Dec 032014


A customers 2007 Toyota Aurion was towed into the workshop after the NRMA diagnosed its 6 speed U660E Automatic Transmission as being lock in Fail Safe mode.  When a Fail Safe condition presents it self the vehicles transmission is placed into a safe condition in the event of a breakdown or malfunction. The engine light or transmission related selector lights may also flash relating to a Code present in either the ECU (Engine Control Unit) or TCM (Transmission Control Unit).


The vehicle presented a constant engine light on the dash so with a diagnostic tool we scanned the ECU module and found the following hard code (A hard code either won’t clear or returns immediately after clearing).

DTC U0101 Lost Communication with Transmission Control Module (TCM).

We then tried to scan the TCM but our scanner would not communicate at all so we tried some other modules to make sure the scanner was compatible with the vehicle.  The other basic modules showed no trouble codes so then we checked for blown fuses.

Fuses – The main fuse box is located in the engine bay and we tested them all and found none where faulty.

Toyota Aurion U660 Transmission Control Module

TCM located on the Transmission.

Power – Next we checked the power at the TCM (Located on the Transmission see image) plug which will need the ignition turned on which was also fine.

Communication – With high speed data traveling between the TCM and the ECU at a great speed any interruption can stop the system so next we disconnected the plugs on the ECU and cleaned them as well as the TCM. We also preformed a hard reset buy disconnecting the battery and discharging any memory in the system. With everything reconnected the fault still remained so it was looking like a faulty module or bad wiring.

Wiring – We knew the module had power but was it sending any signals? an easy check is to probe a CAN-Bus line with a multimeter. Most Can-Bus systems use a low to high volt signal to transmit data on the network if you have anywhere around 3- 5 volts as a guide there is communication on the network. When we probed the TCM there was only 1 volt showing which means there is really no activity from the TCM.

The Problem…

The fault pointed to a faulty/dead Transmission Control Module as the U0101 code suggested so we decided to try a second hand unit. A new one from the dealer was about 4 times the price and not available for a few days.

The Fix…

Replace the TCM with a second had unit which immediately removed the engine light and allowed us to clear the previous hard code in the ECU.


Although we did not find any reason for the TCM to have failed it could be for a number of unknown reasons. Due to it being located on the transmission vibration, heat, water, oil or acid from the battery could have played apart.

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