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 many different circuits of fuel injection system. This monitoring is called "on-board diagnosis". Certain criteria, or "warming conditions", must be met for a trouble code to be entered into PCM memory. The criteria may be a range of: engine rpm, engine temperature, and/or input voltage to PCM. If a problem is sensed with a monitored circuit, and all of criteria or arming conditions are met, a trouble code will be stored in PCM.
It is possible that a trouble code for a monitored circuit may not be entered into PCM memory even though a malfunction has occurred. This may happen because one of trouble code criteria (arming conditions) has not been met.
The PCM compares input signal voltages from each input device with specifications (the established high and low limits of range) that are programmed into it for that device. If input voltage is not within specifications and other trouble code criteria (arming conditions) are met, a trouble code will be stored in PCM memory.
The On Board Diagnostics have evolved to second Generation of Diagnostics referred to as OBDII. These OBDII Diagnostics control functions necessary to meet requirements of California OBDII and Federal OBD regulations. These requirements specify inclusion of a Malfunction Indicator Light (MIL) located on instrument panel for all 1994 and subsequent model-year passenger cars, light duty trucks, and medium-duty vehicles. The purpose of MIL is to inform vehicle operator in event of malfunction of any emission systems and components which can affect emissions and which provide input to, or receive output from, engine controller module.
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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.