Have you ever wondered if there is a way to know if someone cleared the codes on a car? Unfortunately, a seller may reset the check engine light and the codes so that they can sell a car that has underlying problems. In this article, we will show you how you can use an inexpensive OBD-II scanner such as MaxiScan MS300 to tell if some erased the codes.
How to tell if the Check Engine Light was reset
For this test, you will need a basic OBD2 code reader or a Bluetooth adapter and Torque app. There are many OBD2 scanners, some of which sell for under $15 that monitor I/M readiness status. We would recommend taking a look at the following.
Make sure that the scanner that you buy is capable of displaying I/M Monitor Readiness Status. One scanner that has great reviews is on the best selling list and is capable of showing emission status is Autel AutoLink AL319 OBD II CAN Scan Tool. You could also use an ELM327 Bluetooth adapter and a free app for iPhone or Android to check the I/M Monitor Readiness Status.
Instructions: How to check if the Check Engine Light was recently reset
Turn the key to 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 the scanner model that you have.
Scroll down to I/M Readiness menu and select it.
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 - 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.
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).
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 a Mass Air Flow sensor, the light is going to 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 the transmission shows up the symptoms again, such as the next time the transmission slips.
During a driving cycle, the vehicle on-board diagnostics will perform testing of all systems, 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.
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. On board diagnostic will continue to monitor oxygen sensors but also EGR and canister purge.
Slowly decelerate to 20 mph without pressing the brake pedal. Simply coast down to 20 mph. OBDII will check for cylinder misfire 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.
Just because the check engine light (also referred to as CEL/MIL) is not on, doesn't necessarily mean that 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.