P0231
Fuel Pump Secondary Circuit Low
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The P0231 trouble code — Fuel Pump Secondary Circuit Low — indicates that your vehicle's Powertrain Control Module (PCM) has detected a lower-than-expected voltage or signal in the fuel pump's secondary (feedback) circuit. This circuit allows the PCM to verify that the fuel pump is actually receiving power and operating as commanded. When the voltage in this circuit falls below the expected threshold, the PCM logs P0231 and illuminates the Check Engine Light.
The most common culprits behind P0231 are a faulty fuel pump relay, a failing fuel pump itself, or wiring issues such as corrosion or damaged insulation in the fuel pump circuit. Diagnosing this code effectively starts with checking the relay — it's the cheapest and easiest component to test and replace. From there, a multimeter can be used to verify voltage at the fuel pump connector and check for proper ground. A fuel pressure test can help confirm whether the pump is operating within spec, even if it is receiving voltage.
While the vehicle may still drive in some cases, P0231 should not be ignored. A compromised fuel delivery system can cause stalling at highway speeds or during critical moments, creating a safety hazard. Most DIY-capable owners can handle relay and wiring inspections at home, making this a moderately approachable repair. However, if the fuel pump itself needs replacement, the job typically requires dropping the fuel tank or accessing an in-tank pump through an access panel, which may call for professional assistance depending on your vehicle and comfort level.
Severity
Symptoms
- •Engine cranks but is difficult to start or won't start
- •Engine stalls unexpectedly, especially at low speeds or idle
- •Loss of power during acceleration or under load
- •Engine sputtering or hesitating when throttle is applied
- •Fuel pressure readings below specification
- •Check Engine Light illuminated on dashboard
Likely Causes
The fuel pump relay controls power delivery to the fuel pump secondary circuit. A failing or failed relay is one of the most common causes of this code, as it can interrupt voltage to the pump intermittently or completely.
The fuel pump itself may be drawing insufficient current or failing to operate correctly, which the PCM detects as a low-voltage condition in the secondary circuit. Fuel pumps degrade over time, especially in vehicles that are frequently run with low fuel levels.
Damaged, corroded, or frayed wiring between the fuel pump relay and the fuel pump can cause a voltage drop that triggers the P0231 code. This is especially common in older vehicles or those exposed to moisture and road salt.
A loose or corroded ground connection at the fuel pump or in the fuel pump circuit can cause abnormally low voltage readings, leading the PCM to set this fault code.
In rare cases, the PCM itself may be misreading the fuel pump circuit voltage or have an internal fault affecting the fuel pump driver circuit. This is typically a diagnosis of exclusion after other causes are ruled out.
Estimated Cost
Professional Repair
Includes parts + labor
Common Fixes
- Replace the fuel pump relay (inexpensive first step, often $15–$30 part)
- Inspect and repair damaged or corroded wiring in the fuel pump circuit
- Clean or replace corroded ground connections at the fuel pump and chassis
- Replace the fuel pump assembly if pump motor has failed or is drawing incorrect current
- Test and replace the PCM only after all other causes have been eliminated