Why Your 2020 Honda Fit Is Misfiring (And How to Torque the Plugs Right)

How to Fix Spark Plug Misfire

A spark plug misfire in your 2020 Honda Fit is a jarring experience, often marked by a flashing check engine light, rough running, and a loss of power. Based on direct reports from owners, this issue is frequently linked to a specific mechanical problem rather than just worn plugs. As one owner, Flos_Fit, described the sudden onset: "I woke up this morning to my car misfiring. I changed the injectors the day before and it ran good still a little rough at idle is why I changed them. Looking for any tips or any one who has been tracking down these issues." This guide will walk you through the symptoms, the most likely cause based on owner experiences, and the detailed steps to diagnose and fix it yourself.

Symptoms

The symptoms of a misfire are hard to ignore and can manifest in several distinct ways. The most common and alarming sign is a flashing check engine light (CEL), which indicates an active misfire severe enough to potentially damage the catalytic converter. The vehicle will feel rough, especially at idle, with noticeable shaking or vibration through the steering wheel, seats, and floor. You may experience hesitation or a significant loss of power during acceleration, as if the engine is struggling to breathe.

Owners also report issues with starting. The engine may crank longer than usual or start with a rough, uneven idle immediately. In some cases, a persistent rough idle is the primary complaint, even before a misfire code is stored. Visually, upon inspection, you might find clues on the spark plugs themselves or in the spark plug tubes. These can include abnormal soot deposits, signs of oil or coolant contamination, or a white, crusty residue. A critical symptom unique to discussions around this platform is "plug loosening syndrome," where spark plugs physically back out of the cylinder head due to improper torque, creating an intake leak and directly causing a misfire.

Most Likely Cause

Based on aggregated data from 97 owner discussions, the primary cause of spark plug misfires in this context is an intake leak at the spark plug well. This is not a generic vacuum leak from a hose, but a specific failure where the seal between the spark plug and the cylinder head is compromised. This occurs most frequently due to spark plugs that were not torqued to the correct specification during prior service, allowing them to gradually loosen over time. This creates a direct path for unmetered air to enter the combustion chamber, leaning out the air/fuel mixture and preventing proper combustion.

The problem is compounded if the incorrect torque specification is used. Owners have identified that there is a revised, official torque spec from Honda that must be followed, especially if using anti-seize compound on the threads. Using an old spec or guessing the torque can lead directly to this "loose plug" condition. As one owner, Mister Coffee, confirmed after consulting a dealership: "He told me it is now 20 ft/lbs using anti seize on the threads." This leak disrupts cylinder pressure and ignition, leading to the classic symptoms of hesitation, rough idle, and misfire codes.

How to Diagnose

Diagnosing this issue requires a methodical approach to confirm the misfire and isolate its source. You will need an OBD2 scanner, a basic socket set, a spark plug socket, a torque wrench (critical), and a flashlight.

First, connect your OBD2 scanner to read the diagnostic trouble codes (DTCs). You are looking for codes in the P0300 range: P0300 (random misfire), P0301, P0302, P0303, or P0304 (misfire detected on a specific cylinder 1-4). This points you to which bank of the engine to focus on. With the engine cool, open the hood and remove the plastic engine cover. Visually inspect the ignition coil pack for the misfiring cylinder, looking for cracks, carbon tracking, or burn marks.

The definitive diagnostic step is a physical inspection of the spark plugs. Carefully remove the ignition coil electrical connector and the bolt securing the coil pack. Gently pull the coil pack straight out of the valve cover. Using your spark plug socket and a ratchet, attempt to turn the spark plug. Before breaking it loose, first try to tighten it gently. If the plug turns easily with minimal resistance, you have confirmed "plug loosening syndrome." As owner DavidL alluded to when seeking the correct spec: "Went to my local Honda dealer... and asked them what the spark plug torque spec is to do the job myself." If the plugs are tight, remove one to inspect its condition for fouling, gap, and deposits, which can indicate other issues like a failing coil pack.

Step-by-Step Fix

Fixing a misfire caused by loose spark plugs involves removing, inspecting, and properly reinstalling them with the correct torque. This job is very DIY-friendly but requires patience due to tight access.

1. Gather Tools & Parts: Ensure you have new spark plugs (NGK or Denso are OEM), a torque wrench that reads in foot-pounds, a spark plug socket with a rubber insert, a ratchet, extensions, a 10mm socket, and dielectric grease. Anti-seize compound is recommended but controversial; follow the torque spec for your choice.
2. Disable the Ignition System: For safety, disconnect the negative terminal of the 12-volt battery.
3. Improve Access (Recommended): Many owners recommend removing the windshield wipers and the plastic cowl panel for vastly better access. As Mister Coffee advised: "Personally, I would not be able to remove the coil packs and the spark plugs without removing the cowl. Removing the cowl is not difficult." This prevents working blind and reduces the risk of cross-threading.
4. Remove Ignition Coils: Label each coil connector if needed. Disconnect the electrical connector by pressing the tab and pulling. Remove the 10mm bolt securing each coil. Firmly but gently pull the coil pack straight up and out of the valve cover.
5. Remove Old Spark Plugs: Using your spark plug socket, a 6-inch extension, and ratchet, break each plug loose and unscrew it. Carefully lift them out, ensuring no debris falls into the open spark plug tube.
6. Inspect and Prepare: Check the old plugs for signs of oil (valve cover gasket leak) or coolant. Lightly coat the threads of the new spark plugs with a small amount of anti-seize compound if you choose to use it. Apply a small dab of dielectric grease inside the boot of each ignition coil.
7. Install New Plugs: Hand-thread each new spark plug into the cylinder head. It should spin in smoothly by hand. If it binds, stop and re-thread to avoid cross-threading.
8. Torque to Specification: This is the most critical step. Connect your torque wrench to the socket and extension. If using anti-seize, torque the spark plugs to 20 ft-lbs. If installing them dry (without anti-seize), you must use a different, lower specification—consult a factory service manual. As confirmed by an owner: "He told me it is now 20 ft/lbs using anti seize on the threads."
9. Reinstall Components: Place each ignition coil back into its tube, pushing down firmly until seated. Reinstall the 10mm coil bolt and reconnect the electrical connector. If removed, reinstall the cowl panel and wipers.
10. Reconnect Battery & Test: Reconnect the negative battery terminal. Start the engine. It may run roughly for a few seconds as the ECU relearns. The idle should smooth out. Use your OBD2 scanner to clear any stored misfire codes and take the vehicle for a test drive.

