What does it mean?
This generic powertrain diagnostic trouble code (DTC) generally applies to many OBD-II vehicles. This may include, but is not limited to, vehicles from Jeep, Chrysler, BMW, Toyota, Volkswagen, Dodge, Ram, Ford, Chevrolet, Mazda, etc.
When a P068F code is set, it means the Powertrain Control Module (PCM) has detected a high voltage situation in the glow plug control circuit for cylinder #8. Consult a reliable vehicle service resource to determine the location of said cylinder in the code description for your specific vehicle year, make, model, and engine configuration.
Diesel engines use high compression instead of spark to initiate piston movement. Since no spark is involved, the cylinder temperature must be increased to maximize compression. Glow plugs are used in each cylinder to achieve this increase.
Often confused with spark plugs, a glow plug for each individual cylinder is screwed into the cylinder head. Battery voltage is applied to the glow plug element via a glow plug timer (sometimes called a glow plug controller or glow plug module) and/or the PCM. When voltage is properly applied to the glow plug, it will literally become red-hot and help increase the cylinder temperature. Once the cylinder temperature reaches the desired level, the voltage is limited by the control unit and the glow plug returns to its normal state.
If the PCM detects a voltage level for the glow plug control circuit of cylinder #8 that is higher than expected, a P068F code will be stored and a Malfunction Indicator Lamp (MIL) may be illuminated.
Example photo of a glow plug:
P068F Glow Plug
How severe is this DTC?
Any code related to the glow plug will likely be accompanied by driving issues. A stored P068F code should be addressed urgently.
What are some of the symptoms of the code?
Symptoms of a P068F fault code may include:
Excessive black smoke from the exhaust
Engine running problems
Delayed engine starting
Reduced fuel efficiency
Engine misfire codes may be stored
What are some common causes of the code?
Causes
of this code may include:
Faulty glow plug
Open or shorted glow plug control circuit
Loose or faulty glow plug connector
Faulty glow plug timer
What are the P068F troubleshooting steps?
A diagnostic scanner, a reliable source of vehicle information, and a digital volt/ohmmeter (DVOM) will be needed to reach an accurate diagnosis for a P068F code. Use the vehicle information source to look up applicable Technical Service Bulletins (TSBs). If you find a TSB that matches the vehicle’s make and model, the symptoms presented, and the stored code, it will help you make a diagnosis.
You may also need to obtain diagnostic flowcharts, wiring diagrams, connector face views, connector pinout diagrams, component locations, and component testing procedures/specifications from the vehicle information source. All of this information will be necessary to correctly diagnose the stored P068F code.
After performing a careful visual inspection of all glow plug and glow plug control wiring and connectors, connect the diagnostic scanner to the vehicle’s diagnostic port. Now, retrieve all stored codes and freeze frame data and record them for later (in case you need them). Next, I would test drive the vehicle to see if the P068F code resets. Drive until one of two things happens: either the PCM enters readiness mode, or the code resets. If the code resets, proceed with the diagnosis. If not, you are dealing with an intermittent condition that may need to worsen before an accurate diagnosis can be made.
Here’s a tip the service manual won’t give you. A sure way to test glow plugs is to remove them and apply battery voltage. If the glow plug becomes bright red, it is good. If the glow plug does not heat up and you wish to take the time to test it with your DVOM, you will likely find that it does not meet the manufacturer’s resistance specifications. Be careful not to burn yourself or start a fire when performing this test.
If the glow plugs appear to be working correctly, use the scanner to activate the glow plug timer and test for battery voltage (and ground) at the glow plug connector (use the DVOM). If no voltage is present, test the power supply for the glow plug timer or glow plug controller. Test all associated fuses and relays according to the manufacturer’s recommendations. Typically, I have found it preferable to test system fuses and fuse links with the circuit under load. A fuse in a circuit that is not under load may appear functional (when it is not) and lead you down the wrong diagnostic path.
If all fuses and relays appear functional, use the DVOM to test the output voltage at the glow plug timer or PCM (wherever they originate from). If voltage is detected at the glow plug timer or PCM, suspect that you have an open or shorted circuit. You can search for the cause of the discrepancy or simply replace the circuit.