How to Fix Your Honda Fit's Misfire, Rough Idle, and Hesitation

How to Fix Spark Plug Issues

For 2020 Honda Fit owners, spark plug problems can manifest as frustrating drivability issues, from a rough idle to a complete loss of power. While the 2020 model is generally reliable, improper installation, incorrect parts, or related issues like intake leaks can lead to significant problems. As one owner on FitFreak shared after a repair attempt, "The spark plug saga continues... today on All My Sparkies. 'You wanna tighten the spark plug? Okay, go ahead and do it... that is if you can get past the end of the coil pack that is now stuck in there.'" This guide, built entirely from real owner experiences, will help you diagnose and fix these common spark plug-related troubles.

Symptoms

Owners report a distinct set of symptoms when spark plug issues arise. The most common and noticeable is engine misfiring. This isn't just a minor hiccup; it can cause the engine to run extremely rough, trigger the check engine light (CEL), and even put the vehicle into a protective "limp mode" that severely limits power. One driver experienced this directly: "I woke up this morning to my car missfiring. I changed the injectors the day before and it ran good still a little rough at idle is why i changed them." This shows how a pre-existing rough idle can be a precursor to a full misfire.

Beyond the misfire, the driving experience often degrades significantly. You may feel a persistent vibration through the steering wheel, seat, or floorboard, especially at idle or under acceleration. This is frequently described as a "herky jerky" sensation, where the car surges and stumbles instead of providing smooth power. Performance issues like hesitation during acceleration are also red flags. An owner who installed aftermarket plugs noted, "On a side note, I did use the colder plugs and honestly... something seems off. The day after I put them in, it seemed like there was noticeable hesitation."

Finally, these problems often lead to secondary symptoms that hit your wallet. Poor combustion from faulty or incorrect spark plugs directly leads to reduced fuel economy. You might find yourself visiting the gas station much more frequently without any change in your driving habits. In severe cases, prolonged misfiring can also cause the engine to run hotter than normal, as the unburned fuel can ignite in the exhaust system, leading to overheating warnings. Dashboard warning lights, primarily the check engine light, are almost always illuminated, storing diagnostic trouble codes (DTCs) related to misfires or fuel trim.

Most Likely Cause

Based on the collective data from owner discussions, the primary cause of spark plug-related drivability issues in this vehicle is the installation of incorrect spark plugs. This encompasses several critical mistakes. The most direct error is using plugs with the wrong physical dimensions or heat range. An owner discovered this dramatically: "Finally found out why my gas mileage was crappy. My bud checked out my spark plugs... when I switched to NGK spark plugs he measured them and the ones installed before were 1CM shorter then the NGK ones!" A plug that is too short can't properly ignite the air-fuel mixture, leading to weak combustion, misfires, and poor efficiency.

Another facet of "incorrect installation" is improper torque. Spark plugs that are not tightened to the manufacturer's specification can work loose, causing a loss of compression and a massive misfire. Conversely, over-tightening can damage the threads in the cylinder head—a far more expensive repair. The debate over tools highlights this concern, with one owner advising, "you got to be careful with the harbor freight torque wrenches," emphasizing the importance of accuracy. Furthermore, while not a fault of the plug itself, an intake leak (a vacuum leak in the intake system after the mass airflow sensor) is a cause identified by owners that creates symptoms identical to a bad spark plug. This leak introduces unmetered air, leaning out the air-fuel mixture and causing a rough idle, hesitation, and misfires that can be mistakenly blamed on the ignition system.

How to Diagnose

A proper diagnosis will save you time and money by ensuring you fix the right problem. You will need a basic socket set, a spark plug socket (likely 5/8" or 16mm for your Fit), a torque wrench, and an OBD2 code scanner.

Step 1: Read Diagnostic Trouble Codes (DTCs). Connect your OBD2 scanner to the port under the dashboard. The most relevant codes will be P0300 (random misfire), P0301, P0302, P0303, or P0304 (misfire detected on a specific cylinder 1-4). Note the cylinder number if provided. Also, check for codes like P0171 (System Too Lean), which could point directly to an intake leak.

Step 2: Perform a Visual and Auditory Inspection. With the engine cold, open the hood and listen for a distinct hissing or sucking sound around the intake manifold, throttle body, and all associated vacuum hoses. This is a sign of an intake leak. Visually inspect these hoses for cracks, dryness, or disconnections.

Step 3: Inspect the Spark Plugs (Physical Diagnosis). This is the most telling step. Carefully remove the ignition coil from the cylinder with the misfire code (or all four for a complete picture). Use your spark plug socket and extension to remove the plug.

