Why Your Honda Accord Won't Start When Hot (The $15 Relay Fix)

How to Fix Relay Issues

A failing main relay is a notorious and well-documented source of frustrating, intermittent problems in the 2010 Honda Accord. Owners often chase phantom issues like fuel starvation, no-starts, and electrical gremlins, only to find the culprit is a small, inexpensive box under the dashboard. This relay, often called the fuel pump relay or PGM-FI main relay, is critical for supplying power to your car's fuel pump and engine control systems. When it fails, it can mimic a wide array of serious and expensive problems. As one owner shared their diagnostic journey: "First potential symptom was intermittent check engine light (solid, but not always on). Then breakdown where rpm's dropped to 500, no power for 5-10 seconds, then engine shuts off." — WonderingWhatsUp2. The fix, fortunately, is often straightforward and affordable once you've correctly identified the relay as the source.

Symptoms

The symptoms of a failing main relay are notoriously intermittent and heat-sensitive, making diagnosis tricky. The most common report is a no-start condition, particularly when the engine or interior cabin is hot. The car may crank strongly but never fire up, as if the fuel pump isn't running. This can happen after a short drive to the store or on a warm day. Crucially, the issue often resolves itself once the vehicle cools down, leading owners to believe the problem has mysteriously vanished—until the next hot start.

Beyond the classic hot-start failure, a failing relay can cause a cascade of other erratic behaviors. Owners report episodes of fuel starvation while driving, where the engine suddenly loses power, RPMs drop dramatically, and the vehicle may stall. An intermittent check engine light that comes and goes without a consistent code is another red flag. Some have noted a correlation between the check engine light turning off and the fuel pump audibly priming, allowing the car to start. Issues with other systems powered through the same interior fuse panel, like power mirrors or headlights, have also been loosely associated, pointing to broader electrical instability.

Perhaps the most telling symptom involves the vehicle's security system. The main relay also supplies power to the circuit for the chipped key immobilizer. Therefore, a key symptom is when you turn the ignition to "ON" and the green key security light on the dashboard does not illuminate as it should. Without this signal, the Engine Control Module (ECM) will not allow the car to start, even though everything else seems normal. As one owner confirmed after their fix: "I thougt that relay was just for the fuel pump, but it also sends power to the ECM / imobolizer. Replaced it during a hot part of the day when the green key light would not come on and it immefiately fixed the issue." — observant1.

Most Likely Cause

The primary cause of these relay failures is not a mechanical fault with the relay's clicking mechanism but an internal electrical fault within the relay unit itself. The main relay in your vehicle is not a simple, single relay; it's a integrated module containing multiple relays and circuitry on a small printed circuit board (PCB). The most common point of failure is the solder joints that connect the relay components to this board.

Over years of use, the relay is subjected to constant heat cycles—heating up when the car is running and cooling down when it's off. This repeated expansion and contraction causes the solder joints to fatigue, develop tiny cracks, and eventually fail. These cracks can break the electrical connection, but they may also make intermittent contact. This explains why the problem is heat-sensitive: as the relay warms up, the metal expands, opening the cracked solder joint and breaking the circuit. When it cools, the joint contracts and makes contact again. A technical contributor on a forum explained it precisely: "The cause is that a solder joint on the circuit board gets weak over time and cracks due to heat/cooling cycles." — PAhonda. This failure mode directly leads to the intermittent, temperature-dependent symptoms that plague owners.

How to Diagnose

Diagnosing a bad main relay requires a methodical approach focused on its heat-sensitive nature. You will need a basic set of tools: a flashlight, a trim removal tool or flat-head screwdriver (wrapped in tape to prevent scratches), and possibly a multimeter if you want to confirm voltage loss. The most effective diagnostic test is the "heat test." The next time your car fails to start—especially when hot—locate the main relay immediately. It is found under the dashboard, above the driver's left footwell, mounted to the interior fuse panel.

Once you've found the relay, carefully remove it from its socket. The goal is to heat the relay module itself to see if the fault is inside. You can do this by using a hairdryer on a low setting to gently warm the relay's plastic case for 60-90 seconds. Do not overheat it or use an open flame. Alternatively, if the car is already hot from a no-start episode, the relay is already at the suspected failure temperature. After heating it, quickly plug the relay back into its socket and try to start the car. If it starts immediately after this heating (or cooling) intervention, you have strong evidence the relay is faulty.

For a more electrical confirmation, use a multimeter. With the key in the "ON" position, you can probe the appropriate pins in the relay socket to check for power output to the fuel pump circuit. However, given the intermittent nature, the physical heat test is often more reliable. Pay close attention to the dashboard during your diagnosis. Turn the key to "ON" and watch for the green key immobilizer light. If it does not illuminate during a no-start event, the main relay is the prime suspect, as it powers that circuit. One owner's experience highlights this clue: "Replaced it during a hot part of the day when the green key light would not come on and it immefiately fixed the issue." — observant1.

