Code
P1234
MINI
P — Powertrain
Fuel pump (FP) relay, primary circuit - low
Views:
UK: 1
EN: 3
RU: 2
AI status
Completed
Completed
100%
Causes
- Blown fuse for fuel pump/relay supply
- Faulty fuel pump (FP) relay
- Corroded, loose, or damaged connector or wiring (power or ground) at relay or pump
- Short to ground in the relay control circuit
- Open or high-resistance ground
- Low battery voltage or poor battery connections
Symptoms
- Engine cranks but does not start (no fuel pressure)
- Intermittent no-start or stalls
- Fuel pump does not run or runs intermittently
- Check Engine Light (MIL) illuminated
- Long cranking times or hard starting
- Possible reduced engine performance or limp mode if fuel supply is affected
What to check
- Read freeze-frame and live data; confirm P1234 and any related codes or events (no-start, cranking voltage)
- Check battery voltage and charge state
- Inspect fuse(s) for fuel pump/relay and replace if blown; check fuse contacts for corrosion
- Visually inspect relay, relay socket, and wiring for corrosion, melted insulation, or loose pins
- Swap fuel pump relay with a known-good identical relay (if available) and re-test
- Measure voltage at the relay supply terminal with key ON and while cranking (expect approx. battery voltage)
Signal parameters
- Battery voltage at relay supply: ~11.5–14.5 V (key ON/crank)
- Switched relay output to pump: near battery voltage when relay energized (within ~1 V of battery)
- Relay coil/control input: either ground-switched or +12 V commanded by module; command should toggle on key ON/crank
- Control signal idle: typically open/high-impedance when not commanded (no solid low if open-circuit)
- Expected continuity to pump: low resistance path from relay output to pump (few ohms for wiring; pump coil resistance varies)
Diagnostic algorithm
- Verify code and conditions: Use scanner to confirm P1234 is current/pending and review freeze-frame data and related DTCs.
- Check battery and fuses: Verify battery voltage and inspect/replace fuel pump/relay fuses and fusible links as needed.
- Visual inspection: Inspect relay, relay socket, wiring harness, connectors at relay and fuel pump for damage or corrosion.
- Swap relay: Replace or swap with a known-good identical relay and attempt to start; note any change.
- Measure at relay socket: Back-probe supply terminal (should be battery voltage) and control terminal while turning key to ON and during crank. Record voltages and compare to expected.
- Check ground: Verify good ground at relay and pump with continuity and voltage-drop tests under load.
- Verify pump feed at tank: With helper cranking or using a remote-start command, measure voltage at fuel pump connector. If no voltage but relay commanded, trace between relay and pump for open/short.
- Pump bench test: If feed and control appear correct but pump does not run, bench-test pump to confirm operation and current draw.
- Trace wiring: If voltage at relay output is low or drops under load, inspect for high-resistance connections, damaged wiring, or shorts and repair wiring as required.
- Module driver check: If wiring and relay are good but control signal is absent or incorrect, test/verify module driver outputs per factory procedures and consider ECU/BCM diagnosis or replacement as a last step.
- Clear codes and retest: After repairs, clear DTCs and perform full functional test (multiple key cycles, cranking, and road test) to confirm issue is resolved.
Likely causes
- Failed or stuck fuel pump relay
- Blown or high-resistance fuse supplying the relay
- Poor ground or corroded connector at relay or fuel pump
- Wiring short or open in relay control/supply circuit
- Battery low or weak causing low feed voltage
- Control module output (driver) fault
Fault status
Status
Fuel pump relay primary circuit voltage below expected threshold. Relay output or control path low—fuel pump may not receive proper feed; DTC stored.
Repair difficulty: Medium
Diagnostic time: 0.5-2.0 hours
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