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Overview of vehicle diagnostics
All modern cars have hundreds of sensors built-in. They receive and send information to several on-board computers, which then analyse the information. Note that an ECU is merely one such computer which collates error codes from specific areas.
The ‘brain’ of a modern passenger car, is its PCM or Powertrain Control Module. It is a control unit that consists of the ECU and the Transmission Control Unit or TCU. In some premium cars manufactured by Chrysler, a third computer is also present: a Body Control Module or BCM.
This is a key point of difference between standard engine diagnostics and more comprehensive car or vehicle diagnostics.
To further complicate things, BMW and Citroen use different terminologies: ‘Injector Control Unit’ and ‘Digital Motor Electronics’ respectively.
Note that these are all PCMs.
Since a PCM acts as the central component, all sensors report directly to it. It controls all the parts of a car, while its subsidiary ECU is responsible primarily for engine-specific issues.
Once an issue is detected, the ‘check engine light’ comes on. There can be several reasons behind this, but the most likely suspects are the following:
- Transmission problems
- Exhaust system issues
- Errors in ignition timing
- Poor fuel injector performance
- Poor condition of ignition coils
- Inlet air temperatures and low coolant levels, among others.