How to Turn Off The Check Engine Light, Service Engine Soon or Malfunction Indicator Lamp

If you're performing this procedure in an effort to pass an emissions test when the check engine light or service engine soon light is on or illuminated, read on and in detail to learn how the check engine light reset procedure works.

You may turn off the check engine light using a scan tool or by disconnecting your car's, truck's, van's or SUV's battery.

You may have already read information describing how to turn off the check engine light or service engine soon light prior to the emissions test in order to pass the smog inspection. We'd like vehicle owners to understand that by turning off the check engine or service engine soon light, essentially deleting all emissions data from your vehicle's engine control module (ECM), the smog machine will not know your vehicle has had or currently has any emissions faults AND all Emission Monitors will be erased as well. Without "completed" emission monitors a 1996 and newer vehicle has no chance of passing the smog check.

We will describe the reset or "turn light off" procedure so you may understand the OBDII emissions testing process and how the check engine light or service engine soon light apply to the emissions test. Perform these procedures at your own risk.

Using a Trouble Code Scan Tool

The simplest and safest way to turn off the check engine light is by using an OBD II Trouble Code Scan Tool. You may buy a scan tool at your local auto parts store. It can be used to retrieve stored trouble code(s), clear trouble code(s) and turn off the check engine light, read live streaming engine data (on some models). The OBDII Scan Tool method of turning off your vehicle's check engine light only applies to 1996 & newer model vehicles.

If you've already scanned your vehicle's OBD-II computer and retrieved stored trouble codes, you may want to lookup the trouble codes before you delete them. We highly recommend you keep a detailed record of code(s), including freeze frame data the ECU may have stored while setting them. You will need this information should you need to repair your vehicle. Freeze Frame Data is sensor and actuator measurements, in the form of voltage and/or frequency, which the ECU recorded at the moment it detected a problem with your vehicle's emissions system; and triggered the check engine light or MIL on. Note: Most simple scan tools do not have the ability to display freeze frame data even though your vehicle may have collected this data and stored it.

Using The Battery Disconnect Method

The battery disconnect method will work on most vehicles. Some late model vehicles will not clear trouble codes by disconnecting the battery. On these vehicles trouble codes are stored in the ECM's ROM, and only a scan tool will clear trouble code(s) and turn off the check engine or malfunction indicator lamp (MIL).

Instructions on How to Turn Off The Check Engine Light:

Step 1. If not using a scan tool, locate your vehicle's battery. Disconnect the 12v negative cable (black wire) from battery terminal (-). Leave it disconnect for 5 minutes. While the battery is disconnected, cycle the ignition switch to ON position 3 times. If using a scan tool to turn off the check engine light, follow manufacturers directions. You will need to find your vehicle's OBD II Data Link Connecter (DLC) and connect your scan tool to the DLC, then follow the steps indicated on your scan tool or scan tool manual.

  • You've just erased important emissions related data which is invaluable to a smog technician attempting to repair your vehicle.
  • You also deleted very important "readiness flag" data which takes days, if not weeks to compile.

Step 2. Ensure ignition switch is OFF. Reconnect the battery 12v negative cable (black wire) to battery terminal (-).

Step 3. Cycle ignition switch to ON position and wait 1 minute, then start your vehicle. Check Engine Light or Service Engine Soon Light should be reset and turned OFF.

  • If you are successful in turning off the check engine light, this means your vehicle has not stored it's emissions data in the ROM section of it's computer and you have deleted all data pertaining to your vehicle's engine and emissions systems, which may include security, radio & system settings.

Step 4. Drive your vehicle for one week under normal driving conditions. During this period the emissions computer is gathering data and re-learning the condition of your vehicle's emissions components and systems. The vehicle is undergoing what is referred to as a Drive Cycle. The emissions computer OBD-II (on-board diagnostics) system must complete at least one drive cycle.

A drive cycle is a sequence of internal tests which the emissions computer runs while your vehicle is being driven. This ensures all emissions systems are functioning properly. Proper "readiness flags" are set as the computer completes it's cycles. Test cycles are unique to a vehicle. Certain cycles run under very strict parameters, and may require extended driving time to trigger on. Cycle data and readiness flag information is available through your dealership's service department. The data vary widely.

  • During your drive, maybe even as soon as you start the engine, your vehicle realizes an emissions fault, and turns on the Check Engine or Service Engine Soon light.
  • Emissions computer refuses to set a required readiness flag. No check engine light, but no chance of passing the emissions test either. However, you will not know this until your vehicle is smog tested. If the particular emissions test center doesn't offer a free re-test, you forfeit your inspection fee.

Step 5. Take the smog test. Important Note! Your vehicle might not be completely ready for the smog check inspection. Required parameters may have not been set by the emissions computer.

  • Your vehicle might not be ready for the emissions test. There will be no way of knowing this until the smog test is complete. Your normal driving pattern (if not for an extended length of time) may have failed to trigger the emissions computer tests needed to set the required readiness flags.

Passing the Emissions Test After Turning Off the Check Engine Light, Malfunction Indicator Lamp, or Service Engine Soon Lamp

What are my chances of passing the emissions test after resetting the emissions computer and turning off the service engine soon light or check engine light?

A slight chance of passing the emissions test after a reset does exist, but it is very slim. The trick is to get your vehicle smog checked before your engine computer detects the check engine light illuminating trouble code, and only after the required readiness flags have set. The chances of the engine computer setting the required readiness flags before detecting the trouble code and illuminating the check engine light are slim to none. And so our opinion... time is better spent finding a reputable smog check repair station to diagnose the check engine or service engine soon light, then time spent trying to avoid detection and/or repairs.

Even after evading check engine trouble detection, passing the emissions test will require your vehicle's exhaust sample (1999 & older vehicles only), which will be collected by the smog machine, fall within California Air Resource Board specifications, and all required emissions components be functioning properly.

Passing the entire smog test requires your vehicle pass the visualfunctional, and emissions portions of the test. One may be able to hide trouble code information by deleting data, but the vehicle's exhaust will not be able to hide high emissions.

Important Note: California law allows only a vehicle's registered owner or a State certified smog repair station conduct emissions related repairs. We recommend you ensure the auto repair shop you visit is smog check certified. Smog check investigators will not get involved with faulty emissions repairs performed at non-emissions certified auto repair shops. Nor will they reimburse your costs if you end up applying for the CAP assistance program.


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