable if certain systems are not installed.
- If the codes were recently reset, you would notice that several emissions monitoring systems will show as Completed while others will be Not Ready or Pending (or maybe even Fail).
We hope you find the How To Find Out If Check Engine Light Was Erased by Seller guide helpful. Check thesetroubleshooting and repair guidesfor more help on your vehicle.
Can a mechanic reset the check engine light and clear codes without fixing the problem?
To know if a mechanic cleared the check engine light without fixing the problem, you can do the following:
- Check the vehicle’s maintenance records: The mechanic should have documented the check engine light issue and any repairs they made in the vehicle’s maintenance records.
- Monitor the check engine light: If the problem isn’t fixed, the check engine light is likely to turn back on. Keep an eye on it and see if it illuminates again.
- Retrieve the trouble codes: You can use an OBD-II scan tool to retrieve the vehicle’s diagnostic trouble codes. This will give you a clear picture of what the problem is and whether it was fixed or not.
- Test drive the vehicle: Take the vehicle for a test drive and pay attention to any symptoms or warning signs that the problem still exists, such as rough idling, reduced power, or unusual noises.
- Get a second opinion: If you’re still unsure if the problem was fixed, you can take your vehicle to a different mechanic for a second opinion. They can diagnose the issue and confirm whether the check engine light was erased without fixing the problem.
If the mechanic did clear the check engine light without fixing the problem, it’s important to address the issue promptly to prevent further damage to the vehicle.
Frequently Asked Questions
How long do the check engine codes take to show up again?
Once someone clears the ECU (DTC) codes, the check engine light may come up within minutes or after one week. The reason is that the ECU performs several start-up tests during the next few driving cycles. The light will turn on within minutes if there is a problem with a critical sensor, such as a Mass Air Flow sensor. If the problem is with the EVAP system, the light may come on only after the car performs the necessary tests, which require several driving cycles. If there is a problem with the transmission, the light will come up again when it shows the symptoms again, such as the transmission slips.
How to perform the OBD2 driving cycle?
During a driving cycle, onboard diagnostics will perform all systems testing, but you must drive the car following an exact pattern. Driving cycle instructions could vary depending on the make. Here are some general instructions on how to go through a driving cycle.
Before you start the cycle, you need to start with a cold engine. Turn off all electronic consumers such as A/C, radio, and unnecessary lights.
Start the engine and let it idle for 150 seconds. During this time, OBDII will check oxygen sensor circuits, EVAP purge, and air pump.
Accelerate to 55 mph and drive at this speed for three minutes. The onboard diagnostic will continue monitoring oxygen sensors and EGR and canister purge.
Slowly decelerateto 20 mph without pressing the brake pedal. Coast down to 20 mph. OBDII will check for cylinder misfires and fuel delivery.
Accelerate to 60 mph and drive at this speed for five minutes.
Remove your foot from the gas and decelerate to a complete stop without pressing the brake pedal.
Could there be any problems with the car if the check engine light is off?
Just because the check engine light (also called CEL/MIL) is not on doesn’t necessarily mean there aren’t any issues. It could be that the light was recently reset or the trouble codes are pending. Pending trouble codes means the vehicle has detected something wrong, but it needs to monitor that system a little longer before it triggers the check engine light.
Written and fact-checked by:
YOUCANIC Techs YOUCANIC articles are written by authors who collectively have over 60 years of experience in the automotive field and years of hands-on experience on vehicles. These authors are certified ASE G1 Automotive Maintenance engine professionals (the highest level certification for vehicle maintenance and repair). They hold associate’s and bachelor’s degrees, as well as having gone on to further study to receive a master’s and doctorate degrees. These authors collaborate on research, writing, editing, and fact-checking every article to ensure that it is accurate and up to date and delivers the best counsel and information to YOUCANIC readers. View all posts
