OBDII Explained

OBDII stands for “On-Board Diagnostics, 2nd Generation.” The 1990 Federal Clean Air Act required that the OBD computer systems of every vehicle sold in the United States be standardized starting with 1996 light duty vehicles. The OBDII standards mandate a 16-pin diagnostic connection port with standardized communication that allows for a single connector to read the computer systems of every vehicle subject to the CAA mandate. The OBDII standard has grown as all vehicles sold in the United States are brought within compliance. The following timeline shows federal OBDII implementation for different vehicle classes: Below, we discuss the specific operation of the OBDII computer system and why a vehicle may fail an OBDII inspection:




How does OBDII work?

The OBDII computer monitors a vehicle’s emission control systems in real-time and is capable of informing a motorist or technician of a systemic issue the moment it occurs. The system operates through a series of indicator lights, drive cycles, trouble codes and readiness monitors. During an inspection, an emission analyzer scan tool plugs into the diagnostic connector that is attached to the OBDII computer and communicates with the vehicle. The OBDII computer relays to the scan tool whether it has discovered errors in the emission control systems. The emission analyzer then determines whether the vehicle is being operated in compliance with emission standards.

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Reasons for Failing an OBDII Inspection

The most common reasons for a vehicle to fail an OBDII inspection are: OBDII diagnostic connector is missing, damaged or inaccessible OBDII communication fails MIL remains illuminated while the engine is running OBDII monitors are not ready.

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Malfunction Indicator Light (MIL)

If a problem is detected, the MIL will illuminate on the dashboard as a variation of one of these symbols:



If the MIL illuminates and remains lit, the OBDII computer has detected an issue in an emission control system. These issues can range in severity, and it is recommended that the motorist immediately assess the reason for the MIL being on. Common reasons for the MIL to illuminate are:
  • Malfunctioning component(s) that regulate fuel/air ratio (e.g. oxygen sensors)
  • Issues with the Exhaust Gas Re-Circulation (EGR) valves
  • Dirty air filter
  • Engine misfire (e.g. spark plugs)
  • Leaks in the vacuum system
  • Catalytic Converter error or failure
  • Issues with evaporative control(s), such as a poor-fitting gas cap





If the MIL illuminates, it is a possible indication of a very serious engine issue.

Do not ignore this warning!
Ignoring the MIL may cause further engine damage, thousands of dollars in repair costs and may void any warranty you hold on the vehicle. Auto manufacturers can identify when an MIL illuminated and will know if the issue was addressed. To save a potentially minor issue from becoming a significant one, perform the following if the MIL illuminates:

1. Pull your vehicle over to the side of the road at the soonest and safest possible moment (it is recommended that you exit any highway you may be on, if it is safe to do so);
2. Turn off your engine;
3. Check your gas cap to make sure it is properly secure (twist it until it clicks);
4. If the gas cap was already secure, take your vehicle to an automotive professional immediately;
5. If the gas cap was not secure, you are still encouraged to take your vehicle to an automotive professional and have a diagnostic test run to ensure the issue is resolved.

If your vehicle is under warranty, it is recommended that you call your dealership or warrantor prior to restarting your engine. Doing so may absolve you of responsibility if the issue is covered by warranty.

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Flashing MIL – STOP YOUR VEHICLE IMMEDIATELY!

If your MIL 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.
STOP DRIVING THE VEHICLE IMMEDIATELY!
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.

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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.

TheThe MIL de-illuminating does not necessarily mean the issue has gone away! It may mean the vehicle has not completed a drive cycle and may 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 battery 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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