How to Check If the Fuel Pump Is Bad

A quick way to check for a bad fuel pump is to verify commanded fuel pressure against specification with a gauge or scan tool, confirm proper power and ground at the pump, listen for the 2–3 second “prime” sound at key-on, and rule out simpler causes such as a blown fuse, faulty relay, clogged filter, or wiring issues. Once those basics are covered, measure pressure and flow under load and compare to factory data; low pressure/flow with proper voltage and ground typically confirms a failing pump.

What a Failing Fuel Pump Looks Like

Before grabbing tools, note the symptoms. Fuel pumps tend to fail gradually—especially under heat and load—but can also die suddenly. These clues help you decide whether to focus on the pump or look for other causes like ignition or airflow issues.

• Long crank or no-start, especially after the vehicle sits or when hot
• Whining or humming from the tank that changes with engine load
• Engine stumbles, surges, or hesitates on acceleration or highway passing
• Loss of power on hills, high-speed cruising, or under heavy load
• Random stalling that improves after cooling down
• Lean codes and high positive fuel trims; misfires at higher RPM/load

While these signs point toward a supply issue, they overlap with clogged filters, restricted lines, failing relays, and sensor faults. That’s why testing pressure, power, and grounds is essential.

Tools and Data You’ll Need

You can diagnose most fuel pump issues with basic tools and reliable service information. Modern vehicles often allow pressure checks through a scan tool without opening the fuel system.

• OBD-II scan tool that reads fuel trims and (if supported) fuel pressure/commanded duty cycle
• Fuel pressure gauge with the correct adapter(s) for your vehicle, or an approved in-line test kit
• Digital multimeter (DMM) for voltage, continuity, and voltage drop tests
• 12V test light or fused jumper for relay/pump activation (as appropriate)
• Back-probing pins, wiring diagrams, and factory service specs for fuel pressure and flow
• Safety equipment: eye protection, gloves, rags, catch pan, fire extinguisher

Having both pressure specs and wiring diagrams lets you verify not only that the pump can make pressure and flow but also that it’s receiving proper power and ground from the vehicle.

Step-by-Step Diagnosis (Gasoline Vehicles)

The following sequence takes you from non-invasive checks to definitive pump testing. Always refer to factory procedures for your exact model.

1. Listen for prime: Turn the key to ON (engine off). You should hear a 2–3 second hum from the tank. No sound doesn’t prove a bad pump but raises suspicion for fuses, relays, wiring, or the pump itself.
2. Scan for codes and data: Check for DTCs and look at short- and long-term fuel trims. High positive trims under load suggest low fuel supply. If your scan tool reports fuel pressure, compare to spec at idle and while snapping the throttle.
3. Check fuses and relays: Inspect the fuel pump fuse and swap the pump relay with an identical known-good relay if available. Some cars use a fuel pump control module—scan for related module codes.
4. Verify power and ground at the pump: Back-probe the pump connector or test at the relay output. With the pump commanded on or while cranking, you should see near battery voltage on the power feed and minimal voltage drop on ground (<0.1–0.2V ideally; under 0.5V max under load).
5. Measure fuel pressure: Attach a gauge to the service port (if equipped), or use an approved in-line adapter. Compare key-on, engine-off (KOEO), idle, and under-load readings to factory specs. For many port-injected systems, 40–60 psi is typical; consult exact specs. Pressure that’s low and doesn’t rise with load indicates supply issues.
6. Assess pressure drop and hold: After shutting the engine off, pressure should hold reasonably steady for several minutes. Rapid drop can indicate a leaking check valve in the pump, leaking injector, or regulator fault.
7. Volume/flow test: If pressure is borderline, perform a volume test per service info (for example, measure the amount of fuel delivered over a set time). Adequate pressure with low flow can still cause starvation under load.
8. Command the pump: Use a scan tool’s bi-directional control or a fused jumper to run the pump directly. If the pump won’t run with confirmed power/ground, it’s failed. If it runs but pressure is low, suspect the pump, filter, or line restriction.
9. Differentiate pump vs. restriction: If procedure allows, compare pressure before and after the filter or check return/regulator function. A clogged filter or pinched line can mimic a bad pump. Only perform “deadhead” or block-off tests if explicitly approved by the service manual.
10. Check current draw (advanced): With an amp clamp, measure pump current while running. A worn pump often draws higher current or shows erratic ripple. Compare to factory normal values.

If voltage/ground are good and both pressure and flow are below spec, the pump is the likely failure. If voltage is low or ground drop is high, repair the circuit first; a healthy pump can’t perform without proper electrical supply.

