OBDII Explained


If your MIL light is flashing, the OBDII computer has identified that the engine is seriously misfiring. This condition may cause the catalytic converter to overheat and catch on fire.
Pull over to the side of the road at the soonest and safest possible moment, turn off your engine and not restart your vehicle. Have it towed to an automotive professional for assessment. This could save you thousands of dollars and may save your vehicle!

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Drive Cycles

MIL illumination and de-illumination is based on drive cycles. Every manufacturer pre-programs a required drive sequence into their OBDII computers. During that drive sequence, the OBDII computer assesses the emission control systems and performs a number of self-diagnostic tests. The sequence involves a series of engine ignitions and kills, certain mileage requirements and completed tests for each emission control component – a drive cycle. If a drive cycle is completed and no emission control issues are discovered, the MIL will remain de-illuminated. If an emission control issue is found, the MIL will illuminate.

Drive cycles are specific to each manufacturer and may take between 3 and 7 days to complete, depending on individual driving habits and vehicles. Back to top

Diagnostic Trouble Codes (DTCs)

Once the MIL illuminates, the OBDII system will automatically store a DTC that indicates the reason the MIL was illuminated. When a diagnostic test is run on the car, the scan tool used will read the stored diagnostic code(s) and provide the reason for the MIL illumination. This helps the technician identify what is wrong with the vehicle and provides a guide for what repairs may need to be performed.

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What if the MIL turns off?

The MIL may de-illuminate for two reasons:
  • The battery is disconnected from the engine; or
  • The OBDII system automatically turns it off.
If the battery is disconnected from the engine (often done during repairs) the OBDII system will reset. This will de-illuminate the MIL, erase all DTCs and unset all readiness monitors. If the OBDII system automatically de-illuminates the MIL, then the vehicle has completed an entire drive cycle without an issue being detected. An example of this is if the gas cap had been loose and was properly re-fitted, after a complete drive cycle, the OBDII computer may de-illuminate the MIL if the gas cap evaporative controls return to normal. If the MIL does de-illuminate automatically, pay close attention to it for the next week to ensure it does not illuminate again.

The MIL de-illuminating does not necessarily mean the issue has gone away! It may mean the vehicle has not completed a drive cycle and my yet detect a problem.

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Readiness Monitors

You may be asking yourself, “So, I can disconnect my battery and pass my emissions inspection?” No. Readiness monitors prevent this from being possible. When the MIL is reset and the DTCs are erased (often after the batter has been disconnected), readiness monitors are also reset. Readiness monitors are emission control components that measure whether each, individual emission control system has been properly tested. If the required self-diagnostic tests and drive sequences have not been completed after an MIL is de-illuminated, the readiness monitors will remain unset (not ready) until the drive cycle is completed. If a car is brought in for emissions testing and is not ready, it may be denied an inspection until the required readiness monitors are set (ready). This will only waste yours and the technician’s time, and you will have to return to have your vehicle inspected after a drive cycle is completed and all required readiness monitors are ready. Drive cycles are specific to each manufacturer and may take between 3 and 7 days to complete, depending on individual driving habits and vehicles.

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