# P0301: Cylinder 1 Misfire Detected **Verdict:** Drive to a shop this week Ignition and misfire - Generic OBD-II code, applies to all gasoline cars and light trucks sold in the US since 1996. - **Typical cost:** $100-$1,500 - **Urgency:** This week - **Most likely:** Spark plug or ignition coil on cylinder 1 - **DIY?:** Steps 1-3 ## Quick answer P0301 means the engine computer detected that cylinder 1 is misfiring, so it is not burning fuel properly. A worn spark plug or a failing ignition coil on cylinder 1 is the most common cause. A quick coil swap test usually shows whether the coil is to blame. ## What does P0301 mean? Cylinder 1 is the engine's reference cylinder. The computer uses the camshaft and crankshaft sensors to know exactly when cylinder 1 is on its power stroke, then counts through the firing order to locate every other cylinder. On most inline engines cylinder 1 sits at the end with the timing belt or chain. On V6 and V8 engines it is the front cylinder of bank 1, and bank 1 is by definition the side that holds cylinder 1. P0301 sets when the crankshaft slows briefly at the moment cylinder 1 should fire, often enough to matter. A severe misfire that could overheat the catalytic converter flashes the light and can set the code on the first trip; a milder one usually has to repeat on a second drive cycle. Because one cylinder is named, the cause is usually a part that serves only cylinder 1: its coil, plug, injector or valves. ## What are the symptoms of P0301? - Check engine light on, sometimes flashing - Rough idle or engine shaking - Loss of power and hesitation - Worse fuel economy ## What causes P0301? Ranked from most to least likely. Check them in this order: the cheap, common causes first. 1. **Failing ignition coil on cylinder 1** (most common). Coil-on-plug engines give cylinder 1 its own coil, and heat and vibration slowly break down its windings and boot. Failure shows first under load or in damp weather, then spreads to idle. A P0351 coil A code alongside P0301 points strongly to the coil or connector. Swapping the cylinder 1 coil with cylinder 2 confirms it in minutes. 2. **Worn or fouled spark plug in cylinder 1** (common). A spark plug with a worn electrode, cracked porcelain or oil or carbon fouling needs more voltage than the coil can supply. Engines that burn oil or have a leaking valve cover foul plugs faster. Pull the cylinder 1 plug and compare its gap and color with another cylinder's plug; a clear difference is your clue. 3. **Clogged or failing fuel injector on cylinder 1** (less common). If the coil and plug test good, the cylinder 1 injector may be clogged, sticking or not getting its electrical pulse. Port injectors gum up on engines that sit unused, and direct injectors can clog with carbon. A misfire worst at idle and cold start suggests fuel. A shop swaps injectors, checks the pulse or runs a balance test. 4. **Low compression in cylinder 1** (rare). A burnt exhaust valve, stuck rings, a worn cam lobe or a head gasket leak lets cylinder 1 lose pressure, so it misfires whatever ignition parts are fitted. The misfire is usually steady at every speed. Higher-mileage engines are most at risk. A compression test, then a leak-down test, shows whether cylinder 1 reads well below the rest. 5. **Vacuum leak near cylinder 1** (rare). A leaking intake manifold gasket at the cylinder 1 port, or a cracked vacuum fitting on its runner, lets unmetered air into that one cylinder. It runs lean and misfires mainly at idle, often smoothing out at higher rpm. Plastic intake manifolds can warp or crack with age. A smoke test shows the leak as smoke escaping near that runner. ## How do you diagnose P0301? 1. Find cylinder 1 using a diagram for your engine; on many inline engines it is at the end with the timing belt or chain. 2. Swap the cylinder 1 coil with cylinder 2, clear codes and drive; if the code changes to P0302, the coil is bad. 3. Pull the cylinder 1 spark plug and check for wear, cracks or oil fouling; swap plugs if unsure. 4. If the misfire stays on cylinder 1, have the injector tested or swapped. 5. Run a compression test on cylinder 1 if ignition and fuel check good. ## How do you fix P0301, and what does it cost? | Fix | Typical cost | What it involves | | --- | --- | --- | | Replace the cylinder 1 ignition coil | $150-$350 | Often a 15-minute job on four-cylinder engines. | | Replace spark plugs | $120-$450 | Replace them as a set, not just one. | | Replace the cylinder 1 fuel injector | $250-$700 | Direct-injection injectors cost more. | Full price breakdowns: [ignition coil replacement cost](/cost/ignition-coil-replacement), [spark plug replacement cost](/cost/spark-plug-replacement), [catalytic converter replacement cost](/cost/catalytic-converter-replacement). ## Is it safe to drive with P0301? > **Drive to a shop this week.