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How A Fuel Pump Relay Work & Symptoms Of A Bad Relay

The fuel pump relay is a vital component in your vehicle’s fuel system. It primarily switches the power on and off to the fuel pump, which is crucial for the combustion engine to run. This relay is usually located inside the fuel tank and is responsible for maintaining the necessary pressure in the fuel lines to pass on gasoline to the engine.

When the ignition is turned on, the fuel pump relay is activated, supplying a consistent voltage to the fuel pump. This helps to build up pressure in the fuel rail, ensuring that the engine receives the required fuel to start and operate efficiently. However, if the relay malfunctions, it can lead to several start up issues.

Recognizing the symptoms of a bad or failing fuel pump relay is crucial for every driver. Some telltale signs include the engine failing to start, stalling while driving, or fluctuating fuel pressure. These symptoms can diagnose a problem in the relay, which might require replacement. The costs for replacement can vary, including both the parts and labor.

It’s also important to perform regular maintenance on the fuel pump relay as part of your vehicle’s care routine. Checking its condition during regular vehicle maintenance and understanding these symptoms can help prevent unexpected vehicle problems and ensure smooth operation. Remember, being proactive with such components can save you time and money in the long run.

What is a Fuel Pump Relay?

The fuel pump relay is a necessary component in the combustion engine model of a car. Its primary function is to control the supply of electricity to the fuel pump. When you turn on the ignition, the relay gets activated, ensuring that the engine receives the fuel it needs to run. It’s a part of the power control module, and when the ignition is shut off, it powers down the fuel pump. This subtle yet crucial component plays a significant role in your vehicle’s ability to start and function properly.


Identifying the location of the fuel pump relay is crucial in troubleshooting and starting a car with a bad relay. In most vehicles, the relay is typically found within a fuse box. This box is often situated in the engine bay, resembling a long black box that contains not just the relay but also various other fuses and relays. However, the exact location can vary across different car models. It’s not uncommon to find the fuel pump relay under the dashboard, near the steering column, or even on the firewall.

Location of Bad Fuel Pump Relay

For those not familiar with the intricate parts of their vehicle, the owner’s manual proves to be an invaluable resource. It provides the exact location of the fuel pump relay for specific vehicles, ensuring a quick and efficient approach. Alternatively, consulting a repair manual can offer more detailed guidance, especially for those interested in DIY repairs or just wanting to understand their car better. In any case, knowing where to find this small but vital component can significantly hasten any repair or diagnostic process.


The fuel pump relay plays a critical role in the start cycle of a car’s combustion engine. When you turn the key in the ignition to start the car, the relay gets activated and begins to supply a current to the fuel pump. This current, a small amount of voltage supplied consistently, is essential for the engine to start running. Once the engine is up and running, the fuel pump relay shuts off, and the electricity needed by the fuel pump is then supplied by the oil pressure sending unit. If the ignition is shut off, the relay is engaged again, this time to power down the fuel pump. Understanding this process is vital for diagnosing issues related to a bad fuel pump relay.

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Symptoms of a bad fuel pump relay

Engine does not start

One common symptom of a faulty fuel pump relay is an engine that refuses to start. You might turn the key, hear the engine crank, but it just won’t start. This issue often stems from a lack of power being supplied to the fuel pump. It’s crucial to be aware that a no-start problem can be caused by a vast number of issues, so a thorough diagnosis is necessary to pinpoint the problem.

Another common set of symptoms indicating a bad fuel pump relay includes stalling of the engine, performance issues, and the check engine light appearing on the dashboard. If your car won’t start at all and you can’t hear the fuel pump when you turn on the ignition, it’s a strong indication of relay failure. Reasons for this failure range from dust accumulation to electrical power surges that can affect the relay’s well-being. A detailed list of signs to look for can guide you in identifying a failing fuel pump relay.

