Technical Description of OBD-II Trouble Code
Open Circuit in ECM/PCM Power Relay Sense Circuit
What does that signify?
This is a generic diagnostic trouble code (DTC) that is applicable to many OBD-II vehicles (1996-newer). This includes vehicles from various manufacturers such as Chevrolet, Ford, VW, Jeep, Audi, Chrysler, Dodge, Cadillac, etc. Although it is a generic code, the specific repair steps may vary depending on the year, make, model, and powertrain configuration of the vehicle.
When a code P0688 is stored, it indicates that the powertrain control module (PCM) has detected an abnormality in the relay that supplies it with voltage. In this case, the relay sensor circuit is open, which can be interpreted as disconnected.
The PCM power relay is responsible for safely applying battery voltage to the appropriate PCM circuits. It is a contact type relay that is activated by a signal wire from the ignition switch. This type of relay typically has a five-wire design. One wire carries constant battery voltage, another wire is for ground, a third wire carries a signal from the ignition switch, a fourth wire supplies voltage to the PCM, and the fifth wire is the power relay sense circuit. The PCM uses the power relay sense circuit to monitor the voltage of the power relay.
If the PCM detects an open power relay sense circuit, it will store a code P0688 and may illuminate the malfunction indicator lamp (MIL).
Here is an image of a typical opened PCM powertrain control module:
What is the severity of this DTC?
A P0688 code should be considered severe and addressed accordingly. It can result in a no start condition and/or various drivability issues.
What are some of the symptoms of the code?
Symptoms of a P0688 trouble code may include:
What are some of the common causes of the code?
Some possible causes for this code may include:
What are some P0688 troubleshooting steps?
To diagnose a code P0688, you will need a diagnostic scanner and a digital volt/ohmmeter (DVOM).
You will also need a reliable source of vehicle information that provides diagnostic flow charts, wiring diagrams, connector face views, connector pin-out charts, and component locators. Additionally, you will require component and circuit testing procedures and specifications. All of this information is necessary for a successful diagnosis of a code P0688.
Start by connecting the scanner to the vehicle diagnostic port and retrieving all stored codes and freeze frame data. Make a note of this information as it may be useful if the code is intermittent.
After recording all relevant information, clear the codes and, if possible, test drive the vehicle until the code is reset or the PCM enters readiness mode.
If the PCM enters readiness mode, the code is intermittent and will be more difficult to diagnose. The condition that caused the P0688 code to be stored may need to worsen before an accurate diagnosis can be made. However, if the code fails to reset and there are no drivability symptoms, the vehicle can be operated normally.
Consult your vehicle information source for any technical service bulletins (TSB) that match the stored code, vehicle (year, make, model, and engine), and exhibited symptoms. If you find a relevant TSB, it may provide helpful diagnostic information.
If the P0688 code is immediately reset, visually inspect the system-related wiring and connectors. Repair or replace any broken or unplugged harnesses as necessary.
If the wiring and connectors appear to be functional, use your vehicle information source to obtain the appropriate wiring diagrams, connector face views, connector pin-out charts, and diagnostic flow charts.
Once you have the relevant information, test all system fuses and relays to ensure that the PCM power supply relay is receiving battery voltage.
If constant (or switched) voltage is not present at the power relay connector, trace the appropriate circuit back to the fuse or relay from which it originates. Repair or replace any defective fuses or fusible links as necessary.
If power relay supply input voltage and ground are present on all appropriate terminals, use your DVOM to test the relay output performance at the appropriate connector pins. If the power supply relay output circuit voltage is not adequate, suspect that the relay is defective.
If the PCM power supply relay output voltage is within specifications on all terminals, test the corresponding relay output circuits at the PCM.
If a relay output voltage signal is discovered at the PCM connector, suspect a defective PCM or a PCM programming error.
If there is no relay output voltage signal discovered at the PCM connector, there is an open circuit.