Direct-Injection Nuances (LPFP vs. HPFP)

Gasoline direct-injection (GDI) systems use a low-pressure in-tank pump (LPFP) and a mechanically driven high-pressure pump (HPFP). You’ll need to judge both sides—often without opening the high-pressure circuit.

• Monitor pressures via scan tool: Low side (LPFP) is commonly in the 50–90 psi range; high side can span 500–3,000+ psi at idle/load depending on model. Always compare to your vehicle’s spec and operating mode.
• If low side is low: Suspect the in-tank pump, wiring, control module, filter, or a tank pickup issue. The HPFP can’t create rail pressure if the LPFP starves it.
• If low side is normal but high side is low: Suspect HPFP wear, a leaking pressure regulator, rail leak, or injector leakage. Avoid opening high-pressure lines; use scan data and commanded vs. actual comparisons.
• Command tests: Many GDI vehicles allow you to command LPFP duty cycle and monitor response. A weak response or slow pressure rise indicates a failing LPFP or restricted filter.

Because high-pressure fuel can exceed 2,000 psi, do not crack open high-pressure lines. Diagnostic decisions should rely on scan data and approved tests only.

Diesel Considerations

Diesel systems are sensitive to air, water contamination, and pressure integrity. Testing resembles GDI in that you evaluate both low- and high-pressure sides with a scan tool and approved procedures.

• Lift pump vs. high-pressure pump: Verify the low-pressure (lift) supply first; many no-starts are due to a weak or non-functioning lift pump or clogged filter/water separator.
• Prime and bleed per factory procedure: Air in lines can mimic pump failure. Use the hand primer or electric prime routine and verify stable low-side pressure.
• Scan rail pressure: Compare commanded vs. actual. If low-side is correct but rail pressure won’t build, suspect the high-pressure pump, leaks, or injector return overflow.
• Contamination check: Look for metal debris in filters and water contamination; these can destroy pumps and injectors and require system-wide remediation.

Because diesel pressures are extreme and contamination risks are high, follow factory safety procedures closely and consider professional testing if rail pressure won’t track command.

Ruling Out Look-Alikes

Several faults can duplicate bad fuel pump symptoms. Excluding them prevents unnecessary pump replacement.

• Clogged fuel filter or restricted line/strainer
• Faulty fuel pump relay, control module, inertia switch (where equipped), or poor grounds
• Vacuum leaks causing lean conditions and stumbles
• Failing crankshaft/camshaft sensors causing intermittent stall/no-start
• MAF/MAP sensor errors skewing fueling under load
• Faulty fuel pressure regulator on return-style systems
• Contaminated or incorrect fuel, especially after a recent fill-up
• Ignition faults (coils, plugs) that show up as high-load misfires

If any of these are present, correct them first and retest. A healthy pump can’t compensate for upstream restrictions or electrical control failures.

Common OBD-II Codes Tied to Fuel Pump or Pressure

Diagnostic trouble codes offer solid clues. The following codes often appear in fuel pump-related failures, though they don’t confirm a pump by themselves.

• P0087: Fuel rail/system pressure too low
• P0089: Fuel pressure regulator performance
• P0191: Fuel rail pressure sensor range/performance
• P0230/P0231/P0232: Fuel pump primary/secondary circuit faults
• P0627/P0628: Fuel pump control circuit open/low
• P2635: Fuel pump “A” low flow/performance
• P0171/P0174: System too lean (banks 1/2), often due to low fuel supply or vacuum leaks
• P228C/P228D (GDI): Fuel pressure regulator control performance

Use codes to guide testing, then confirm with pressure, flow, and electrical checks before replacing parts.

Safety Essentials

Fuel system work involves fire risk and high pressures. Take proper precautions every time you test.

• Work in a well-ventilated area away from sparks, cigarettes, or hot surfaces
• Wear safety glasses and gloves; keep a class B fire extinguisher nearby
• Relieve fuel pressure per service manual before opening low-pressure lines; never open high-pressure GDI/diesel lines
• Use only fused jumpers and proper back-probing techniques; avoid piercing insulation unnecessarily
• Support the vehicle securely if accessing the tank or pump module
• Clean spills immediately and dispose of fuel-soaked materials properly

Adhering to these basics reduces the risk of injury and prevents damage to the vehicle and environment.

