P0302: Causes, Symptoms, and What to Fix First

Diagnose and resolve cylinder 2 misfires quickly with this DIY guide

Quick Answer (For SEO Snippets)

P0302 is a diagnostic trouble code indicating a persistent misfire in engine cylinder 2. To fix it efficiently, follow this priority order of common causes and solutions:

Replace the cylinder 2 spark plug (most frequent fix, $5–$20 part cost).
Test and replace the cylinder 2 ignition coil (swap with a neighboring cylinder to confirm; $50–$150 part cost).
Clean or replace the cylinder 2 fuel injector (use a top-tier fuel system cleaner first; $80–$200 part cost).
Perform a compression test to rule out internal engine damage (e.g., worn rings, blown head gasket; $100–$200 professional test cost, or $20–$50 for a DIY gauge).

Key P0302 Symptoms: Rough idle, loss of acceleration, flashing check engine light (severe misfire), increased fuel consumption, rotten egg exhaust smell.
Average Fix Cost: $10–$200 for DIY repairs (spark plug/coil), $300–$1,500+ for professional internal engine or catalytic converter repairs.

Before You Start

Confirm Cylinder Numbering: Cylinder 2 location varies by engine type:
- Inline 4-cylinder: Cylinder 1 is closest to the engine’s front (timing belt/chain side); cylinder 2 is adjacent.
- V6/V8: Check your vehicle’s service manual—front-wheel-drive models often place cylinder 2 on the passenger-side bank, while rear-wheel-drive models may use the driver-side bank.
Prevent Catalytic Converter Damage: A persistent misfire can overheat and destroy the converter. Avoid driving more than 50 miles with P0302 active.
Safety Prep: Disconnect the negative battery terminal to avoid electrical shocks, and wear safety glasses and nitrile gloves.

Tools and Parts You’ll Need

Essential Tools

OBD2 scanner (to confirm P0302 and monitor misfire data)
Spark plug socket (16mm or 18mm, matching your vehicle’s plugs)
Ratchet + extension bar
Ignition coil tester or multimeter
Fuel system cleaner (e.g., Chevron Techron) or fuel injector flow tester
Compression gauge
Torque wrench (critical for proper part installation)

Common Replacement Parts (Order by Frequency of Need)

Cylinder 2 spark plug (e.g., NGK Iridium or Denso Platinum—verify fitment with your vehicle’s year/make/model/engine size)
Cylinder 2 ignition coil (OEM or reputable aftermarket like Delphi or ACDelco)
Fuel injector (if cleaning fails)
Spark plug wires (only for older distributor-based ignition systems)

Note: Always confirm part fitment on trusted retailers (AutoZone, RockAuto, Amazon) using your vehicle’s specific details.

Step-by-Step Diagnostic and Fix for P0302

Follow this order from easiest/cheapest to most complex/expensive to minimize time and cost:

Step 1: Confirm P0302 and Cylinder 2 Location

Plug in your OBD2 scanner, clear existing codes, and idle the engine for 5 minutes. If P0302 reappears, it’s a consistent cylinder 2 misfire.
Locate cylinder 2 using your engine’s diagram and mark it with tape to avoid confusion.

Step 2: Inspect and Replace the Cylinder 2 Spark Plug

Spark plug wear is the #1 cause of P0302.

Remove the ignition coil (or spark plug wire) from cylinder 2.
Use a spark plug socket to remove the plug. Inspect for worn electrodes, oil fouling (valve seal issues), or carbon buildup.
Install a new spark plug. Critical: Torque to your vehicle’s exact specification (service manual—typically 20–30 ft-lbs for 4-cylinders, 15–25 ft-lbs for smaller engines). Over-torquing cracks cylinder heads; under-torquing causes repeat misfires.
Reconnect components, start the engine, and rescan for codes. If P0302 is gone, you’re done.

Step 3: Test and Replace the Cylinder 2 Ignition Coil

Faulty coils are the second most common cause.

Quick Swap Test: Swap the cylinder 2 coil with a neighboring cylinder (e.g., 1 or 3). Clear codes, idle the engine, and rescan. If the misfire code shifts to the swapped cylinder (e.g., P0301), the original coil is bad.
Formal Resistance Test: Use a multimeter to check coil resistance. Compare readings to service manual specs (0.5–2 ohms primary, 5–15 k ohms secondary).
Replace the faulty coil and torque mounting bolts to manufacturer specs (8–12 ft-lbs). Rescan to confirm the fix.

Step 4: Check Fuel Delivery to Cylinder 2

Clogged injectors cause lean or rich misfires.

First, Clean: Add a full bottle of top-tier fuel system cleaner to your tank, fill with premium gas, and drive 50–100 miles. Rescan for codes.
Injector Flow Test: If cleaning fails, use a flow tester to compare cylinder 2’s fuel delivery to other cylinders. Replace the injector if flow rates are inconsistent.
Note: Some vehicles require intake manifold removal to access injectors—follow your service manual.

Step 5: Perform a Compression Test (Rule Out Internal Damage)

If all above steps fail, low compression indicates engine issues (worn rings, blown head gasket, burnt valves).

Disable the fuel system (unplug pump relay) and remove all spark plugs.
Thread a compression gauge into cylinder 2’s plug hole. Crank the engine 3–5 times and record the reading.
Compare to specs (125–175 psi for most engines). If 20% lower than other cylinders or below 100 psi, consult a professional.

Common Mistakes to Avoid

Misidentifying Cylinder 2: Wastes time and money. Always double-check your engine’s diagram.
Ignoring Torque Specs: Over/under-torquing spark plugs causes permanent engine damage. Never guess—use the service manual.
Driving With a Flashing Check Engine Light: Severe misfires melt catalytic converters. Pull over immediately.
Skipping Post-Fix Converter Checks: After fixing the misfire, scan for P0420 (catalytic converter efficiency) to catch damage early.
Forgetting Vacuum Leaks: Inspect hoses/gaskets near cylinder 2 for cracks—lean mixtures from leaks cause misfires.

Fitment and Diagnostic Notes

Coil-On-Plug vs. Distributor: Modern vehicles use coil-on-plug systems (each cylinder has its own coil); older models use distributors with wires. Inspect wires for cracks if applicable.
Fuel Injector Cleaners: Generic cleaners are ineffective. Use top-tier options designed to dissolve carbon buildup.
Engine-Specific Variations: Turbocharged or hybrid engines may have unique misfire causes (e.g., boost leaks). Always consult your vehicle’s service manual for model-specific steps.

Safety Precautions

Disconnect the Battery: Prevents electrical shocks or accidental engine cranking when working on ignition/fuel systems.
Cool Engine First: Let the engine cool 30+ minutes before removing spark plugs to avoid burns.
Flammable Hazards: Fuel and oil are flammable. Keep a fire extinguisher nearby and avoid open flames.
Eye/Hand Protection: Wear safety glasses and nitrile gloves to avoid debris, fuel, or chemical exposure.

When to Stop and Call a Professional

DIY fixes work for most P0302 cases, but consult a certified mechanic if:

Low Compression Reading: Indicates internal engine damage (burnt valves, head gasket failure) requiring specialized tools.
Persistent Misfire After DIY Fixes: May signal complex issues (ECM failure, wiring harness damage, hidden vacuum leaks).
Catalytic Converter Damage: Rotten egg smells or reduced power post-fix mean a clogged/melted converter (cost: $500–$2,000+).
Fuel System Uncertainty: Fuel systems are pressurized—avoid leaks or fire risks by letting a pro handle injector or pump repairs.