Step-by-Step Fix

Replacing the main relay is a straightforward DIY job that can be completed in under 30 minutes. Here is a detailed step-by-step guide based on owner experiences.

Step 1: Gather Parts and Tools. Ensure you have the correct replacement relay (see Parts section below) and a flashlight. You may want a trim removal tool, but fingers often work.

Step 2: Locate the Relay. Sit in the driver's seat. Look up and under the dashboard, above where your left foot rests. You will see the interior fuse panel. The main relay is a black, rectangular plastic box (about 2" x 1.5") plugged into this panel. It is often labeled, but it's typically the largest relay module in that cluster.

Step 3: Remove the Relay. You do not need to disconnect the battery for this. Firmly grasp the relay with your fingers. It is held in by friction fit. Pull it straight down and out of its socket. It may require a firm tug. Avoid using excessive force on the surrounding fuse panel.

Step 4: Install the New Relay. Take your new relay and orient it correctly by matching the pin configuration and the shape of the socket. Align the pins and press the relay firmly and evenly up into the socket until it is fully seated. You should hear or feel a soft click.

Step 5: Test the Repair. Before reassembling anything, test the fix immediately. Insert your key and turn it to the "ON" position (but don't start it). Listen for the distinct 2-3 second humming sound of the fuel pump priming. Also, verify that the green key security light on the dashboard illuminates. If both occur, turn the key fully and start the engine. It should fire up normally.

Step 6: Confirm the Fix Under Conditions. The true test is a hot-start. Drive the car until it is at normal operating temperature, then turn it off and attempt to restart it after waiting 5-10 minutes. If it starts reliably, the repair is successful. Owners who have performed this fix often report instant success. As one noted regarding a similar fix for a clicking starter relay: "Now works perfectly. After that, there is the relay under the dash, simply plugs in... both can be done for about $15 all in if you shop around." — AccordingToWill.

Parts and Tools Needed

The only essential part for this repair is the main relay itself. For the 2010 Honda Accord, it is most commonly referred to as the PGM-FI Main Relay or Fuel Pump Relay. While an OEM Honda part is ideal, aftermarket equivalents are widely available and have proven reliable for this fix.

• Primary Part: PGM-FI Main Relay. Honda part numbers can vary, but common references include 39400-SDA-A01 or similar. It is critical to purchase one specified for your exact model year and engine. A standard automotive 5-pin relay is NOT the correct part; you need the specific integrated module.
• Where to Buy: You can purchase this relay from a Honda dealership parts counter, online OEM parts retailers, or auto parts stores like AutoZone, Advance Auto Parts, or RockAuto. As an owner pointed out, shopping around saves money: "2006 HONDA ACCORD 2.4L L4 Starter Motor Control Relay | Shop Now at RockAuto If you don't care about using OEM parts for those, both can be done for about $15 all in if you shop around." — AccordingToWill.
• Tools Needed:
  • Flashlight
  • Trim removal tool or a flat-head screwdriver (for prying if necessary, but use caution)
  • Your hands

Real Owner Costs

The cost to fix this issue is remarkably low, especially when compared to the potential diagnostic fees for chasing its symptoms. The repair is almost entirely in the part cost, with labor being minimal.

DIY Repair Cost: The relay itself is inexpensive. Owners report spending between $15 and $50 for the part. The lower end typically involves aftermarket parts from online retailers or discount auto parts stores. An OEM relay from a dealership will be on the higher end of that range. Your total DIY cost is just the price of the relay.

Professional Repair Cost: If you take the car to a mechanic, the cost will include parts markup and labor. A shop might charge 0.3 to 0.5 hours of labor for diagnosis and replacement. With parts markup, the total bill would likely range from $75 to $150. However, the risk with a professional repair is misdiagnosis. A mechanic not familiar with this common Honda issue might spend time testing the fuel pump, crankshaft position sensor, or ignition switch, running up a significant diagnostic bill before finding the simple relay. One owner's experience underscores the value of a correct, simple diagnosis: fixing the issue themselves for the cost of a cheap part.

The cost-benefit analysis strongly favors the DIY approach. For about $20 and 15 minutes of your time, you can solve a problem that could otherwise lead to a tow truck and a much larger repair invoice.

Prevention

While the main relay failure is ultimately a result of age and thermal cycling, there are a couple of practices that may prolong its life. First, avoid adding unnecessary electrical load that might cause the relay to run hotter. This means being mindful of using high-power aftermarket accessories that are tapped into circuits related to the fuse panel. Second, if you are skilled with a soldering iron, some owners of older Hondas have successfully repaired failed relays by re-soldering the cracked joints on the circuit board. However, for the 2010 model, given the low cost of a new unit, replacement is the most reliable and recommended preventive measure once symptoms appear. Ensuring your vehicle's cooling system is working efficiently also helps keep the overall under-dash temperature lower, potentially reducing stress on all electronic components.

