Damaged or corroded wiring in the CAN bus circuit, particularly at connectors or where the harness flexes.; Faulty transmission control module (TCM) or mechatronic unit, causing it to misinterpret CAN messages.; Poor electrical connections at the TCM, ECM, or other control modules due to loose pins or water intrusion.; A failed control module on the CAN bus (e.g., ECM, ABS) that is disrupting communication.; Low battery voltage or charging system issues causing erratic CAN bus behavior.; Aftermarket electronic devices improperly installed and interfering with the CAN bus signals.

DTC codes / BMW codes / P1747

P1747 · BMWCAN Bus Monitoring

Q: What does code P1747 mean?

CAN Bus Monitoring. P1747 on BMW vehicles indicates that the transmission control module (TCM) has detected a fault in the controller area network (CAN) bus, which is the high-speed communication link between the TCM and other control units such as the engine control module (ECM) and the anti-lock brake system (ABS) module. The TCM monitors the CAN bus for expected messages and signal integrity, and when it does not receive a required message or detects a communication error, it stores this code and may illuminate the warning light.

P1747 on BMW vehicles indicates that the transmission control module (TCM) has detected a fault in the controller area network (CAN) bus, which is the high-speed communication link between the TCM and other control units such as the engine control module (ECM) and the anti-lock brake system (ABS) module. The TCM monitors the CAN bus for expected messages and signal integrity, and when it does not receive a required message or detects a communication error, it stores this code and may illuminate the warning light.

SystemPowertrain

Code typeBMW specific

SeverityMedium

Fix difficultyProfessional

AreaTransmission

Symptoms you may notice

Illumination of the transmission malfunction warning light or the general check engine light.
The transmission may enter limp mode, defaulting to a single gear or a fixed gear ratio.
Harsh or delayed gear shifts, or the transmission may refuse to shift out of a particular gear.
Intermittent loss of communication with other systems, leading to multiple warning lights on the dashboard.
Often no symptoms besides the warning light, especially if the fault is intermittent.

Common causes, most likely first

Damaged or corroded wiring in the CAN bus circuit, particularly at connectors or where the harness flexes.
Faulty transmission control module (TCM) or mechatronic unit, causing it to misinterpret CAN messages.
Poor electrical connections at the TCM, ECM, or other control modules due to loose pins or water intrusion.
A failed control module on the CAN bus (e.g., ECM, ABS) that is disrupting communication.
Low battery voltage or charging system issues causing erratic CAN bus behavior.
Aftermarket electronic devices improperly installed and interfering with the CAN bus signals.

How to diagnose and fix it

Perform a thorough visual inspection of the CAN bus wiring and connectors for damage, corrosion, or loose pins.
Check battery voltage and charging system operation to rule out low voltage as a cause.
Use a scan tool to read all stored codes in all modules and check for communication errors with other systems.
Measure CAN bus resistance (typically 60 ohms between CAN High and CAN Low) to verify bus integrity.
Inspect and test the TCM power and ground circuits, and check for water intrusion in the transmission connector.
If wiring and power are good, perform a software update or reprogram the TCM if applicable.
Substitute a known-good TCM or other suspect module to isolate the fault, following BMW diagnostic procedures.

CAN Bus Faults Often Misdiagnosed as TCM Failure

A common mistake is to replace the transmission control module immediately, but the root cause is frequently a chafed wire in the harness near the transmission or a corroded connector.
Intermittent CAN bus faults can be caused by a failing alternator or voltage regulator, which introduces electrical noise that corrupts digital signals.
Aftermarket accessories such as dash cams or alarm systems tapped into the CAN bus can disrupt communication and set this code without any hardware failure.
The code can also be triggered by a low battery during engine cranking, which causes a voltage drop that the TCM interprets as a CAN bus error.
Always check for related communication codes in other modules (e.g., U-codes) to determine if the fault is isolated to the TCM or affects the entire network.

Can I keep driving with P1747?

If the vehicle is in limp mode or shifting erratically, avoid driving at high speeds or under heavy load and seek service immediately. If only the warning light is on and the car drives normally, you can drive cautiously to a workshop, but do not ignore the code as it may worsen.

Tools that help

Tools we recommend are in the Auto Veteran shop on Amazon. As an Amazon Associate we earn from qualifying purchases.

Guidance is general and written for BMW vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.