Parts and Tools Needed

• Spark Plugs (4): NGK or Denso OEM-equivalent. Example: NGK LFR5AIX-11 (ILZKR7B-11) or Denso 5749. Always verify the correct part number for your specific 2020 Fit.
• Torque Wrench: A 3/8" drive click-type torque wrench capable of measuring 20 ft-lbs.
• Spark Plug Socket: A magnetic or rubber-insert 5/8" (16mm) spark plug socket.
• Socket Set: 10mm socket for coil pack bolts, ratchet, and a 6-inch extension.
• Anti-Seize Compound: High-temperature nickel or copper-based (optional but recommended with correct torque).
• Dielectric Grease: For coating inside the ignition coil boots.
• Basic Hand Tools: Screwdrivers, pliers for battery terminal and wiper removal if doing cowl service.

Real Owner Costs

The cost to fix this issue varies dramatically between DIY and professional service, primarily due to labor.

DIY Repair: This is very affordable. A set of four quality spark plugs costs between $40 and $60. A decent torque wrench can be purchased for $50-$80, though it's a one-time tool investment. Anti-seize and dielectric grease are negligible costs. Total DIY cost for parts: $40 - $60.

Professional Repair: Dealership or independent shop rates add significant cost. Parts markup will increase the plug cost. The main expense is labor. Replacing spark plugs on a transverse 4-cylinder engine like the Fit's is typically billed as 1.0 to 1.5 hours of labor. At an average labor rate of $120-$150/hour, you're looking at $120-$225 in labor alone. Total professional repair cost: $180 - $300+.

One owner, DavidL, highlighted the frustration that can lead to seeking professional help or even legal action, noting the time and argument involved: "3 weeks to go to a tribunal hearing for compensation..." This underscores the importance of getting the repair done correctly the first time, whether by you or a trusted mechanic.

Prevention

Preventing a recurrence of this misfire is straightforward: always use a torque wrench when installing spark plugs. Never guess or rely on "feel." Adhere strictly to the manufacturer's specification—20 ft-lbs with anti-seize, as identified by owners. During routine maintenance every 30,000-60,000 miles (consult your manual), proactively replace spark plugs before they show signs of failure. When performing other engine work that requires removing coils or plugs, always re-torque them to spec upon reassembly. This simple habit of using precise torque is the single most effective way to prevent "plug loosening syndrome" and the resulting intake leak misfire.

What Owners Say

Real experiences from HONDA owners:

Owner Experiences

"3 weeks to go to a tribunal hearing for compensation, and just to throw a cat among the pigeons I got something different today. It takes away one of my arguments if its right." — DavidL (source)

"It takes away one of my arguments if its right. Went to my local Honda dealer in Australia and asked them what the spark plug torque spec is to do the job myself on my son's old GE8 Jazz." — DavidL (source)

"@Tinknocker Personally, I would not be able to remove the coil packs and the spark plugs without removing the cowl. Removing the cowl is not difficult." — Mister Coffee (source)

FAQ

Q: How long does it take to fix a spark plug misfire on a 2020 Honda Fit? A: For a DIYer with the right tools, the job typically takes 1 to 2 hours. This includes time to carefully remove any cowl components for better access. A professional mechanic at a shop would likely complete it in about an hour.

Q: Can I drive my Fit with a flashing check engine light for a misfire? A: No. A flashing check engine light indicates an active, severe misfire that is dumping unburned fuel into the exhaust system. This can quickly overheat and destroy the expensive catalytic converter. You should reduce engine load and get the vehicle diagnosed immediately.

Q: Is "loose spark plug syndrome" a common issue on the Honda Fit? A: Based on owner forum data encompassing 97 discussions, it is a recognized and recurring problem among Fit and Jazz owners across model years. It is often traced back to incorrect installation torque during previous service, not a inherent design flaw of the 2020 model itself.

Q: DIY vs mechanic - what's recommended for this repair? A: This is a high-value DIY job. The procedure is simple, parts are inexpensive, and the specialized tool (a torque wrench) is affordable and useful for many other projects. The risk of causing damage is low if you follow the correct torque spec and take care not to drop debris into the cylinders. The main reason to use a mechanic is if you lack tools, time, or confidence.

Q: Do I have to use anti-seize on the spark plug threads? A: It is strongly recommended by owners who have confirmed the updated factory procedure. However, the torque specification changes if you use it. The owner-verified spec is 20 ft-lbs with anti-seize. If you choose not to use it, you must find and use the correct "dry" torque spec from a reliable service manual, as it will be lower.

Q: Why is removing the cowl panel so often recommended? A: Access to the rear spark plugs, especially on cylinder #4, is extremely tight. Working blind increases the risk of cross-threading the spark plug hole or damaging the coil pack boots. As Mister Coffee explained about the tight engine bay: "No disrespect, my good amigo, but I am not that talented. Even with the cowl off, you are still working 50% blind because the intake stuff is all facing away from you." Removing the cowl provides crucial visibility and working space.