The PCM contains an EPROM (Electrically Programmable Read Only Memory) that contains the calibration information used by the PCM to control fueling, idle speed, ignition timing, transaxle shifts and emissions. This calibration information is based on various aspects of the vehicle such as engine size, vehicle weight, transaxle type, final drive ratio, etc.

The EPROM is programmed (Flashed) with this critical information. Replacement PCM's used for service come without the programming, which must be performed by the technician using the latest diagnostic tools and programs. Additionally, in certain situations, the diagnosis may require the technician to reprogram (Flash) the PCM in an attempt to eliminate a DTC or condition. Therefore, it is crucial that the latest service publications (bulletins, service manuals, etc.) be referenced for information concerning any new procedures or EPROM programs.

If the current program or an attempted reprogramming fails on the PCM, it will set DTC P0602. Refer to the repair instructions portion of this section for information pertaining to PCM service procedures.

Important:  The PCM contains a data storage unit which can save critical data even when battery power is removed. This unit, the Electrically Erasable Programmable Read Only Memory (EEPROM) is used by the PCM to store TP Sensor learned value, Transaxle Shift Adapt values, and Transaxle Oil Life Index.

These values are stored in the EEPROM during the first 30 seconds after the key is turned OFF. Therefore, you should wait at least 30 seconds after key OFF before disconnecting the battery or PCM connectors to allow these values to be stored in the EEPROM.

The EEPROM is soldered into the PCM and cannot be serviced separately.

Important: The service PCM will not contain data. The EPROM must be flashed after the service PCM is installed.

This powertrain has very complex components with equally complex operating characteristics. Therefore, it is essential that the Powertrain diagnostic procedures given in this and other service manual sections be used for diagnosis before any repairs are made. While the PCM may appear on the surface to be the source of a problem, it may, in fact, be operating properly based on incorrect inputs it receives. Powertrain diagnostic procedures are designed to arrive at the CORRECT repair as QUICKLY as possible. DO NOT, under any circumstances, replace a PCM unless directed to do so by diagnostic procedures in the service manual, other service information sources, or through technical assistance.

Service of the PCM will consist of either replacement of the PCM and/or flashing the EPROM. If a diagnostic procedure calls for the PCM to be replaced, the PCM should be checked first to see if it is the correct part. The EPROM can also be flashed when called for. When flashing the EPROM, also refer to the latest service publications for updated programming procedures and information.

After replacement of a PCM and/ or flashing the EPROM, clear PCM DTCs and check the vehicle for proper operation. Poor engine performance, surging, loss of cruise control, improper SERVICE ENGINE SOON MIL operation, no starts, and/or hard starts may be caused by an improperly flashed EPROM, in which case DTC P0602 may set and the vehicle may not start.

Notice: In order to prevent internal PCM damage, leave the ignition OFF when installing or removing the PCM connectors and disconnecting or reconnecting the power to the PCM (battery cable, PCM pigtail, PCM fuse, jumper cables, etc.).

The EPROM and the EEPROM are soldered into the PCM and cannot be serviced separately.

Since the data contained in the EEPROM is specific to the vehicle that the PCM is in, the EEPROM should reset if the PCM is installed in another vehicle.

When this reset occurs, the following data will be erased or reset

Resetting transaxle oil life will trigger the Change Trans Fluid message. Resetting engine oil life will trigger the Change Engine Oil Now message. The TP sensor learn can be relearned by ensuring the throttle is closed and turning the ignition ON for 30 seconds. Do not move the throttle or brake pedals during the TP learn. The transaxle oil life may be reset using the transaxle oil life override or it may automatically reset to 100 depending on the vehicle. The engine oil life may be reset to 100 by displaying the Oil life left and pressing the Reset button. Engine oil life may be reset to near it's previous value with a scan tool. The transaxle adapts will be relearned during normal operation. The PCMs learned odometer reading must be reset with a scan tool on vehicles without a "smart" (serial data equipped) Instrument Panel (IP). "Smart" IPs communicate their odometer value to the PCM at ignition ON.