P0231 Code: Fuel Pump Secondary Circuit Low
P0231: Test Feedback Under Load
Separate low secondary-circuit voltage from fuel-pressure and pump-performance faults.
DIAGNOSTIC FOCUS
- FAULT TYPE
- Fuel-pump secondary feedback path
- FIRST CHECKS
- Command, relay/module, loaded voltage and pump current
Secondary can mean monitored output/feedback after a relay or module; topology differs between relay, returnless and smart-pump systems.
P0231 means Fuel Pump Secondary Circuit Low. The controller saw the monitored fuel-pump secondary circuit remain low relative to the commanded pump state or expected feedback. The code does not prove low fuel pressure or a failed pump motor, and it is different from a primary control-circuit code.
Save the complete fault event before clearing it: fuel-pump command/duty, secondary feedback, system voltage, low-side pressure if exposed, engine speed/load, start/stall conditions and companion P0230, P0232, P025A or P0627 faults. Map the relay or fuel-pump control module, load-test feed and ground at the pump, compare command with feedback and pressure, then identify wiring, relay/module or pump current as the cause.
What Does the P0231 Code Mean?
In a simple relay system, the secondary circuit is the high-current pump output and may also be monitored by a feedback wire. Returnless systems can use a fuel-pump control module and variable duty instead of a single relay.
P0231 says the monitored secondary state is low. A failed relay contact, voltage drop, short to ground, open feedback, weak supply, overloaded pump or module fault can create that state.
Fuel pressure can be low because the circuit cannot deliver power, but pressure is a separate physical result. Conversely, a pressure problem from restriction can exist with normal secondary electrical feedback.
Save whether the pump was requested and what secondary feedback or module status reported.
Measure feed and ground at the pump under command; unloaded voltage does not prove current capacity.
Relate secondary-circuit evidence to measured fuel pressure/flow without treating either one as the other.
P0231 Symptoms, Severity, and Driving Safety
The pump may not build supply pressure, but ignition, immobilizer and other causes need separate evidence.
A dropping secondary circuit can interrupt fuel delivery under vibration or load.
Voltage drop or weak pump current capacity may appear when demand rises. Avoid unsafe full-load tests.
A relay or module feedback fault may be historical while the pump presently runs.
Heat and terminal resistance can make the fault appear after a drive and disappear after cooling.
This is not explained by P0231 and requires immediate leak control before electrical testing.
Crank-no-start, stalling in traffic, loss of power, visible fuel, strong odor or unstable pressure requires immediate attention and often towing. Fuel vapor plus high-current wiring makes improvised testing unsafe.
P0231 Causes: Relay Output, Feedback Wire, Module, or Pump Load?
Follow the command from controller to relay/module and the high-current path to the pump. Measure loaded voltage, ground and current before condemning the pump or fuel-pressure hardware.
Burned Relay Contacts or Socket
The relay coil may work while high-current contacts or terminals drop voltage.
How to prove it: Command the pump and voltage-drop the input/output and socket under load.
Check firstOpen/High-Resistance Secondary Wiring
Corrosion, chafing or loose terminals can reduce pump voltage and feedback.
How to prove it: Measure both sides of the circuit under pump current and isolate the failed section.
Check firstPoor Pump Ground
A ground path may pass continuity yet develop significant voltage drop under load.
How to prove it: Measure loaded ground drop at the pump and inspect shared ground effects.
Verify directlyOverloaded or Failing Pump Motor
Worn bearings or internal electrical faults can pull supply down or trigger module protection.
How to prove it: Compare startup/running current and pressure/flow with the OEM procedure.
Verify directlyFuel-Pump Control Module Fault
Variable-speed systems can fail internally or lose command, power, ground or communication.
How to prove it: Read module codes and prove its inputs/outputs before replacement or programming.
Verify directlyFeedback/ECM Driver or Calibration Issue
A feedback circuit can be low even if pump pressure is acceptable, or a documented control issue may apply.
How to prove it: Prove the feedback wire and module powers/grounds and check VIN-specific service information last.
Final branchDo not jumper unknown relay cavities or apply battery voltage to a smart pump module. Secondary-circuit topology, current limits and command strategy vary.
Using UR800 to Organize P0231 Evidence
The iCARZONE UR800 Full-System Bidirectional Scan Tool with ECU Coding supports full-system diagnostics, available DTCs, freeze frame, ECU information and live data. Save pump request/duty, secondary feedback, fuel-pump module codes/status, system voltage, low-side pressure where available, engine speed/load and start/stall snapshots. Compare command, electrical response and pressure on the same timeline.
The official product page advertises bidirectional control and ECU coding. Coverage varies by VIN, ECU and software. A scan command cannot prove loaded conductor capacity, ground voltage drop, pump current, physical fuel pressure/flow or safe depressurization. Coverage and output tests vary by VIN.