Engine stalls

A failing fuel pump relay often manifests as the engine stalling. This occurs when the relay suddenly loses connection or stops delivering power to the fuel pump while the vehicle is running. The interruption of fuel supply causes the engine to stall, and it may refuse to restart if the relay has completely failed. In some cases, you might be lucky and the car restarts after a short while, indicating a faulty relay that requires immediate inspection. It’s vital to understand that the fuel pump relay ensures the combustion chamber receives adequate fuel at the proper pressure. Bad solderings within the relay can suddenly disrupt this balance, leading to issues in engine function. Replacing the relay is often necessary to ensure your car works perfectly again.

Check Engine Light on

A check engine light illuminating on your dashboard can be a common sign of a bad fuel pump relay. This occurs when the engine control module (ECU) monitors the car’s sensors and detects wrong or incorrect values. The ECU, acting as the vehicle’s brain, suspects an issue with the fuel pressure, often aided by readings from the fuel pressure sensor. If the fuel pump relay suddenly fails, it disrupts the normal fuel flow, prompting the ECU to light up the check engine light and store a trouble code relating to the fuel system. This warning is the car’s way of telling you to inspect and diagnose the problem for safety and efficiency.

Rough Acceleration

Rough acceleration in a vehicle can often indicate a fuel pump relay going bad. This component is crucial for the power delivery to the engine, much like the heart in a human body that pumps oxygen and blood. When the relay is failing, it affects the steady flow of fuel to the combustion chamber, leading to problems in starting or maintaining a smooth run. Such issues in your vehicle might not always be due to the relay; they can also happen because of several other reasons, like a choked filter. Hence, it’s important to diagnose the issue accurately before replacing any parts in your car. Identifying the root cause ensures the correct fix and prevents unnecessary replacements.

Loss of Power

Loss of power while driving is a key indicator that your car’s fuel pump relay might be the culprit. This sudden loss of power, especially when slowing down or without any apparent reason, can cause your vehicle to stall or come to a halt in the middle of the road, leaving you potentially stranded. It’s a scenario where the engine feels like it’s running normally one moment and then loses power the next. A faulty fuel pump relay can cause the engine not to start at all, appearing completely dead due to no fuel pressure. This situation means the relay has failed to give power to the fuel pump to build up the necessary pressure. If you experience these issues, an inspection and repair are highly recommended to mitigate the risk of unexpected stops.


No noise from the fuel pump

A telling symptom of a bad fuel pump relay is the absence of noise from the fuel pump when you turn on the ignition. Normally, when you turn the ignition lock, you should hear a distinctive whirring noise from the rear of the car, indicating that the fuel pump has started and is building pressure in the fuel rail. This sound is usually a low volume hum or whine that can be heard both from inside and outside the vehicle, near the fuel tank. It’s a sign of a healthy fuel pump actively working.

However, if the fuel pump relay fails, it will fail to receive or transmit power to the fuel pump, rendering it silent. If you can’t hear this noise within 2-3 seconds of turning the ignition, it might point to a problem with the relay. In such cases, it’s crucial to listen closely near the fuel tank for any noise. The absence of it suggests that the relay is not operating as it should, and a check or replacement might be necessary.

No noise from the fuel pump

How to diagnose a bad fuel pump relay

Visual inspection

Diagnosing a bad fuel pump relay begins with a visual inspection. This initial step is crucial in identifying any obvious problems. Start by locating the black fuse box in the engine bay. The fuel pump relay is usually a cube-shaped object that connects with prongs like an electrical plug. Carefully pry it out using a flathead screwdriver if needed, and disconnect it to inspect its terminals and socket. Look for signs of corrosion or overheating, which can impede the flow of a small but consistent current. If you find any, clean the corroded terminals and socket with contact cleaner, ensuring to protect your hands and eyes with gloves and goggles.

The relay’s contact and coil are the most common parts to burn out or break. Check all fuses for any burn out or breakages. Sometimes, issues with the relay can be due to simple problems such as a broken fuse. If you discover bad soldering inside the relay’s case, it can be resoldered. This is often a more straightforward fix than replacing the entire unit.

