What does it mean?
OBD II fault code P068A is a generic trouble code that is defined as “ECM/PCM Power Relay De-Energized Performance – Too Early”, and is set when the PCM (Powertrain Control Module) detects an improper de-energizing procedure for the relay that supplies it with power. While modern automotive control modules such as Powertrain Control Modules are more reliable now than they have ever been, their long-term reliability depends on several factors.
Foremost among these is that all currents must be stable, within specifications, and most importantly, that when power is removed from any control module when the vehicle is switched off, power must be removed gradually to prevent voltage spikes in some electronic circuits. Almost all vehicles use a five-pin relay to supply the PCM with battery power, and in practice, this relay works in the same way as any other relay.
When the ignition is turned on, a low current is fed to the internal coil-driven switch, which then closes to allow battery current to flow to the PCM through the relays’ electromagnetic coil/switch. However, unlike four-pin relays, the five-pin relay that supplies the PCM with power has a sensing circuit (accounting for the fifth pin) that senses the rate at which power is removed from the relay when the ignition is turned off.
As a practical matter, all control modules contain capacitors that perform several different functions, depending on where in the circuit they are installed. For instance, some capacitors filter currents; others store electrical energy, while others keep some circuits powered up for a few seconds after power is removed from the control module. One example of this is the phenomenon of an audio system that remains on for a few seconds after it is turned off.
Thus, to prevent damage to internal components when capacitors release all their stored energy in a single burst when the ignition is turned off the relay is de-energized slowly to “bleed off” the energy that is stored in capacitors in a controlled fashion. This process is somewhat analogous to how a personal computer is de-energized with on-screen prompts.
Suddenly removing power from the computer by pushing the power button does not allow electronic circuits to de-energize in a controlled fashion, and we have all seen operating systems crash as a result of shutting down a computer improperly. On cars, much the same thing happens: components can be damaged, software can be corrupted, and in some cases, one or more control modules can lose their programming completely.
To prevent this from happening, all PCM’s monitor the rate at which they are de-energized, which typically happens too fast, or too soon after the ignition is turned off. Thus, when a PCM detects an improper de-energizing procedure, it will set code P068A, and illuminate a warning light as a result.