One single microprocessor called the Central Processing Unit (CPU) is dedicated to maintaining control over the entire computer. The CPU performs all of the calculations and logistical decisions. Operating instructions for the CPU are preprogrammed into other memory locations and are ‘read-only’ programs. These programs are permanent and generally cannot be altered by service personnel in the field.
A program is a set of instructions arranged in a specific order to accomplish a specific task. Each instruction in the program is assigned to a specific location or address within the computer’s memory. Only the address of where the instruction is stored is retained in the CPU. When the CPU requires information to perform a calculation it looks for the address of the required data and then copies the data from the memory location. This copy is retrieved and temporarily retained by the CPU for processing. Retrieving programs in this manner ensures that the information programmed in memory is retained and does not change.
Related Symptoms
The following symptoms can be caused by open circuits, dead-shorts to ground, short to power or excessive resistance in the power and ground circuits; data line communication malfunctions and /or component failure:
CAUTION
Always turn the ignition off when installing or removing the ECM or PCM connectors in order to prevent damage to the components.
CAUTION
In order to prevent any possible electrostatic discharge damage to the ECM/PCM , touch a good engine ground with your hands before handling the ECM or PCM and avoid touching the ECM/PCM connector pins.
Note: OBD-II compliant PCM's are sealed to prevent unauthorized tampering with circuits. In some cases the PCM will need to be updated to either update the software or modify a program. This is a procedure that may be limited to the vehicle manufacturer.
CAUTION