Use a fused/load-safe circuit method, current probe, rated fuel-pressure/flow equipment and the exact wiring diagram. Keep sparks away and do not bypass crash or safety shutoff logic.
iCARZONE UR800 Full-System Bidirectional Scan Tool with ECU Coding
Preserve fuel-pump command, module feedback and pressure context before jumping relays or replacing the pump.
Price and availability checked September 2, 2026. Prices shown are USD. Recheck the current offer, stock and required vehicle/function coverage before purchase.
How to Diagnose P0231 With Command, Feedback, and Pressure
P0231 diagnosis must preserve the distinction between electrical feedback and physical fuel pressure. Map the system, command it safely and make simultaneous loaded voltage, current and pressure observations.
P0231 Quick Diagnostic Path
-
1
Capture Pump-Control Evidence
- Save P0231 status, freeze frame, all pump/module codes and the start/stall event before clearing.
- Record command/duty, secondary feedback, system voltage, pressure data, speed/load and whether the fault is current or history.
-
2
Identify Relay, Module, and Secondary Path
- Use VIN wiring to locate primary control, secondary output/feedback, pump, grounds and safety shutoff logic.
- Confirm whether the system is fixed-speed relay, returnless variable-speed or smart-pump controlled.
-
3
Inspect Safely and Check Shared Supply
- Look for odor or leaks before electrical work, then inspect fuses, relay/module connectors, grounds and harness routing.
- Do not bypass inertia/crash controls or energize exposed fuel-system wiring.
-
4
Load-Test Secondary Feed and Ground
- Command the pump with the approved method and measure relay/module output, pump feed, ground drop and current.
- An unloaded battery-voltage reading is not a pass; stop for heating, arcing or abnormal current.
-
5
Compare Electrical Results With Pressure/Flow
- Use rated equipment to confirm low-side pressure or flow at the required condition.
- If loaded electrical values are correct but pressure is wrong, follow filter, regulator and pump hydraulic tests separately.
-
6
Repair and Verify Starting and Load Operation
- Repair the proven relay/socket, harness, ground, module, pump or feedback issue using sealed fuel-safe procedures.
- Confirm reliable prime/start, stable loaded voltage/current and pressure, normal safety shutdown and no returning P0231.
How to Fix P0231 and Verify the Repair
Repair Relay, Socket, or Secondary Wiring
Restore high-current contacts, sealed terminals and routing where voltage drop is proven.
Restore Pump Ground or Shared Supply
Correct loaded ground/feed faults and verify other affected circuits.
Replace a Proven Pump or Control Module
Use command, loaded current and pressure/flow evidence to justify the exact component.
Repair a Verified Feedback/Controller Issue
Address the feedback circuit or controller only after pump power and hydraulic output are proven.
Control and Loaded-Circuit Diagnosis
VIN-specific quoteSystem mapping, command/feedback and voltage-drop tests establish scope.
Relay/Connector/Harness Repair
VIN-specific quoteHeat damage and access determine labor.
Pump/Module and Tank Access
VIN-specific quoteFuel handling, integrated sender and programming can affect parts and time.
Pressure/Flow and Road Verification
VIN-specific quoteRated gauges plus start/load confirmation should be itemized.
Ask the estimate to separate relay/harness, ground, module, pump and pressure-system work. P0231 alone does not justify a pump-module assembly.
Verification needs a reliable prime and loaded electrical/pressure response at the original condition. A pump that runs from a jumper does not prove the vehicle circuit or safety logic.
P0231 Pump Architectures, Fuel Safety, and DIY Limits
Ford's 2024–2025 OBD summary defines P0231 as fuel-pump secondary circuit low in a single-speed monitored relay system and separates it from P0232 high.
Secondary may be relay output/feedback or a module-controlled pump path.
Low feedback can cause low pressure, but pressure restriction can exist with a healthy circuit.
Crash/inertia shutoff and prime strategy must remain intact during testing.
Ford investigation material also lists P0231 under fuel-component monitoring. Other vehicles use modules and duty control, so relay details and thresholds are not portable.
- Save command, feedback and start conditions.
- Inspect for odor/leaks before testing.
- Check accessible fuses/connectors by diagram.
- Stop for stalling or visible fuel.
- Jumper unknown relay/module circuits.
- Open tank or pressurized fuel connections.
- Measure pump current without fused methods.
- Bypass crash shutoff or program modules.
Frequently Asked Questions About P0231
What does P0231 mean?
What is the secondary circuit?
Does P0231 mean the fuel pump is bad?
Can a clogged filter set P0231?
Can I jumper the fuel-pump relay?
Can the vehicle be driven?
Can UR800 diagnose P0231?
How is the repair verified?
Sources and Technical Review Notes
- Ford 2024–2025 Gasoline OBD System Summary — Defines P0231 and explains single-speed monitored relay secondary-circuit operation.
- NHTSA Ford Investigation Appendix — Lists P0231 Fuel Pump Secondary Circuit Low in OEM monitoring data.
- Ford OBD System Operation Summary — Provides output-driver feedback concepts and fuel/relay diagnostic context for circuit monitoring.
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