Lost Communication With Gear Shift Control Module "A"

What it means

Another module, usually the transmission module (TCM) or the engine computer, has stopped receiving messages from the gear shift control module over the CAN bus. This module exists on vehicles with an electronic gear selector or shift buttons, and passes the driver's gear selection to the transmission. When communication is lost, selecting a gear or leaving park may not be possible, and the cause is usually module power, its connector or the bus wiring.

Related codes: U0101U0100U0001P0705P0850

Symptoms5

• Unable to select a gear, or the gear selector does not respond.
• Gear indicator on the cluster flashing or missing.
• Vehicle stays in park, or shifts to park on its own on some vehicles.
• No start because the park signal does not arrive.
• Transmission warning message and check engine light.

Possible causes6

• Weak battery or low voltage while running.
• Spilled liquids such as coffee or soft drinks reaching the module through the selector.
• Blown module power fuse.
• Loose or corroded connector under the gear selector.
• Open or shorted bus wiring to the module.
• Internal failure of the gear selector module.

How to diagnose5

1. Read codes from all modules; if many modules report lost communication, start with the battery and the bus, and if only the gear shift control module is missing, focus on it.
2. Try to communicate with the gear shift control module directly with a scan tool; no response points to lost power, ground or wiring to it.
3. Locate the gear shift control module from the manufacturer information, which is usually built into or near the electronic gear selector, and its name and location vary by make, then check its power fuse and measure power and ground at its connector.
4. Inspect the module connector for water, corrosion or loose terminals, and check the bus wiring to it.
5. Disconnect the battery and measure resistance between the CAN bus wires at the module connector, comparing with the manufacturer specification to find an open or short.

How to fix6

1. Charge or replace the battery, and clean and tighten its terminals.
2. Replace the blown fuse after finding why it blew.
3. Clean the module connector and repair the ground or damaged bus wiring.
4. Update the module software if the manufacturer has an update.
5. Replace the gear shift control module if proven faulty with good power and bus wiring, then program and code it to the vehicle.
6. Clear codes in all modules and confirm the code does not return.

Repair cost

In Saudi riyals; the highlighted part is the expected range for this fault.

The cost is low for a battery, fuse or connector, rises if the module was damaged by spilled liquid, and is highest if the gear selector assembly must be replaced and programmed.

Prices are rough estimates for common cars at independent workshops and vary with the car and the source of the part.
Prices last updated: September 2026

Frequently asked questions