When to Replace the Pump

Replace the pump if you confirm proper power and ground, yet pressure and/or flow remain below spec under load, or the pump won’t run when directly commanded. Also consider replacement if pressure bleed-down points to a failed internal check valve and other potential leaks are ruled out. On high-mileage vehicles, replacing the in-tank filter/strainer and inspecting the tank for debris is prudent during pump replacement.

Cost and Parts Quality

Quality matters. OEM or reputable aftermarket pump modules typically last longer and include updated strainers, seals, and level senders. Expect parts costs to vary widely by vehicle; labor rises if the tank must be lowered. Avoid low-cost, no-name pumps that often have short service lives.

Summary

To check if a fuel pump is bad, verify it electrically and hydraulically: listen for prime, scan for codes and fuel trim clues, confirm power/ground, and measure pressure and flow against factory specs under real operating conditions. Rule out relays, filters, restrictions, and sensor issues. On direct-injection and diesel systems, compare commanded and actual pressures on both low and high sides via scan data. When voltage is good but pressure/flow are not, the pump is the likely culprit.

How do you test a bad fuel pump?

To check if a fuel pump is bad, first listen for the typical humming sound when you turn the key to the ON position; its absence or a loud whine can indicate a problem. You can then use a fuel pressure gauge to test the pressure at the fuel rail, comparing the reading to the manufacturer’s specifications for your vehicle. If the pressure is too low or non-existent, the pump is likely failing. Other signs of a bad fuel pump include engine sputtering, loss of power, stalling, difficulty starting, and poor fuel economy. 
1. Listen for the Fuel Pump 

• Check for the priming sound: Turn the ignition key to the ON position (without starting the engine) and listen for a few seconds. 
• Listen near the fuel tank: A normal, healthy fuel pump emits a low, whirring or humming sound as it builds pressure. 
• What to listen for:
  • No sound: This could indicate a failed pump or a blown fuse/relay. 
  • Loud, whining sound: This suggests the pump is struggling and may be failing, notes Highflowfuel.com and AutoZone.com. 

2. Check Electrical Components 

• Locate the relay and fuse: Find the fuel pump relay and fuse in your vehicle’s fuse box.
• Inspect for damage: Remove the fuse to check if it’s blown.
• Test the relay: A faulty relay can also prevent the pump from working. You can test it by swapping it with a similar, known-good relay from a non-critical system, according to YouTube.

3. Perform a Fuel Pressure Test 

• Connect a fuel pressure gauge: This is the most reliable way to confirm a pump issue. 
• Find the test port: On many vehicles, you can connect the gauge to a test port (often a Schrader valve) on the fuel rail. 
• Compare pressure to specifications: Start the engine and observe the pressure reading on the gauge. 
• Low pressure indicates a problem: If the pressure is significantly lower than what your manufacturer specifies, the fuel pump needs to be replaced, says AutoZone.com. 

4. Observe for Symptoms

• Stalling and difficulty starting: A weak or failing pump can cause your car to stall or struggle to start, especially when hot, notes stevenagetyresandservice.co.uk. 
• Loss of power: You may notice a significant drop in power, particularly when accelerating or driving uphill. 
• Engine sputtering: This can happen if the pump isn’t delivering enough fuel to the engine. 
• Decreased fuel economy: A faulty pump might also contribute to an increase in fuel consumption. 

How do I know if my fuel pump is to be replaced?

You might need a new fuel pump if you hear a whining noise from the gas tank, the engine struggles to start or cranks for a long time, the engine stalls, loses power under acceleration or while driving uphill, experiences surges or dips in power, or you notice a significant drop in fuel efficiency. A check engine light with codes like P0087 (low fuel pressure) or a fuel pressure test showing insufficient fuel can also confirm a failing pump.
 
Sounds

• Whining/Whirring Noise: Opens in new tabA high-pitched whine from the fuel tank area can indicate a struggling or faulty pump. 
• No Priming Sound: Opens in new tabIf you normally hear a humming or whirring sound for a few seconds when turning the ignition on, but it’s now gone, it could be a fuel pump issue. 

Starting & Driving Issues

• Trouble Starting: The engine may crank but fail to start, or it may take a very long time to start. 
• Stalling: The engine may unexpectedly stall while driving or at stop lights, especially when the fuel pump overheats from frequent low fuel levels. 
• Loss of Power: The vehicle may lose power or feel sluggish, particularly when going uphill or carrying a load. 
• Engine Sputtering: The engine may jolt, stutter, or sputter at steady speeds because the pump can’t deliver enough fuel consistently. 
• Sudden Surges: In some cases, an excessively strong pump can deliver too much fuel, causing the engine to surge unexpectedly. 

