Technical Description of OBD-II Trouble Code
The glow plug circuit for Cylinder #1 is experiencing high voltage.
What does that mean?
This generic powertrain diagnostic trouble code (DTC) is applicable to various OBD-II vehicles, including but not limited to Jeep, Chrysler, BMW, Toyota, Volkswagen, Dodge, Ram, Ford, Chevrolet, Mazda, etc.
When code P066B is set, it indicates that the powertrain control module (PCM) has detected a high voltage situation in the glow plug control circuit for Cylinder #1. To determine the location of this cylinder in your specific year, make, model, and engine configuration, consult a reliable vehicle service resource.
Diesel engines rely on high compression instead of spark to initiate piston movement. Since there is no spark involved, the temperature of the cylinder must be increased to maximize compression. Glow plugs are used in each cylinder to achieve this temperature increase.
Glow plugs, which are often mistaken for spark plugs, are threaded into the cylinder head for each individual cylinder. Battery voltage is applied to the glow plug element through a glow plug timer (also known as a glow plug controller or glow plug module) and/or the PCM. When voltage is correctly applied, the glow plug will glow red hot and help increase the cylinder temperature. Once the desired temperature is reached, the control unit restricts the voltage and the glow plug returns to its normal state.
If the PCM detects a higher than expected level of voltage in the glow plug control circuit for Cylinder #1, it will store a code P066B and may illuminate the malfunction indicator lamp (MIL).
Example photo of a glow plug:
What is the severity of this DTC?
Any code related to glow plugs is likely to cause drivability issues. It is important to address a stored code P066B with urgency.
What are some of the symptoms of the code?
Symptoms of a P066B trouble code may include:
What are some of the common causes of the code?
Possible causes for this code may include:
What are some P066B troubleshooting steps?
In order to accurately diagnose a code P066B, you will need a diagnostic scanner, a reliable source of vehicle information, and a digital volt/ohmmeter (DVOM). Use the vehicle information source to search for applicable technical service bulletins (TSB) that match your vehicle make, model, symptoms, and the stored code.
You may also need to obtain additional information such as diagnostic flow charts, wiring diagrams, connector face views, connector pin out charts, component locations, and component testing procedures/specifications from the vehicle information source. All of this information will be necessary for a correct diagnosis of the stored code P066B.
After visually inspecting all glow plugs and glow plug control wiring and connectors, connect the diagnostic scanner to the vehicle diagnostic port. Retrieve all stored codes and freeze frame data, and record them for future reference. Next, test drive the vehicle to see if the code P066B is reset. Continue driving until either the PCM enters readiness mode or the code is reset. If the code is reset, proceed with the diagnosis. If not, you may be dealing with an intermittent condition that may need to worsen before an accurate diagnosis can be made.
Here is a tip that the service manual will not provide: a reliable way to test glow plugs is to remove them and apply battery voltage. If the glow plug glows bright red, it is functioning properly. If the glow plug fails to heat up, you can use your DVOM to test it for resistance and determine if it meets the manufacturer’s specifications. Be cautious to avoid burns or starting 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 a 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 related fuses and relays as recommended by the manufacturer. It is best to test system fuses and fusible links with the circuit loaded, as a fuse for a circuit that is not loaded may appear functional when it is not.
If all fuses and relays appear to be functional, use the DVOM to test the output voltage at the glow plug timer or PCM (wherever it originates). If voltage is detected at the glow plug timer or PCM, suspect an open or shorted circuit. You can investigate the cause of the discrepancy or simply replace the circuit.
