Technical Description of OBD-II Trouble Code
High Circuit Communication in TCM
What is the meaning of P0866?
This diagnostic trouble code (DTC) is a generic powertrain code that applies to many OBD-II vehicles (1996-newer). This includes vehicles from Dodge, Honda, Volkswagen, Ford, and others. While it is a generic code, the specific repair steps may vary depending on the year, make, model, and powertrain configuration.
If your vehicle has stored a code P0866 and the malfunction indicator lamp (MIL) is illuminated, it indicates that the powertrain control module (PCM) has detected a high voltage condition in the transmission control module (TCM) communication circuit.
While some transmission control modules for OBD-II equipped vehicles are integrated into the PCM, most are standalone components. TCM related codes may also be stored as P-codes, B-codes, or U-codes, depending on the vehicle’s make and model. If a P0866 code is stored, there is a high chance that there are other PCM and/or TCM codes stored as well. ABS codes related to the antilock braking system may also be stored.
Electronic transmission control systems in OBD-II equipped vehicles are controlled using a network of computers called control modules. These control modules constantly communicate with each other via the controller area network (CAN).
The PCM uses input signals from various engine and transmission sensors to calculate the automatic transmission shift strategy. A high-pressure pump inside the transmission forces fluid through the valve body and into the sprag assembly, which lubricates and cools the transmission. The high-pressure fluid allows the clutches to separate from the sprag momentarily, enabling smooth gear ratio changes. Electronic pressure control (EPC) solenoids help regulate fluid pressure.
Electronic pressure (EP) sensors provide the PCM with important data about fluid pressure at different points in the transmission.
Electronic shift solenoids are used to interrupt the flow of high-pressure fluid, allowing the transmission to shift gears when commanded. The PCM uses voltage input signals from the transmission input speed sensor and transmission output speed sensor to determine if the transmission is shifting efficiently.
Similar to other automatic transmission malfunctions, a transmission control system code can be caused by either an electrical or mechanical failure. If the PCM detects a malfunction that requires MIL illumination, a code P0866 will be stored and the MIL may be illuminated.
An example of a typical TCM transmission control module
What is the severity of this DTC?
If this code is stored, you may have noticed that your transmission shifts much more harshly than normal. If your vehicle has exhibited a code P0866, it should be diagnosed as soon as possible.
What are some of the symptoms of the code?
Symptoms of a P0866 trouble code may include:
What are some of the common causes of the code?
Causes for this code may include:
What are some P0866 troubleshooting steps?
To accurately diagnose a code P0866, you will need a diagnostic scanner, a digital volt/ohmmeter (DVOM), and a reliable source of vehicle information. An oscilloscope can also be helpful in diagnosing various sensor outputs.
You can save time by searching for technical service bulletins (TSB) that match the stored code, vehicle information (year, make, model, and engine), and exhibited symptoms. This information can be found in your vehicle information source. If you find the appropriate TSB, it may provide a quick solution to your diagnosis.
After connecting the scanner to the vehicle diagnostic port and retrieving all stored codes and relevant freeze frame data, write down the information in case the code is intermittent. Then, clear the codes and test drive the vehicle until one of two things happens: the code is restored or the PCM enters readiness mode.
If the PCM enters readiness mode at this point, the code may be more difficult to diagnose because it is intermittent. The condition that caused the P0866 to be stored may need to worsen before an accurate diagnosis can be made. If the code is restored, continue with the diagnosis.
You can obtain connector face views, connector pinout charts, component locator charts, wiring diagrams, and diagnostic flow charts related to the code and vehicle in question using your source of vehicle information.
Perform a visual inspection of the related wiring and connectors. Repair or replace any wiring that is cut, burned, or damaged.
Use the DVOM to test voltage and ground circuits at the TCM and/or PCM. If no voltage is detected, check the system fuses. Replace any blown or defective fuses as necessary and retest.
If voltage and ground are detected at the TCM, test the corresponding circuit at the PCM connector. If no voltage is detected there, suspect an open circuit between the component in question and the PCM. If voltage is discovered there, suspect a defective TCM, PCM, or a programming error.
