Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
On-Board Diagnostics
The PCM has been programmed to monitor any circuit or system that has an effect on vehicle emissions, or is used by the PCM to determine the proper functionality of these systems. This monitoring is called "on-board diagnosis."
Certain criteria or, "arming conditions", must be met before a trouble code will be entered into the PCM memory. The criteria range from engine RPM, engine temperature, and/or input voltage to the PCM. If a problem is detected with a monitored circuit, and all of the criteria or arming conditions are met, a trouble code will be stored in the PCM.
It is possible that a trouble code for a monitored circuit may not be entered into the PCM/ECM memory even though a malfunction has occurred. This may happen because one of the trouble code criteria (arming conditions) has not been met.
The PCM compares input signal voltage from each input component to specifications (the established high and low limits of the range) that are preprogrammed for that component. If the input voltage is not within specifications, and other trouble code criteria (arming conditions) are met, a trouble code will store in the PCM memory.
The On Board Diagnostics have evolved to the second Generation of Diagnostics referred to as OBDII. These OBDII Diagnostics control the functions necessary to meet the requirements of California OBDII and Federal OBD regulations. These requirements specify the inclusion of a Malfunction Indicator Light (MIL) located on the instrument panel for all 1994 and subsequent model-year passenger cars, light duty trucks, and medium-duty vehicles. The purpose of the MIL is to inform the vehicle operator in the event of the malfunction of any emission system or component failures that can affect emissions and which provide input to, or receive output from, the PCM.
MIL Lamp Strategy
I/M Readiness OK to test = Key On Engine OFF - MIL Lamp will remain on until the vehicle is started or Ignition is turned off.
I/M not ready for testing = Key On Engine OFF - MIL Lamp on solid for (15) seconds then MIL Lamp will flash on/off for (5) seconds then it will remain on until the vehicle is started or the Ignition is turned off.
Use the Manual With the Right Hardware
Pair factory procedures with proven DIY tools so the instructions are easier to execute.
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.