P0231 Fuel Pump Secondary Circuit Low Voltage

P0231 is an OBD-II diagnostic trouble code that means the engine control module (ECM/PCM) has detected lower-than-expected voltage in the fuel pump secondary circuit.

In practical terms, the ECU is monitoring the electrical side of the fuel pump system and sees that the voltage reaching or being returned from the monitored circuit is not behaving as expected.

The important distinction is that P0231 is primarily an electrical fault code. It does not automatically mean that fuel pressure is low, that the fuel pump itself has failed, or that the engine is not receiving enough fuel.

The actual cause can be somewhere between the fuse, relay, fuel pump control module, wiring, connector, ground, and fuel pump.

What Does “Secondary Circuit Low Voltage” Mean?

The wording of P0231 can be confusing because “secondary circuit” does not necessarily mean that the vehicle has two fuel pumps.

Depending on the vehicle, the fuel pump system may contain:

• A conventional fuel pump relay

• A fuel pump control module

• A dedicated electronic pump driver

• A PWM-controlled fuel pump

• A fuse and relay arrangement

• A variable-speed fuel pump system

• An ECM-controlled fuel pump circuit

The exact architecture depends on the manufacturer.

“Low voltage” means the control module has detected an electrical voltage condition below the expected value in the monitored fuel pump secondary circuit.

It is therefore possible to have P0231 even when the fuel pump is still operating.

For example, the pump may receive enough voltage to run at idle but experience a significant voltage drop when the engine demands more fuel. In that situation, the vehicle may start and drive normally under light load but hesitate, lose power, or stall during acceleration.

Why Is the Fuel Pump Circuit So Important?

The fuel pump has to deliver fuel from the tank to the engine at the pressure and flow required by the fuel system.

On a traditional system, the pump may operate at a relatively fixed speed. Modern vehicles, however, increasingly use electronic fuel pump control.

The ECU or fuel pump control module may vary pump speed according to:

• Engine load

• Engine speed

• Fuel pressure

• Accelerator position

• Fuel temperature

• Starting conditions

• Emissions requirements

Because of this, simply measuring whether the pump turns on is not always enough.

A pump can run while the electrical circuit still has a problem.

A weak power connection, corroded connector, damaged ground, failing relay, or excessive resistance in the wiring can cause voltage to drop when pump current increases.

Common Causes of P0231

There is no single component responsible for this code. Several different faults can create the same electrical condition.

Weak or failing fuel pump relay

On vehicles equipped with a conventional fuel pump relay, worn relay contacts can create excessive resistance. The relay may still click and operate, but the voltage delivered to the pump may be lower than expected.

Damaged fuel pump wiring

A partially broken wire, corroded section, damaged insulation, or high-resistance connection can cause voltage loss between the power source and the pump.

This can become especially noticeable under high electrical load.

Poor fuel pump ground

The ground side of the pump circuit is just as important as the power side.

A loose, corroded, contaminated, or damaged ground connection can create a voltage drop and cause the control module to detect an abnormal secondary circuit.

Corroded or loose connectors

Fuel pump connectors are particularly important because they are often exposed to heat, vibration, moisture, and contamination.

A connector may look acceptable externally while its terminals have become loose or developed corrosion.

Failing fuel pump

A worn fuel pump motor can draw abnormal current or develop internal electrical problems.

A pump that is beginning to fail may operate correctly when cold but become unreliable as it heats up.

It can also cause voltage behavior that differs from what the control module expects.

Fuel pump control module problem

Some modern vehicles use a dedicated Fuel Pump Control Module (FPCM) instead of simply switching the pump directly through a relay.

A fault inside the module, its power supply, ground, or communication with the ECM can contribute to P0231.

Fuse or power supply problem

A damaged fuse connection, poor fuse-box terminal, or problem in the main power supply can introduce resistance into the pump circuit.

A fuse that has not blown does not necessarily prove that the entire power path is healthy.

ECM/PCM fault

The engine control module is possible, but it should normally be considered after the rest of the circuit has been properly tested.

Replacing the ECU simply because P0231 is present is usually poor diagnostic practice.

Symptoms You May Notice

P0231 can produce very different symptoms depending on how severe the voltage problem is.

