Mastertemp 400 error on PS LED light.

Welcome to TFP.

Check the wires between the PS and the PCB for continuity.

Any signs of rodents in the heater chewing on wires?

If the wires are good then the PCB is bad.

I recommend you pay the price for a genuine Pentair PCB. People have had mixed results using clone boards. Read Pentair MasterTemp Heaters - Further Reading