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How To Find Out If Check Engine Light Was Erased by Seller

By YOUCANIC Staff, 06 December, 2019

Have you ever wondered if there is a way to know if someone cleared the check engine light on a car?

In this article, we will show you how to use an OBD2 scanner to find out if the check engine light was recently reset. 

How do people reset the check engine light? 

check engine light was reset

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 clear all the Diagnostic Trouble Codes (DTCs) from the On-Board Diangntoic computer at least temporarily. This procedure also reset the various systems in the car to Not Ready state.  Depending on the problem, the check engine light could come back right away or could take up to two weeks to turn on. 

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 a cheap OBD2 code reader. A Bluetooth ELM327 OBD2 adapter combined with a free app (Torque, Dash Command, Free OBD2, etc.) can be used. 

Make sure that the scanner that you buy is capable of displaying I/M Monitor Readiness Status. One scanner with great reviews is on the best selling list and can show emission status is Autel AutoLink AL319 OBD II CAN Scan Tool.    

  1. Turn the key to the ON position. Do not start the car. 
  2. Locate the OBD-II port under the dashboard and plug in the OBD2 scanner. 
  3. The scanner should power on once connected to the OBDII port. Press Read or Enter on the scanner, depending on the scanner model that you have.  
  4. Please scroll down to the I/M Readiness menu and select it. 
  5. Look at all the systems. If most of them say NOT READY, it means 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 it has passed the test. 
    • Not Ready / Pending - The monitoring of this system is not completed yet. You need to continue 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. 

know if check engine light was reset im emissions status

  •  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). 

Frequently Asked Questions

How long does it take for the check engine codes to show up again?

Once someone clears the ECU (DTC) codes, the check engine light may come up within minutes, or it can come up after one week. The reason is that the ECU performs several tests on start-up and during the next few driving cycles. If there is a problem with a critical sensor, such as a Mass Air Flow sensor, the light will turn on within minutes. 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, the vehicle onboard diagnostics will perform all systems testing, but it requires that you 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. 
  1. 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. 
  2. Start the engine and let it idle for 150 seconds. During this time, OBDII will check oxygen sensor circuits, EVAP purge, and air pump.
  3. Accelerate to 55 mph and drive at this speed for three minutes. The onboard diagnostic will continue to monitor oxygen sensors but also EGR and canister purge. 
  4. Slowly decelerate to 20 mph without pressing the brake pedal. Coast down to 20 mph. OBDII will check for cylinder misfire and fuel delivery. 
  5. Accelerate to 60 mph and drive at this speed for five minutes.
  6. 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 referred to as CEL/MIL) is not on doesn't necessarily mean there aren't any issues. It could be that the light was recently reset or that the trouble codes are pending. Pending trouble codes mean that the vehicle has detected something wrong, but it needs to monitor that system a little longer before it triggers the check engine light.