PCMs typically fail for one of two reasons: voltage overloads (often due to a short in a solenoid or actuator circuit) or environmental factors (corrosion, thermal stress or vibration). If the shorted solenoid or actuator is not found and repaired, the voltage overload it creates may damage the replacement PCM, too.
As for environmental factors, water is the main thing to avoid. If water gets inside a PCM, it can short circuits and set up irreversible corrosion that ruins electronic connections. Most remanufacturers will not even attempt to repair a PCM if the vehicle it came out of was submerged in a flood. Replacement is the only option. Thermal stress and vibration can form microcracks in circuit boards (which are repairable). This often has more to due with the ruggedness of the circuit design than operation factors in the vehicle itself.
Because a powertrain control module can be very expensive to replace, almost all aftermarket replacement PCMs are "remanufactured" units. A PCM is not rebuilt in the same way that an alternator or water pump is rebuilt because there are no mechanical parts that wear out. Remanufacturing in this case usually means testing the powertrain control module, isolating and repairing any faults that may be found, then retesting the powert5ain control module to make sure everything works correctly.
A remanufactured PCM is typically sold one of two ways: on an exchange basis from stock, or on a custom rebuild basis. If a particular PCM is not in stock or is unavailable, you may be able to send an old PCM to a remanufacturer for repair. Turn around time is typically a few days and the cost is about the same as an exchange unit except there is no core charge). The hard part is finding a remanufacturer who can test and repair your powertrain control module.
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