DariusThor

The Thermostat Housing broke and the Coolant subsequently escaped. I do not know if the Engine overheated, but it is not Ceased.
Even before replacing the Thermostat Housing, the Car would crank but no start. There is no spark on all 4 coils and code P0688.
I checked all Fuses. I replaced the relay for the ECU with a new one from Audi. I bought a used ECU Programmed for my Audi. I also tried out 1 new coil to see if it would spark, but did not spark either. Replaced the relay under the ECU for the coils with one from O'Reillys, but did not change anything (put the old relay for the coils back in and returned the new one to O'Reillys).
But still only cranks and no start, no spark on all four coils and still code P0688.
Thank you for Sharing your Wisdom🙏

DariusThor

P0688 Definition: PCM/ECM Power Relay Sense Circuit/Open

Here’s the definition of P0688 divided into its three distinct parts.

ECM/PCM Power Relay

ECM and PCM are interchangeable terms. They stand for engine control module and powertrain control module, respectfully. For this article, we will be referring to your A4’s main computer as the PCM exclusively.

The PCM Power relay circuit provides the proper voltage to the PCM when you turn the key to the on position. Here’s what’s typically involved in the circuit:

1. A ground wire
2. Power supply from the battery
3. Power from the ignition when the key is turned to the “on” position
4. Power relay sense circuit
5. Power to PCM

The sense circuit lets the PCM monitor the voltage at the PCM power relay circuit.

Sense Circuit Open

The sense circuit lets the PCM monitor the voltage at the PCM power relay. When your Audi A4 has P0688, it has detected an open circuit. If there is an open detected between the PCM and the PCM power relay, you’d have P0685.

Cybersombosis

If the wiring or relay is in question, have you tried bypassing the relay so the ECU gets power? If the fuse is popping, there may be a short somewhere so bypass the relay at your own risk.

DariusThor

I will try to bypass the Relay, but Fuses are all okay. Although, does it still make sense in that case?

Cybersombosis

What your experiencing doesn’t make any sense so I would inspect the wiring harness to the ECU for corrosion and for continuity from the relay plug to the ECU harness. I would use an outside 12v source to power up the ECU and see if the car will start. Check for bent pins in the ECU harness.