I can help decode them and suggest some diagnostics.
P0231 is low voltage on the fuel pump circuit. The
PCM monitors the fuel pump monitor. The test fails if the
PCM commands the fuel pump ON and B+ voltage is not detected on the FPM circuit.
Possible causes:
Open B+ circuit to the fuel pump relay
Open FP PWR circuit between the fuel pump relay and its connection to the FPM circuit
Damaged fuel pump
Damaged
PCM (engine will start)
Pinpoint Tests
- Disconnect the inertia fuel shut-off switch.
- Disconnect the PCM.
- Install a jumper wire between the PCM harness connector pin 80 (FSV) and ground.
- Connect a digital multimeter between PCM harness connector pins 40 (FPM) and ground.
- Key on. The fuel shut-off valve relay will activate and voltage will be greater than 10.0 volts.
- Observe voltage for an indication of a fault while completing the following (the voltage will change suddenly when a fault is detected, indicating an open):
- Shake, wiggle, bend the B+ circuit to the fuel shut-off valve relay;
- Lightly tap the fuel shut-off valve relay to simulate road shock;
- Shake, wiggle, bend the FSV PWR circuit between the fuel shut-off valve relay and the FSVM splice.
• Inspect the fuel shut-off valve relay connector for corrosion and/or damaged pins.
P0571 is a brake switch plausibility error. This DTC sets when the brake switch does not toggle during the key ON engine running (KOER) test.
Possible causes:
BPP circuit open
BPP circuit short to ground
Concern in modules connected to the BPP circuit
Damaged brake switch
Incorrectly adjusted brake switch
I don't have a listing for P2121 in the PCED reference.
Given the rest of the symptoms you are describing I suspect an electrical fault in the wiring somewhere that is of a major nature or a faulty PCM - although the latter would tend to throw a lot of other DTC's as well and the U codes lead me to believe it's probably in the CANBUS wiring.