Error Code 54:- Water/Air Intercooler
This is a very common problem reported by owners of water/air intercooler cars (ST165, ST185RC & ST205).
Often this is very simple, just a matter of checking the coolant level in the intercooler reservoir. After topping up, you need to reset the ECU to clear the error code. You can do this by removing the EFI fuse from the fuse box for a few seconds, or disconnecting the negative terminal on the battery. This is necessary as the ECU will limit boost levels if code 54 is detected.
Error Code 54 may indicate a problem with the intercooler water circulation pump. If all else fails, you may need to replace the bearings in the pump as described in this article.
Some owners prefer their intercooler running constantly, rather than the haphazard on/off that the stock ECU seems to dictate. This will cause a problem with the ECU detecting code 54.
There is a write up on pump overhaul in the maintenance section