• Check the Gap: Use a feeler gauge to measure the gap between the center and ground electrode. Compare it to your vehicle's specification (typically between 0.039-0.043 inches or 1.0-1.1mm for the 2020 Fit). An incorrect gap affects performance.
• Check the Type and Length: This is critical. The plug should be the exact part number specified for the 2020 Honda Fit (e.g., NGK DILFR6A11G or equivalent). Compare the overall length and the design to a known correct plug. As the owner found, a plug that is 1cm shorter is definitively wrong.
• Check the Condition: Look for signs of oil (indicating a valve cover gasket leak), heavy carbon deposits (rich mixture), or a white, blistered electrode (overheating).

Step-by-Step Fix

If you've diagnosed incorrect spark plugs or a simple intake leak, here is the repair process based on owner experiences.

Tools & Prep: Gather a ratchet, extensions, your spark plug socket, a torque wrench, a gap tool, dielectric grease, and the correct new spark plugs. Ensure the engine is completely cool to the touch.

1. Disconnect the Battery: Always start by disconnecting the negative terminal of the battery for safety.
2. Remove the Ignition Coils: Unclip the electrical connector from each coil pack. Remove the 10mm bolt securing each coil, then gently twist and pull the coil straight up and out of the valve cover.
3. Remove the Old Spark Plugs: Blow out any debris from the spark plug wells with compressed air to prevent dirt from falling into the cylinder. Insert your spark plug socket with an extension and carefully loosen each plug. Remove them by hand once loose.
4. Inspect and Gap New Plugs: Before installation, verify the new plugs are the correct part number. Check the gap with your tool and adjust it only if necessary by gently bending the ground electrode. Most quality plugs come pre-gapped, but always verify.
5. Install New Plugs by Hand: Carefully thread each new spark plug into the cylinder head by hand. Do not use the ratchet at this stage. If it does not thread in smoothly, stop and realign it. Cross-threading the aluminum head is a catastrophic error.
6. Torque to Specification: This is the most critical step. Connect your torque wrench and tighten the plugs to the manufacturer's specification. For the 2020 Honda Fit's L15B engine, this is typically 13 lb-ft (18 Nm). Do not guess. As one owner discussed regarding tool investment, "For me I just can't spend the money on more expensive torque wrenches when I'm only maintaining one car. The one you were talking about pre-set to 18 ft-lbs is $150." While a dedicated preset wrench is ideal, a reliable, calibrated click-type torque wrench is a must-have for this job.
7. Reinstall Coils and Connectors: Apply a small dab of dielectric grease inside the rubber boot of each ignition coil. This prevents moisture and ensures a good connection. Reinsert each coil, secure it with its bolt, and reconnect the electrical connector.
8. Reconnect Battery and Test: Reconnect the negative battery terminal. Start the engine and listen for a smooth idle. Use your OBD2 scanner to clear any stored codes, then take the car for a test drive to ensure the hesitation or misfire is gone.

For an Intake Leak: If you found a hissing sound or cracked hose, the fix is simpler. Identify the leaking component—often a dry-rotted vacuum hose, a cracked intake boot, or a faulty PCV valve hose. Replace the damaged part with a new, OEM-quality component. After replacement, clear the engine codes and test drive. The difference in idle quality is often immediate.

Parts and Tools Needed

• Spark Plugs (x4): NGK DILFR6A11G (Laser Iridium) or the exact OEM equivalent. This is non-negotiable for proper operation.
• Torque Wrench: A reliable 3/8" drive click-type torque wrench capable of accurately measuring 13 lb-ft. Inaccurate tools can cause the very problems you're trying to fix.
• Spark Plug Socket: A magnetic or rubber-lined 5/8" (16mm) deep-well socket with a thin wall to fit in the well.
• Ratchet and Extensions: A standard 3/8" drive ratchet and a 3" or 6" extension.
• Feeler Gauge or Gap Tool: For verifying the spark plug gap.
• Dielectric Grease: A small tube for coating ignition coil boots.
• Compressed Air (optional but recommended): To clean the spark plug wells before removal.
• OBD2 Scanner: Essential for reading and clearing diagnostic trouble codes.

Real Owner Costs

The cost to fix this issue varies dramatically between DIY and professional repair, and depends heavily on the root cause.

DIY Repair (Incorrect Spark Plugs):

• Parts: A set of 4 OEM NGK iridium spark plugs typically costs between $45 - $65.
• Tools: If you need to purchase a torque wrench, add $50 - $150 for a quality tool, though this is a one-time investment.
• Total DIY Cost: $50 - $215, with the higher end reflecting the initial purchase of a proper torque wrench.

