What is OBDII

The OBDII diagnostic system is the next step in self testing protocols for cars and trucks. This protocol applies for vehicles from 1996 onward, and it is meant to diagnose and identify problems with your powertrain. This is done through your Powertrain Control Module (PCM) otherwise known as the vehicle’s ECU. This computer controls every aspect of your engine, from your fuel curves and ignition events to your diagnostic OBDII interface.

The software was rewritten to enable the PCM to carry out the responsibilities to meet these required guidelines. The PCM now contains a Task Manager. The Powertrain Control Module (PCM) monitors many different circuits in the fuel injection, ignition, emission and engine systems. If the PCM senses a problem with a monitored circuit often enough to
indicate an actual problem, it stores a Diagnostic Trouble Code (DTC) in the PCM’s memory.


If the code applies to a non-emissions related component or system, and the problem is repaired or ceases to exist, the PCM cancels the code after 40 warmup cycles. Diagnostic trouble codes that affect vehicle emissions illuminate the Malfunction Indicator Lamp (MIL). Refer to Malfunction Indicator Lamp in this section.

Certain criteria must be met before the PCM stores a DTC in memory. The criteria may be a specific range of engine RPM, engine temperature, and/or input voltage to the PCM.
The PCM might not store a OBDII code for a monitored circuit even though a malfunction has occurred. This may happen because one of the DTC criteria for the circuit has not been met. For example, assume the diagnostic trouble code criteria requires the PCM to monitor the circuit only when the engine operates between 750 and 2000 RPM.

OBDII Sensors and how they work

Suppose the sensor’s output circuit shorts to ground when engine operates above 2400 RPM (resulting in 0 volt input to the PCM). Because the condition happens at an engine speed above the maximum threshold (2000 rpm), the PCM will not store a DTC. There are several operating conditions for which the PCM monitors and sets DTC’s. Refer to Monitored Systems, Components, and Non-Monitored Circuits in this section.

NOTE: Various diagnostic procedures may actually cause a diagnostic monitor to set a DTC. For instance, pulling a spark plug wire to perform a spark test may set the misfire code. When a repair is completed and verified, use the DRB III scan tool to erase all DTC’s and extinguish the MIL. Technicians can display stored OBDII codes by using the
DRB III scan tool. Refer to Diagnostic Trouble Codes in this section. For DTC information, refer to charts in this section.

The PCM is responsible for efficiently coordinating the operation of all the emissions-related components. The PCM is also responsible for determining if the diagnostic systems are operating properly. The software designed to carry out these responsibilities is call the ’Task Manager’

The Task Manager protocol requires that vehicles falling under OBDII guidelines utilize several different system monitors. These monitors are a vital part of your vehicle’s operation, and are listed below.

  • Comprehensive Component Monitor (inputs/outputs for powertrain management that affect emissions, but do not have a specific major monitor)
  • Fuel Control Monitor (fuel compensation required to maintain stoichiometric ratio rich/lean)
  • Misfire Monitor (change in crankshaft speed)
  • Oxygen Sensor Heater Monitor (response and performance of oxygen sensors)
  • Catalyst Monitor (Performance and efficiency of catalyst)
  • Evaporative Emissions Monitor (performance of and leaks from EVAP system)
  • Exhaust Gas Recirculation Monitor (flow performance of EGR system)


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