What does it mean?
P1623 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Transmission Control Module Missing on Bus .
Symptoms
- MIL (check engine lamp) on together with a transmission warning lamp
- Gearbox locked in one gear, no upshifts, harsh engagement or transmission limp
- Scan tool cannot connect to the transmission module at all
- Speedometer or gear position display blank or dashed
- Cruise control and stability systems disabled
- On some vehicles a no-start, because the range signal is not available
Likely causes
- TCM (transmission control module) unpowered: blown fuse, open supply wire or a bad ground
- Bus wiring to the TCM open, shorted or corroded, very often at the gearbox connector
- Water or gearbox oil wicking into the transmission harness connector, one of the most common causes
- TCM internal failure
- A single faulty module dragging the whole network down
- Bus termination resistance out of specification after a wiring repair
- TCM removed, replaced or not coded after a gearbox change
How to fix it
- Attempt communication with every module and note whether others are missing as well
- Check the transmission module fuses and measure supply voltage and ground quality at the module
- Measure the bus resistance at the diagnostic socket with the ignition off; a value far from the expected terminated figure points to bus wiring
- Unplug the TCM and measure the bus again; if the network recovers, the module or its branch is the fault
- Open and inspect the gearbox harness connector for oil, water and green corrosion on the pins
- Repair the harness, dry and reseal the connector properly rather than just taping it
- Replace and code the TCM only after supply, ground and bus wiring have all been proven