Diagnosing the fuel pump relay can be relatively easy with the right tools. After the visual inspection, locate the relay in the fuse box and lift it to check the numbers 30, 85, 86, and 87. Use a multimeter to cycle the ignition and turn on the voltage. Check for intermittent problems, ensuring you have constant voltage on specific pins and that they are properly grounded as indicated by your engine control unit. If these pins show abnormal readings, it might indicate an issue with the relay. In cases where the diagnosis points to a faulty relay, replacing it is often the most effective solution.


After a thorough visual inspection to identify the culprit, the next step in diagnosing a failed fuel pump relay is to test it using a digital multimeter. Set your multimeter to the lowest range on the Ohms scale or to continuity. Connect one lead to one of the power circuit pins of the relay and the other lead to the other power terminal. The meter should ideally read infinite resistance. If you detect zero ohms or any resistance value other than infinity, it’s a clear sign that the relay has shorted and needs to be replaced. In some cases, if the multimeter makes a beeping sound while testing, this also indicates a short and necessitates replacement of the relay. This test is a reliable method to ascertain the health of your fuel pump relay.

Test Of Bad Fuel Pump Relay

Fuel Pump Relay Replacement Cost

The replacement cost for a fuel pump relay can vary depending on your car model and where you choose to have the work done. On average, you can expect the cost to range between $90 and $150. This includes both the parts and labor costs. The parts alone typically cost between $20 and $50, with the remaining cost attributed to labor. The fuel pump relay is a relatively small electronic device that can be acquired either online or at your nearest automotive spare parts shop.


When considering the labor cost, it generally accounts for at least an hour of work, though it can be more in certain cases. The amount your repair shop charges will depend on the mechanic’s hourly pay rate or the per-hour cost. Despite being a critical component, the fuel pump relay is not typically an expensive device. For most vehicles, you can buy a new relay for an average cost that aligns with the aforementioned range. However, it’s always wise to get a few quotes to ensure you’re getting the best deal for your specific vehicle’s needs.

Cleaning A Fuel Pump Relay

Cleaning a fuel pump relay is an essential maintenance job that every car owner should learn. Over time, the relay can become dirty, especially if it’s located in the engine bay where it’s exposed to heat from the engine, dust, and dirt. These elements can impede the relay’s performance. In rare cases, if the relay is placed outside the fuse box, it’s even more susceptible to debris, further affecting its functionality. Regular inspection and cleaning are necessary to maintain optimal performance.

The process involves careful handling of this electrical car part. Take off the relay and use a blower to blow off any dust and dirt for about 1-2 minutes. Never use water for cleaning; instead, ethanol or a specialized electrical contact cleaner should be used to thoroughly clean the part. After cleaning, apply ethanol to the relay and rub it gently to remove any remaining dirt. Once done, leave the relay in an airy place to dry. This will prevent it from getting dirty too quickly and ensure the relay remains free from contaminants that could affect its operation.


In essence, no fuel pump relay means no fuel pump, and consequently, no fuel being delivered to the engine. This situation makes it virtually impossible for a car to start. The fuel pump relay is essential in initiating the fuel delivery process, and without it, the engine cannot run. It’s not a case of hopes and dreams; the relay is a critical component for starting the car.

To start a vehicle with a fuel pump problem, first, let the engine cool off for about 8 hours. This period allows the fuel lines to even out and the pump to potentially reset. If the car gets started, immediately drive it to a mechanic shop ASAP. Continually driving with a bad fuel pump can lead to further damage or even pose a risk of fire.

A temporary fix for a faulty fuel pump involves bypassing the fuel pump relay. First, locate the relay within your vehicle’s relay box. Swap it with another relay of the same type from the box. This action effectively bypasses the fuel pump’s electrical circuit, allowing the pump to operate temporarily until a more permanent solution can be implemented.

The fuel pump relay is typically activated for three to five seconds when the ignition is switched on or when the driver’s door is opened. It is a basic four-pin, normally-open relay, meaning its contacts are open with no current flow through the winding. This design allows it to control the fuel pump’s operation effectively.

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