I recently purchased a 2004 base C5. (Salvaged but purely cosmetic) the engine and transmission work properly. The DIC had multiple codes stored. Too many to list here. But a Chatgtp search narrowed it down to a communication problem or voltage issue. The battery cables were trashed. So I replaced them. Cleaned the grounds below the battery box, then pulled and examined the pcm. No corrosion or bent pins were found. Cleared out all codes from all module and took a test drive. Recheck the DIC and only one code returned P0654 Com loss to the tach. A scan tool plugged into the ALAD connector can read a tach signal, but no dash tach. All other gauges and the HUD work properly with the exception of the tach on both the dash as well as the hud. Since the fuel and ignition systems are working, i believe I can eliminate the cam and crank sensors and signals. Fuel ,oil, temp and voltage gauges all work properly, so the signal from the PCM is getting to the dash. What's my next step in tracking down this issue? What are the connectors between the pcm and the dash do I need to check next? If the other gauges work it would be logical the data transmission line should be working. I'm at a bit of a loss as to were to head next.
the tach as well as all the other gauges sweep when the key is turned to the run position then the tach drops to zero. All other gauges read normal when the engine is started
addendum:
After replacing the signal wire from the pcm to the IPC, the tach and hud now work properly. Thanks for all the input!

 

Does your tachometer sweep when you turn the key on? It could very well be the tachometer itself has failed.