Possible symptoms include:

• Check Engine Light

• Hard starting

• Longer-than-normal cranking

• Engine hesitation

• Poor acceleration

• Loss of power under load

• Engine stalling

• Intermittent no-start condition

• Rough running

• Fuel pressure that drops during acceleration

• Reduced engine performance

• Limp-home operation on some vehicles

In mild cases, the driver may notice almost nothing other than the Check Engine Light.

In more serious cases, the engine may start normally and then stall when fuel demand increases.

Why Can the Car Run Normally With P0231?

This is one of the most interesting aspects of this fault.

Imagine a fuel pump circuit with a poor electrical connection.

At idle, the pump requires relatively little effort and the available voltage may still be sufficient for it to operate.

When the driver accelerates hard, fuel demand increases. The pump may need to work harder, and the electrical load changes.

If there is excessive resistance in the circuit, the voltage available to the pump can fall significantly.

The result can be a vehicle that:

• Starts normally

• Idles normally

• Drives normally at low load

• Develops hesitation during acceleration

• Loses power at higher engine load

• Eventually stalls

This is why a basic “the pump is running, so the pump circuit is good” test can be misleading.

P0231 Does Not Automatically Mean Low Fuel Pressure

The word “Low” in P0231 refers to the electrical condition being monitored.

It should not automatically be interpreted as:

“The fuel pressure is low.”

There can certainly be a relationship between the two.

For example, low voltage at the pump can reduce pump speed and consequently cause low fuel pressure.

But the diagnostic process should separate these two questions:

Is the electrical voltage reaching the pump correct?

and

Is the fuel pressure produced by the pump correct?

Both should be checked when the symptoms indicate a fuel-delivery problem.

A Voltage Drop Test Can Be Extremely Useful

A voltage drop test can reveal problems that an ordinary voltage measurement may miss.

Suppose a technician measures voltage at a connector with the pump disconnected and sees an apparently normal value.

That does not necessarily prove that the circuit can deliver sufficient current.

A damaged connection may show normal voltage with almost no load but lose significant voltage once the pump is operating.

For this reason, the circuit should be evaluated under the appropriate operating conditions using the manufacturer's diagnostic procedure.

The power side and ground side should both be checked.

This can help locate:

• High-resistance connectors

• Weak relay contacts

• Damaged wiring

• Poor grounds

• Corroded terminals

• Fuse-box problems

The exact acceptable voltage-drop limits should come from the vehicle manufacturer's specifications rather than a universal number.

How P0231 Should Be Diagnosed

A good diagnosis starts with the entire fuel pump system rather than immediately replacing the pump.

1. Scan all stored codes

Read every DTC stored in the ECM and other relevant modules.

A communication, power supply, fuel pressure, or fuel pump control code stored alongside P0231 may provide an important clue.

2. Check the freeze-frame data

Look at the conditions under which P0231 was recorded.

Pay attention to:

• Engine RPM

• Engine load

• Vehicle speed

• Battery voltage

• Coolant temperature

• Fuel-related data

• Whether the engine was starting, idling, or accelerating

If the code repeatedly appears during high-load operation, that can point toward a circuit problem that becomes significant when pump demand changes.

3. Identify the exact fuel pump architecture

Determine whether the vehicle uses:

• A relay

• A fuel pump control module

• Direct ECM control

• PWM pump control

• Another manufacturer-specific arrangement

This prevents incorrect assumptions about what “secondary circuit” means on that particular vehicle.

4. Inspect the wiring and connectors

Inspect the complete accessible circuit.

Look for:

• Corrosion

• Loose terminals

• Burnt connectors

• Melted plastic

• Damaged insulation

• Previous repairs

• Broken wires

• Poor grounds

• Water intrusion

The fuel tank connector and fuel pump module connector deserve particular attention.

5. Check power and ground

Verify the pump's power supply and ground according to the manufacturer's procedure.

Do not rely only on an unloaded voltage measurement.

The circuit needs to be evaluated while the pump is operating.

6. Check for voltage drop

Measure voltage drop across the relevant power and ground portions of the circuit under load.

A significant drop can identify resistance that would otherwise be difficult to find.

7. Test the relay or fuel pump control module

If a relay is used, inspect its operation and terminals.

If an electronic fuel pump control module is installed, its power, ground, command, output, and communication should be checked according to the manufacturer's diagnostic procedure.

8. Evaluate the fuel pump

Only after the electrical supply has been properly assessed should the pump itself be condemned.