Professional Repair (Shop):

• Parts & Labor for Spark Plug Replacement: A shop will charge 1-1.5 hours of labor. Total cost usually ranges from $180 - $300 for a standard plug change.
• Diagnosis & Intake Leak Repair: If the issue is an intake leak, diagnosis might take 0.5-1 hour ($75-$150). Replacing a vacuum hose may cost $100 - $200 total, depending on the hose's location and cost.
• Worst-Case Scenario: If incorrect plugs were over-torqued and stripped the threads in the cylinder head, a shop would need to perform a "heli-coil" thread repair. This job can easily exceed $500 - $800 due to the intensive labor required to remove the cylinder head or use specialized threading tools in place.

Prevention

Preventing spark plug issues is straightforward with disciplined maintenance.

1. Use Only OEM-Spec Parts: Always purchase the exact spark plug part number listed in your owner's manual or from a reputable Honda parts dealer. Avoid generic "fits-all" plugs or performance "colder" plugs unless you are specifically tuning the engine and understand the consequences.
2. Torque Correctly, Every Time: Never install spark plugs without a torque wrench. The 13 lb-ft specification is not a suggestion. This prevents both loose plugs and catastrophic thread damage.
3. Follow the Maintenance Schedule: Replace your spark plugs at the interval specified by Honda (typically around 60,000-100,000 miles for iridium plugs). Don't wait for symptoms to appear.
4. Inspect Related Systems: When performing routine maintenance, quickly inspect vacuum hoses and the intake air duct for cracks or brittleness. Catching a small intake leak early can prevent drivability problems.

What Owners Say

Real experiences from HONDA owners:

Owner Experiences

"I woke up this morning to my car missfiring. I changed the injectors the day before and it ran good still a little rough at idle is why i changed them." — Flos_Fit (source)

"On a side note, I did use the colder plugs and honestly... something seems off. They day after I put them in, it seemed like there was noticeable hesitation." — Goobers (source)

"Finally found out why my gas mileage was crappy. My bud checked out my spark plugs to see there was any oil on them but luckily there wasn't but when I switched to NGK spark plugs he measured them and the ones installed before were 1CM shorter then the NGK ones!" — derykisonder (source)

Lessons Learned

⚠️ "Quote: Originally Posted by SiXiam I agree you got to be careful with the harbor freight torque wrenches. Most reviews seem to have the opposite problem that you've experienced." — michel54 (source)

Real Repair Costs

"For me I just can't spend the money on more expensive torque wrenches when I'm only maintaining one car. The one you were talking about pre-set to 18 ft-lbs is $150." — michel54 (source)

FAQ

Q: How long does it take to replace spark plugs on a 2020 Honda Fit? A: For a DIYer with the right tools and some mechanical experience, the job typically takes 45 minutes to 1.5 hours. Most of the time is spent carefully removing the coils, hand-threading the new plugs, and torquing them correctly. A professional mechanic can often complete it in under an hour.

Q: Can I drive my car with a misfire? A: It is strongly advised not to. Driving with a persistent misfire can damage the catalytic converter, as unburned fuel is dumped into the exhaust system, causing it to overheat and melt internally. This can lead to a repair costing over $1,000. If the check engine light is flashing, this indicates a severe misfire that requires you to stop driving immediately to prevent damage.

Q: Are spark plug issues common on the 2020 Honda Fit? A: The 2020 Fit itself is not known for a widespread, inherent spark plug defect. However, the problems are common as a result of incorrect service—using the wrong plugs or improper installation during previous maintenance. As one vigilant new owner noted, "I believe that they never replaced the air filter or even wiper blades, they both are very old looking... I knew to dive into this right away," highlighting how prior owner neglect can lead to these issues.

Q: DIY vs mechanic—what's recommended for this repair? A: For a straightforward spark plug replacement with the correct parts, this is a very accessible DIY job for someone comfortable with basic tools. The key is having and using a torque wrench. If you are unsure about diagnosing the problem (e.g., differentiating between a bad plug and an intake leak), or if you suspect previous work may have damaged the cylinder head threads, it is wise to consult a professional mechanic for a proper diagnosis first.

Q: What does a "colder" spark plug do, and should I use them? A: A "colder" plug has a shorter insulator nose, which dissipates heat from the tip faster. They are designed for high-performance or forced-induction engines that run at higher temperatures. As one owner experimenting with them found, "something seems off... it seemed like there was noticeable hesitation." For a stock 2020 Honda Fit, using a colder plug than specified can lead to fouling, misfires, and poor performance. Stick with the OEM heat range.

Q: Could my coil packs be the problem instead of the spark plugs? A: Absolutely. A failing ignition coil can cause an intermittent or constant misfire. The diagnostic process is similar. If you find the spark plugs are correct, gapped properly, and in good condition, swapping the coil from the misfiring cylinder with one from a good cylinder is a common test. If the misfire code moves to the other cylinder, you've found a bad coil.