In 2008, Audi a6 engine module had no signal.

1. Sensor failure: The engine control module needs to receive signals from various sensors to monitor the running state of the engine. If any sensor is damaged or fails, the engine control module may not receive the correct signal, resulting in no signal from the engine module.

2. Electrical problem: ECM needs correct voltage and grounding to work normally. If there is a problem with power supply or grounding, there may be no signal from the engine control module.

3. Module failure: The engine control module itself may also fail, resulting in no signal output. This usually requires checking and replacing the module.