Error Code P0866 is defined as TCM Communication Circuit High. This is a generic trouble code, meaning it applies to vehicles equipped with the OBD-II system, particularly those made from 1996 up to present. This includes vehicles from but not limited to Dodge, Ford, Honda, Volkswagen, etc. Specifications on the definition, troubleshooting, and repairs may vary depending on the manufacturer, year model, and powertrain configuration.
This code means the powertrain control module (PCM) has determined a high voltage condition in the communication circuit of the transmission control module (TCM).
While some TCM (for vehicles equipped with OBD-II) are integrated into the PCM, many are stand-alone components. In some cases, TCM related codes can be stored as P-codes, B-codes, or even U-codes. This, of course, depends on the make and model of the vehicle. When Error Code P0866 is stored, this could mean there are stored PCM or TCM codes as well, as well as Antilock Braking System (ABS) code too.
Vehicles equipped with OBD-II have electronic transmission control systems controlled by a network of computers (control modules). These modules communicate constantly and are controlled by the Controller Area Network (CAN).
To calculate the shifting strategy of the automatic transmission, the PCM uses input signals from different engine and transmission sensors. Aside from lubricating and cooling the transmission, there’s also a high-pressure pump inside the transmission that forces fluid through the valve body and into the sprag assembly. The high-pressure fluid allows the clutches to momentarily separate from sprag so that gear ratio can change smoothly. To help with regular fluid pressure, there are one or multiple Electronic Pressure Control (EPC) solenoids.
The PCM gets data regarding fluid pressure at various points in transmission from the Electronic Pressure (EP) sensors.
Electronic shift solenoids are used to interrupt the flow of high-pressure fluid so that the transmission shifts gears when commanded. The voltage input from both the transmission input and output speed sensor are then used by the PCM to determine efficient transmission shift.
As with other error codes that refer to automatic transmission malfunction, the transmission control system code may be caused by an electrical or mechanical issue. If the PCM determines a malfunction that activates the Check Engine light, then this code may be stored.
Aside from activating the Check Engine light, other symptoms for this code include:
- Transmission unable to shift
- Erratic transmission shift patterns
- Transmission slippage
- Delayed engagement of the transmission
- Increased fuel consumption
- ABS disabled
- Other transmission related codes
- Open or shorted circuits in CAN harness
- Faulty vehicle speed sensor
- Faulty transmission sensor
- Faulty mechanical transmission
- Defective or programming error in PCM or TCM
How to Check
As with most codes, a digital volt/ohmmeter (DVOM), and reliable vehicle information are needed to diagnose this error code accurately. An oscilloscope can be a huge bonus for checking various sensor outputs too.
Start your diagnosis by searching the Technical Service Bulletins (TSB) this code, along with the vehicle (year, make, model, engine, etc.) and related symptoms. This could come in handy.
First off, connect the DVOM scanner to the diagnostic port of the vehicle. Retrieve all stored codes, as well as the pertinent freeze frame data. Write these data down before clearing the codes and taking the vehicle for a test drive. During the test drive, wait until two things happen; the code restored or the PCM enters readiness mode.
If the PCM enters readiness mode, then it could be more difficult to diagnose since the code is intermittent. That means the conditions may need to worsen before you can accurately diagnose the problem. If the code is restored in the test drive, then you can proceed with the following diagnosis.
Check the connector face views and pinout charts, component locator charts, wiring diagrams, and the diagnostic flow chart that pertains to the code with the vehicle information source.
Next, inspect the related connectors and wirings. Repair or replace any damaged, burnt, or faulty wirings.
Then, test the voltage and ground circuits at the PCM or TCM using the DVOM. If there is no voltage present, then check the system fuses. Replace blown or defective fuses as needed, then retest.
If there are voltage and ground detected at the TCM, then test the corresponding circuit at the PCM connector. If there’s no voltage, that means there’s an open circuit between the component in question and the PCM. If there is a voltage, then the TCM or PCM may be defective or has a programming error.
How to Fix
- Repair or replacement of faulty wiring or damaged connectors
- Replacement of faulty or damaged control modules (note that some control module require reprogramming and integration in to CAN bus system after replacement)
- Professional reprogramming of one or more control modules with the manufacturer, with supplied software and updates (in rare cases)
Since this code can immobilize the vehicle unexpectedly, the vehicle must not be driven, especially in traffic.
Most communication error codes can be attributed to a programming issue.