OBD-II trouble code
P0231: Fuel Pump Secondary Circuit Low
SAE J2012 APR2002: fuel pump secondary circuit low. On the 1999 Ford PCED the monitor wants B+ on the fuel-pump monitor wire when the pump is commanded on. The pump motor is not in that cause list. The printed pair is the relay.
Quick facts
- System
- Powertrain
- Category
- Fuel & Air Metering
- Severity
- High severity
- Drivable
- No — stop driving until repaired
- Repair cost range
- $90 – $110 (fuel pump relay replacement, parts and labor, RepairPal national average updated 3 September 2026; the in-tank pump, the Mustang low-speed circuit, and the Lightning inertia switch are outside this pair)
- DIY difficulty
- Advanced DIY
Browse every code in P0200–P0299, or start from the full code library.
What does P0231 mean?
SAE J2012 APR2002 names P0231 Fuel Pump Secondary Circuit Low. Circuit low is measured with the circuit connected. P0230 is the primary, relay-coil side. This row is the secondary side, the feed after the relay.
The 1999 Ford PCED section 4 says the PCM monitors the fuel-pump monitor (FPM) circuit. The test fails if the PCM commands the pump on and B+ is not detected on FPM. During KOEO self-test the PCM commands the pump on so the test can run. The cause list is an open B+ circuit to the fuel-pump relay, an open FP PWR circuit between the relay and its connection to the FPM circuit, a damaged relay, or a damaged PCM, with the note that the engine will start in that PCM case. The in-tank pump is not named.
Two lines are model-specific and stay there. For a 4.6L Mustang, an open FP PWR circuit from the low-speed fuel-pump relay, through the resistor, to the FPM splice, and the aide says the engine will start. For a 5.4L supercharged Lightning, a damaged inertia fuel shutoff switch, its relay, or the related circuits. Do not look for a Lightning inertia switch on a vehicle that does not have that circuit, and do not look for the Mustang resistor on a single-speed pump.
Natural-gas applications: the same paragraph says the description applies to the fuel shutoff valve monitor and the fuel shutoff valve power circuits. That is the natural-gas note, not a gasoline pump.
An open power feed and a relay that does not close both keep B+ off the monitor wire. A short to ground on that feed can open the fuse and look the same. Test the short to ground with the relay removed and the pump unplugged so a motor winding is not the path you measure. A short to power is the high code, P0232, and it is a different command state.
RepairPal's relay average, $89–$108, is the printed pair because the relay is on the Ford list. The pump average, $1,344–$1,513, is outside it. Depressurize before a tank or a line is opened. This code's Ford test does not start by dropping the tank.
Common causes
- An open B+ feed to the fuel-pump relay, on the 1999 Ford list
- An open FP PWR wire between the relay and the FPM splice, on that list
- A damaged relay, which is the RepairPal job priced below
- A short to ground on the pump feed that opens the fuse, tested with the pump disconnected
- The 4.6L Mustang low-speed relay, resistor, and FPM splice, only on that note
- The 5.4L supercharged Lightning inertia switch or its relay, only on that note
- A damaged PCM, which the Ford cell says can set the code on an engine that still starts
- An in-tank pump, which that cause list does not name
Symptoms
- P0231 stored
- No B+ on the FPM circuit while the pump is commanded on, which is the Ford fail
- A no-start or a stall when the secondary feed is actually open
- An engine that still starts, which the Ford cell allows for the damaged-PCM case and for the 4.6L Mustang note
- A Lightning inertia-switch complaint, only on that vehicle
- A pump quote written before the relay and the FP PWR wire are tested
Diagnostic steps
- 1.Do not keep driving if the engine stalls when the pump drops out.
- 2.Separate this code from P0230 (relay coil) and from P0232 (FPM high while the pump is commanded off).
- 3.On a 1999-PCED Ford, use KOEO so the PCM commands the pump on, and look for B+ on FPM. Identify natural gas before treating the wire as a gasoline pump feed.
- 4.Check the inertia switch only on the 5.4L supercharged Lightning note, and the low-speed relay and resistor only on the 4.6L Mustang note.
- 5.Relay removed: B+ into the relay on the battery terminal of the socket, then the wire from the relay output to the FPM splice for an open. Both ends disconnected, ignition off, for continuity. Short to ground on that feed with the pump unplugged, so the motor is not the measured path.
- 6.RepairPal's relay check is a click plus resistance compared with the relay or the manual. Replace the relay. Do not rebuild it.
- 7.The Ford PCM line is what remains when the feed, the relay, and the FP PWR wire are intact, including the case where the engine still starts. It is not the relay price and not a pump.
- 8.Depressurize only if a pressurized fitting is opened. Verify B+ returns on FPM with the pump commanded on.
Repair cost
$90 – $110 (fuel pump relay replacement, parts and labor, RepairPal national average updated 3 September 2026; the in-tank pump, the Mustang low-speed circuit, and the Lightning inertia switch are outside this pair)
RepairPal's fuel pump relay replacement, updated 3 September 2026, averages $89–$108 (labor $41–$61, parts about $48), rounded to $90–$110. The fuel pump replacement average is $1,344–$1,513 and is outside the pair. The 1999 Ford P0231 causes name the relay and the power feed, not the pump motor. The 4.6L Mustang resistor circuit and the 5.4L Lightning inertia switch are not the national relay average. Check-engine diagnosis is $125–$183 and is separate.
Estimate your repair
Run the numbers for your vehicle
Open the Repair Cost Estimator with check engine light diagnosis preselected. Adjust labor rate and vehicle category to fit your situation.
DIY vs shop
This is an advanced DIY job. It typically requires specialty tools, scan-tool access, lifting equipment, or careful sequencing to avoid causing new failures. Plan for extended downtime and have a backup vehicle. Most owners are better served by a shop that has done this repair before.