To I just got this Audi in a trade, so it's essentially free to me. I've never heard the thing run but I know I'm so close it makes me sick. I'm about to pull my hair out with a thing, and I'm not sure what you next so some help would be appreciated. I do have extensive knowledge about cars and repair, but not much experience in the Audi field, I mostly Volkswagen's. So let's begin

Have the car towed to my house, cranks but no start. Clothes were for crank position sensor , PCM relay sensor open circuit, and no communication for either the TCM or PCM. Replaced the crank sensor, replaced the PCM relay, no change. Same codes upon further inspection I found that the previous owner attempted to replace the fuel injector harness, but apparently something happened midway and he disrupt the thing up with electrical tape and left half of the wires disconnected or exposed under what appeared to be a good wireline. I went in and made proper connections on all wires, and now it still cranks but no start. It just shows the only codes are now P0 600 collar and P0688. It shows P0 600 twice, and I'm about to rip my face off I'm so mad with this thing. It seems like it wants to start, I just won't fire. I could really use some guidance as for what to do or what to check next thanks in advance

 

P0600 means that the ECU has lost the connection with one of its many controllers more than once. When this code is set the ECU triggers the Check Engine Light to be displayed on the dashboard and a failsafe mode enacted. The failsafe mode is to prevent further damage and protect the vehicle.

When a code P0688 is stored, it means that the powertrain control module (PCM), has detected an abnormality in the relay which supplies it with voltage. In this case the relay sensor circuit is open. Open could be translated as disconnected. The PCM power relay is used to apply battery voltage safely to the appropriate PCM circuits. It is a contact type relay that is activated with a signal wire from the ignition switch. This type of relay typically uses a five-wire design. Constant battery voltage is applied on one wire; ground on another. A third circuit carries a signal from the ignition switch and a fourth supplies voltage to the PCM. The fifth wire is the power relay sense circuit. It is used by the PCM to monitor power relay voltage. If the PCM detects an open power relay sense circuit, a code P0688 will be stored and a malfunction indicator lamp (MIL) may be illuminated

Read more at: https://www.obd-codes.com/p0688
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Sounds like there's still a wire disconnected and/or the codes need to be cleared.