What Owners Say

Real experiences from HONDA owners:

Success Stories

"I thougt that relay was just for the fuel pump, but it also sends power to the ECM / imobolizer. Replaced it during a hot part of the day when the green key light would not come on and it immefiately fixed the issue." — observant1 (source)

"Replaced it during a hot part of the day when the green key light would not come on and it immefiately fixed the issue. Relay is under the dash near your left foot up above. -- It is comonly called the fuel pump relay...or that is what I thought that was all it did. - It also sends voltage to the ecm for the chipped key imobilizer ckt." — observant1 (source)

"1998 acccord ex sedan, Ac issues with the actuator So this is weird, all my ac systems were working as of earlier, cold ac, all blower motors selections worked, defroster worked, and the hot/cold selector worked." — Prelude_user_2022 (source)

Owner Experiences

"From ECM picture, the problem was potentially in vicinity of Connector E, pins 19, 20, 21, 29, 30 (I think 31 is empty) https://preview.redd.it/ok7ln85t661g1.jpg?width=480&format=pjpg&auto=webp&s=49db360050be5f3b6926536162d4b91ef14438da" — WonderingWhatsUp2 (source)

"First potential symptom was intermittent check engine light (solid, but not always on). Then breakdown where rpm's dropped to 500, no power for 5-10 seconds, then engine shuts off." — WonderingWhatsUp2 (source)

"Once it starts up, it runs perfectly fine. The other thing I’ve noticed is that the engine light will stay on and when it finally goes off, the fuel pump kicks on and the car cranks right up." — Jaredb63 (source)

Lessons Learned

⚠️ "The 03 used a different relay setup. That main relay is really two relays in one unit, so it is can fail in a differnt way where the immobilizer would still get power." — PAhonda (source)

⚠️ "That main relay is really two relays in one unit, so it is can fail in a differnt way where the immobilizer would still get power. The cause is that a solder joint on the circuit board gets weak over time and cracks due to heat/cooling cycles." — PAhonda (source)

Real Repair Costs

"Now works perfectly. After that, there is the relay under the dash, simply plugs in: 2006 HONDA ACCORD 2.4L L4 Starter Motor Control Relay | Shop Now at RockAuto If you don't care about using OEM parts for those, both can be done for about $15 all in if you shop around." — AccordingToWill (source)

"I pulled mine apart and the starter "nub" was toasty and worn, probably could be cleaned up and re-lubricated, but I had already bought a $10 switch so I simply replaced it." — AccordingToWill (source)

FAQ

Q: How long does it take to replace the main relay? A: The physical replacement takes less than 5 minutes. The entire process, including locating the relay, removing it, and installing the new one, can be comfortably done in 15-20 minutes for a first-timer. Testing the fix by checking for the fuel pump prime and doing a hot-start test will add a bit more time.

Q: Can I drive my Accord with a failing main relay? A: It is not safe to drive with a confirmed failing main relay. The failure is intermittent and can cause the engine to stall without warning while you are driving, leading to a complete loss of power steering and brakes. This creates a dangerous situation, especially in traffic or at high speeds. Have it fixed as soon as symptoms appear.

Q: Is this a common issue on the 2010 Honda Accord? A: While the provided owner data spans several model years, the failure mode of the solder joints inside the PGM-FI main relay is a famously common issue across many Honda and Acura vehicles from the 1990s through the 2000s. The 2010 Accord, sharing similar electrical design principles, is susceptible to this same type of age- and heat-related failure. With 58 discussions noted, it is a recognized pattern among owners.

Q: DIY vs mechanic - what's recommended for this fix? A: This is a highly recommended DIY job. The repair requires no specialized tools, the part is cheap, and the location is easily accessible. The potential cost savings are enormous compared to a shop visit. The only reason to use a mechanic would be if you are completely uncomfortable with any automotive work or if you want a professional to confirm the diagnosis before purchasing the part. However, as the data shows, many owners have successfully diagnosed and fixed this themselves.

Q: My car cranks but won't start, and the green key light isn't on. Is it definitely the main relay? A: This combination of symptoms is the strongest indicator of a failed main relay. The green key light's absence points directly to a lack of power to the immobilizer system, which is supplied through the main relay. While other faults are possible, this specific symptom pattern makes the main relay the primary and most likely suspect. An owner's fix confirms this: "It also sends voltage to the ecm for the chipped key imobilizer ckt." — observant1.

Q: Could a bad main relay cause other electrical problems like my power mirrors or headlights? A: It's possible but less common. The main relay primarily controls power to the fuel pump and ECM/immobilizer. Other accessories like power mirrors and headlights are usually on separate circuits within the same interior fuse panel. However, a severely failing relay causing voltage fluctuations or a poor connection in the fuse panel itself could theoretically lead to intermittent operation of other devices. The core symptoms to watch for are no-starts, stalling, and the missing security light.