Performance & Efficiency

• Reduced Fuel Efficiency: Opens in new tabA failing fuel pump can disrupt the optimal air-fuel mixture, leading to worse gas mileage. 
• Check Engine Light: Opens in new tabA check engine light illuminated with codes like P0087 (system pressure too low) or P0171 (lean mixture) can point to a fuel pump problem. 

What to Do

1. Listen for the Pump: Turn the key to the “on” position without starting the engine and listen for the faint hum of the fuel pump. 
2. Check Fuel Pressure (Advanced): For those with the right tools, a fuel pressure test can definitively show if the pump is delivering insufficient fuel. 
3. Consult a Professional: If you notice these symptoms, it’s best to have a qualified technician perform a comprehensive inspection to properly diagnose the issue, as a malfunctioning fuel pump can lead to dangerous situations and severe engine damage. 

How do you diagnose a bad pump?

To identify a pump problem, listen for abnormal noises like squealing or knocking, and check for excessive vibration. Watch for leaks from seals or connections, and signs of overheating. You should also observe the system’s flow and pressure, looking for a decreased or inconsistent flow rate and pressure readings that don’t match the expected performance. If the pump is a submersible or well pump, check for symptoms like loss of prime, continuous running, or air in the system.
 
1. Listen for Abnormal Noises 

• Squealing or grinding: Often indicates worn-out bearings or a failing seal. 
• Knocking or banging: Can signal air in the system or cavitation, where vapor bubbles collapse and cause damage. 

2. Check for Excessive Vibration 

• A well-functioning pump runs smoothly.
• Excessive vibration can point to misalignment, worn components, or an unbalanced impeller.
• This can accelerate wear on the pump and its parts.

3. Look for Leaks 

• Leaks at seals, glands, or connections: Indicate a failed or improperly adjusted component. 
• Leaking seals can allow contaminants to enter the pump or fluid to be lost from the system. 

4. Monitor Temperature 

• Overheating: A sign of insufficient cooling, contamination, or internal issues like excessive friction.

5. Observe Flow and Pressure 

• Reduced or inconsistent flow: Opens in new tabCan be caused by a clogged intake, a worn impeller, or issues with the supply lines. 
• High or low discharge pressure: Opens in new tabAbnormal pressure readings can indicate clogs, closed valves, or internal damage. 

6. Check Power and System Components 

• Power issues: For electric pumps, check the voltage and amperage, and ensure variable frequency drive (VFD) settings are correct.
• Wear and tear: Inspect the impeller and internal components for signs of wear or damage.
• System changes: Review original system specifications to ensure the pump is still operating within its designed parameters.

How to start a car with a bad fuel pump?

To start a car with a bad fuel pump, try the fuel pump tap method by lightly tapping the fuel tank with a rubber mallet or shoe to temporarily free stuck brushes, or the starting fluid method by spraying starting fluid directly into the throttle body to get the engine to run for a few seconds, giving you time to reach a repair shop. You can also try cycling the ignition key or checking the fuel pump fuse and relay, but these are temporary fixes, and the pump will ultimately need to be replaced. 
Temporary Fixes

• Fuel Pump Tap: Opens in new tabWith the ignition off, locate your fuel tank and give it a few light taps with a rubber mallet or the heel of your shoe to jar the pump’s internal components, which may free stuck brushes. 
• Cycling the Ignition: Opens in new tabTurn the key to the “on” position for a few seconds without starting the engine to allow the pump to prime. Then, turn it off and repeat this a few times before attempting to start. 
• Starting Fluid/Brake Cleaner: Opens in new tabSpray a small amount of starting fluid or brake cleaner directly into the air intake or throttle body, then try to start the engine. This provides a temporary fuel source, allowing the engine to run for a few seconds, giving you time to get to a repair shop. 

Checks Before Trying These Methods

• Check the Fuel Pump Relay and Fuse: A faulty fuse or relay can mimic a bad fuel pump, preventing the engine from getting fuel. 
• Ensure the Battery Is Fully Charged: A dead battery will prevent the car from starting, so rule this out first. 

Important Warnings

• Drive to a Repair Shop Immediately: These methods are only temporary fixes to get you to safety or a mechanic. 
• Do Not Overuse the Starter: Repeatedly trying to start the car can overheat the engine or a faulty pump could catch fire. 
• Call a Tow Truck: If you’re unsure what to do or the car won’t start with these methods, it’s best to have the vehicle towed to a professional for diagnosis and repair.