P1921
No Bus Data From ECM.
Fault Symptoms
CHECK GEARBOX warning lamp is ON.
TCM in Limp-Home mode.
Conditions
The TCM has had contact with the ECM at some time during the same ignition-on period
Fault Criteria
If there has not been communication on the bus between the ECM and the TCM for more than 4 seconds a P1921 diagnostic trouble code is generated
Diagnostic Help
See bus and diagnostic fault finding