54 is IAT(intake air temp) sensor code
2.9l IAT should be on upper intake drivers side I think, about mid-way, two wire sensor.
It gets 5volts from the computer, same 5volt wire goes to TPS, ECT and MAF sensors
It then sends back a lower voltage as it heats up, so sensor has lower resistance cold and resistance gets higher as it warms up, lowing voltage going back to computer.
Yes, if pre-heater for air supply is stuck and pulling air from around exhaust manifold the IAT temp may be too high.
Easy to check that, pull off heat riser tube and put a vacuum hose on its servo, suck on the hose to see if plate moves or not.
Vacuum should move it to close off CIA(cold air intake) tube to pull in pre-heated air from around exhaust manifold, no vacuum releases it and CAI tube from rad support should be used.
Pre-heated air system is not a critical system, so you can plug vacuum hose and leave it in CAI position.
It does help engine warm up faster which lowers emissions, its main purpose, so may be required in Calif