Chapter 9 Strategies of On-Board Diagnostic Continuous Monitors

Circuit Checks

Inspect input/output systems for circuit continuity (opens and shorts) and out-of-range values as preset in the system operating parameters.

Functional Tests

Observes the reaction of an output device to a command from the PCM.

Fuel System Monitor

The software in an OBD II system that monitors fuel system operation.

Long Term Fuel Trim (LTFT)

Designed to modify base fuel calculation, the PCM makes a long-term adjustment to the short-term-fuel-trim (STFT) to maintain the oxygen sensor or air-fuel ratio sensor in the stoichiometric range. Also called block learn.

Rationality Tests

Check to see if the input values from a sensor make sense based on the current operating conditions at the time the test is performed.

Short Term Fuel Trim (STFT)

The PCM immediate response to maintain the fuel delivery system in the stoichiometric range.

Type A Misfire

Can cause immediate damage to the catalytic converter and will flash the MIL.

Type B Misfire

Will cause the emissions to rise above one and one half times the design limits and will turn the MIL on steady.

Type C Misfire

Could cause an inspection/maintenance (I/M) failure but will not damage the catalyst or cause emissions to exceed one and one half times the FTP and will turn the MIL on steady.