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 automotive technician of a systemic issue the moment it occurs. The system provides information through a series of indicator lights, drive cycles, trouble codes, and readiness monitors. During an inspection, an emission analyzer scan tool is plugged into the vehicle's diagnostic connector (attached to the OBDII computer) and communicates with the vehicle. The vehicle's OBDII computer notifies the analyzer whether it has discovered errors in the emission control systems. The analyzer then determines whether the vehicle is functioning in compliance with emission standards.
About the OBD Inspection
Your vehicle's emissions inspection will check correct operation of its pollution controls by evaluating several aspects:
- Check Engine Light
- Ability to Connect
- Ability to Communicate
- Computer Readiness
- Drive Cycle Status and
- Diagnostic Trouble Codes.
Each area is discussed further below.
Check Engine Light
Your vehicle's dashboard gauges include a Malfunction Indicator Light (MIL), commonly known as the Check Engine Light. During your emissions inspection, the vehicle inspector will check the MIL illumination status both visually and electronically. The inspector will also evaluate whether the light is correctly operating.
Correct MIL operation is crucial to safe operation of your vehicle. When you are driving your vehicle, if the vehicle's OBD computer detects a problem, the light will illuminate on the dashboard as a variation of one of these symbols:
If the light illuminates and remains lit, the OBDII computer has detected an issue in an engine control system. These issues can range in severity, and it is recommended that a motorist have their vehicle inspected by a mechanic as soon as possible to determine the reason for the MIL being on. Common emissions-related reasons for illumination include:
- 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 issue.
Do not ignore this warning!
Your engine may be damaged. 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 MIL illumination history, and will know if the issue was addressed.
If your Check Engine Light is FLASHING — It is recommended to safely STOP YOUR VEHICLE IMMEDIATELY!
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 do 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!
Ability to Connect
To start the procedure, the inspector connects a communication cable from the emissions analyzer to the OBDII Diagnostic Link Connector located in your vehicle.
In order to correctly perform the emissions inspection, your vehicle's Diagnostic Link Connector must be operable. If the connector is missing, damaged, or inaccessible, your vehicle will fail its emissions inspection.
Ability to Communicate
Once connected, the emissions analyzer will attempt to communicate with the vehicle's OBD computer. Communications must succeed in order to pass the emissions inspection.
The computer in your vehicle performs diagnostic tests internally on certain emissions systems while you are driving. The vehicle computer monitors these automatic tests and tracks each completed diagnostic monitor's status as Ready or Complete. If a pending problem is detected or any automatic test could not complete, that system's monitor will no longer be Ready or Complete. Sometimes system monitors become Not Ready if the vehicle's battery has been disconnected or if the vehicle computer's memory has been erased. As part of the OBD-II test, the analyzer checks to see how many of these system monitors are "not ready". If too many monitors are Not Ready, then the vehicle will fail the OBD-II test.
Drive Cycle Status
Maintaining your Vehicle's Readiness Status — Ensure you are ready for testing!
MIL illumination and de-illumination is based on drive cycles. Every vehicle manufacturer pre-programs a required drive sequence into its 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.
When readiness monitors are not complete — for example, after the battery is discharged or disconnected, the vehicle must operate for some time to complete the drive cycles before its computer can complete its internal diagnostic tests. Some vehicles require extensive driving to reset certain system readiness monitors. Do not disconnect the battery on your vehicle. If you take your vehicle in for service before your emissions test, ask your technician not to clear the computer's memory, since this will only unset the system readiness monitors and result in your vehicle failing the OBD-II test.
Diagnostic Trouble Codes
In cases where your vehicle's MIL illuminates upon detecting a diagnostic problem, the OBDII system will automatically store a Diagnostic Trouble Code (DTC) identifying the reason the MIL was illuminated. When a diagnostic test is run on the car, the analyzer's OBD scan tool will read the stored DTC(s) and indicate 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.