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The reason suspected for the battery going flat is a fault with the Hazard Warning control.
I have had a problem with the battery in my Toyota Auris Excel Hybrid going flat twice, and my car has been into my local dealer for about a week to be tested.
I had a meeting with a Manager of Toyota Customer Product Support, he talked me through how Toyota deals with this type of problem, and what they suspect the problem is.
First an explanation of the main items talked about as I understand them:
CAN bus (for Controller Area Network) is a car communication network that connects devices to each other around a car, a modern car will have a number of these networks.
ECU (for Electronic Control Unit) is a small computer, connected to the relevant CAN bus, reading and controlling devices connected to the CAN bus.
When the Hazard Warning Switch is operated it puts two signals onto the relevant CAN bus, the first signal alerts the ECU and tells it to wait for the second signal, this second signal instructs the ECU to turn on the Hazard Warning Lights.
It is possible for the Hazard Warning Switch to send the first signal and alert the ECU and instruct it to wait for the second signal, but not to send the second signal, the ECU is then left waiting, as the Hazard Warning Lights have to be able to operate all the time, even when the car is turned off and locked, so the ECU stays waiting all the time.
The power consumption of the ECU is nearly 600mA, the driver can stop, start, lock and unlock the car but this power consumption will be a constant drain, it is only when the car is off, and not charging the battery that this ECU power consumption will show itself, and if the car is off for a prolonged time will completely drain the battery.
Once the battery is flat, there is no power to the ECU, and when the battery is replaced or recharged the ECU has forgotten it is waiting for a signal from the Hazard Warning Switch, so the fault has gone.
To test for this fault, you have to measure the battery load with everything switched off to see if the 600mA ECU load is present, it’s also possible with specialist equipment plugged into the OBD socket, to check the status of the CAN bus and ECU.
At the moment the only way to remove this fault, is to turn the Hazard Warning Lights on then off, powering up then eventually down the ECU normally.
This problem can affect all MKII Auris Hybrids, all other models are not affected.
I have been assured that Toyota take these kinds of issues very seriously, and are working on a resolution to this problem with some urgency.
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