Pump current draw, operating voltage, fuel pressure, and fuel flow can provide valuable information.

A pump that is mechanically worn may also create an electrical problem by drawing abnormal current.

9. Check actual fuel pressure

If the vehicle exhibits hard starting, hesitation, stalling, or loss of power, fuel pressure should be evaluated separately.

Compare measured pressure with the manufacturer's specification and, where applicable, compare commanded and actual pressure.

A Useful Diagnostic Clue: When Does P0231 Appear?

The operating conditions can sometimes tell you more than the code itself.

If P0231 appears immediately when the ignition is switched on, suspect a basic electrical or control problem.

If it appears only after the engine starts, the circuit may be failing under operating conditions.

If it appears during hard acceleration, investigate voltage drop, pump current, fuel pressure, and pump control under load.

If it appears after the vehicle has been driven for a long time, heat-related electrical resistance or a fuel pump that becomes weak when hot may be worth investigating.

If it is completely intermittent, pay particular attention to connectors, grounds, wiring movement, vibration, and temperature.

P0231 and Fuel Pump Failure

A failed fuel pump is certainly possible, but replacing it immediately is not always the correct repair.

Consider a simple example.

The pump itself may be perfectly healthy, but the ground connection near the fuel tank may be heavily corroded.

The result can be:

Good pump + poor electrical connection = insufficient voltage at the pump

The ECU may then detect an abnormal secondary circuit condition.

Replacing the pump in this situation does not repair the underlying electrical problem.

The same principle applies to relays, wiring, connectors, and fuel pump control modules.

P0230, P0231, P0232 and P0233

These codes belong to the same general fuel pump circuit family but describe different conditions.

P0230 – Fuel Pump Primary Circuit Malfunction

A general malfunction is detected in the primary fuel pump control circuit.

P0231 – Fuel Pump Secondary Circuit Low Voltage

The monitored secondary circuit has lower-than-expected voltage.

P0232 – Fuel Pump Secondary Circuit High Voltage

The monitored secondary circuit has higher-than-expected voltage.

P0233 – Fuel Pump Secondary Circuit Intermittent

The monitored secondary circuit behaves abnormally in an intermittent manner.

The exact electrical architecture varies between manufacturers, so the terms “primary” and “secondary” should always be interpreted using the vehicle's wiring diagram and service information.

Can You Keep Driving With P0231?

It depends on the severity of the fault.

If the vehicle is running normally and the code is intermittent, it may still be possible to drive it to a repair facility.

However, a fuel pump electrical problem can become worse without warning.

If you experience:

• Engine stalling

• Severe hesitation

• Major power loss

• Hard starting

• Intermittent no-start

• Fuel pressure dropping under load

the vehicle should not be treated as reliable for normal driving.

A fuel pump that intermittently loses power can cause the engine to shut down unexpectedly, including while driving.

The Correct Repair Depends on the Test Results

P0231 can ultimately be repaired by replacing or repairing very different components.

Depending on the diagnosis, the solution could be:

• Repairing damaged wiring

• Cleaning or replacing a corroded connector

• Repairing a ground connection

• Replacing a fuel pump relay

• Repairing a fuse-box connection

• Replacing a fuel pump control module

• Replacing the fuel pump

• Repairing the pump power supply

• Correcting a control circuit problem

• Repairing an ECM-related fault

There is no universal rule that says P0231 = replace the fuel pump.

The code tells you that the control module sees a low-voltage condition in the monitored fuel pump secondary circuit. The diagnostic work must then determine where that voltage is being lost and whether the pump itself is responsible.

The Most Effective Way to Approach P0231

Think of P0231 as a problem in the electrical path that allows the fuel pump to do its job.

Start with the code and freeze-frame data. Identify how the particular vehicle controls its fuel pump. Then follow the electrical path from the power source through the relay or control module, wiring and connectors, to the pump and its ground.

If the electrical circuit passes its tests, move on to pump performance and fuel pressure.

That approach prevents two common mistakes: replacing a perfectly good fuel pump because of a wiring problem, or assuming that a normal fuel pressure reading means the electrical circuit is healthy.

With P0231, the question is not simply “Is the fuel pump working?” It is “Does the fuel pump receive and return the correct electrical supply under the conditions in which the fault occurs?”