Possible causes

Inertia fuel shutoff (IFS) switch needs to be reset.

Open FPDM ground circuit.

Open circuit to FPDM PWR RLY.

Open FPDM PWR circuit.

Open or shorted FPM circuit (engine should start).

Damaged IFS switch.

Damaged FPDM PWR RLY.

Damaged FPDM.

Damaged PCM.
Also for Mustang:

Open B+ circuit to the constant control relay module (CCRM) pin 11.

Open ground to CCRM pin 18.

Damaged CCRM.

Tech notes

The PCM expects to see one of the following duty cycle signals from the FPDM on the FPM circuit: 1) 50% (500 ms on, 500 ms off), all OK. 2) 25% (250 ms on, 750 ms off), FPDM did not receive a fuel pump (FP) duty cycle command from the PCM, or the duty cycle that was received was invalid. 3) 75% (750 ms ON, 250 OFF), the FPDM has detected a fault in the circuits between the FPDM and the fuel pump.

Symptoms

– Engine Light ON (or Service Engine Soon Warning Light)

P1233 Description

LS6/LS8 and Thunderbird:
For LS and Thunderbird, this DTC indicates the PCM is not receiving the fuel level information on the communication link from the rear electronics module (REM). REFER to Workshop Manual Section 418 for REM self-test/diagnostics. All Others:
The PCM monitors the fuel pump monitor (FPM) circuit from the fuel pump driver module (FPDM). With the key on, the FPDM continuously sends a duty cycle signal to the PCM through the FPM circuit. The test fails if the PCM stops receiving the duty cycle signal.

P1233 Information for specific makes

P1233 FORD