** Drive gently to a shop if the light is steady. If it is flashing or the engine shakes badly, stop driving to protect the catalytic converter. ## Will P0301 fail an emissions test? Any OBD-based emissions test fails a car with P0301 stored and the light on. A steady cylinder 1 misfire also raises hydrocarbon emissions and, left long enough, can damage the catalytic converter and bring a P0420 later. After the repair, let the other readiness monitors complete with several days of mixed driving before the retest. ## After the repair: clearing P0301 After a coil swap test, remember the bad coil moved; replace the one that carried the misfire, wherever it now sits. Clear the code, then drive through the conditions in the freeze-frame data, such as a cold start or highway cruise. The cylinder 1 misfire counter on a scan tool should stay at zero, and P0351 should not return. ## How to prevent P0301 - Replace all spark plugs together at the specified interval so cylinder 1 never runs on an old plug. - Push each coil connector home until it clicks after any plug job, starting with cylinder 1. - Fix valve cover oil leaks before oil reaches the cylinder 1 plug well and softens the coil boot. - Use quality fuel, and follow any intake valve cleaning service your maker recommends for direct-injection engines. ## Codes often seen with P0301 - [P0300](/codes/p0300): Random/Multiple Cylinder Misfire Detected - [P0351](/codes/p0351): Ignition Coil "A" Primary/Secondary Circuit - [P0302](/codes/p0302): Cylinder 2 Misfire Detected - [P0420](/codes/p0420): Catalyst System Efficiency Below Threshold (Bank 1) - [P0171](/codes/p0171): System Too Lean (Bank 1) ## Frequently asked questions ### Where is cylinder 1? It depends on the engine, so check a diagram for your make and model. On many inline engines, cylinder 1 is at the end with the timing belt or chain. On V engines it is the front cylinder of bank 1; on Toyota and Honda transverse V6s that is the rear bank, near the firewall. ### How do I know if it is the coil or the plug? Swap the cylinder 1 coil with cylinder 2, clear the code and drive. If the code changes to P0302, the coil is bad. If it stays P0301, pull the cylinder 1 plug and look for wear, cracks or fouling, or swap plugs the same way to see if the misfire follows. ### Can I drive with a P0301 code? Short, gentle trips are usually fine with a steady check engine light. A flashing light means stop driving, because the misfire is severe enough to overheat the catalytic converter. Even with a steady light, prolonged misfires slowly damage the converter, so plan the repair within days rather than weeks. ### How much does it cost to fix P0301? Most P0301 repairs cost $120 to $450 at an independent shop. A single coil is about $150 to $350, and a full set of plugs $120 to $450 depending on engine access. A cylinder 1 injector runs $250 to $700, and only a confirmed compression problem pushes the bill much higher. ### Do P0301 and P0351 together mean a bad coil? Usually, yes. P0351 is a coil A circuit fault, and coil A fires cylinder 1 on most engines, so the pair points strongly to the coil, its connector or its wiring. Check that the connector is latched and clean, then swap the coil; if the circuit code stays put, inspect the wiring. ### Can a bad spark plug cause P0301? Yes, a worn, cracked or fouled cylinder 1 plug is a common cause. Pull it and look for a wide gap, cracked porcelain, wet fuel or black oily deposits. If the plugs are due, replace the full set, since the other plugs have the same mileage and will wear out soon too. ### Will P0301 clear itself? The light may turn off on its own after a few trips without a misfire, but the code stays in history and the cause is usually still there. Misfires from a failing coil or plug tend to return and worsen, so find and fix the cause instead of waiting. ### Can I fix P0301 myself? Often yes. On most four-cylinder engines with coil-on-plug ignition, the cylinder 1 coil comes off with one bolt and the plug with a socket and extension. On V6 engines where cylinder 1 sits on the rear bank, access is harder, and injector, compression and intake gasket work belongs at a shop. ### Will P0301 pass inspection? No, P0301 with the check engine light on fails an OBD-based emissions test, and the misfire raises tailpipe hydrocarbons too. Repair the cause, clear the code, then drive normally until a scan tool shows all required readiness monitors complete, which usually takes several days of mixed city and highway driving. ### Can P0301 damage my engine? A mild, short-lived misfire rarely harms the engine itself, but unburned fuel can overheat the catalytic converter, and raw fuel can wash oil off the cylinder 1 wall over time. A misfire caused by a burnt valve will get worse with driving, so do not put off the diagnosis for long. ## Sources - [SAE J2012: Diagnostic Trouble Code Definitions](https://www.sae.org/standards/content/j2012_201812/) - [EPA: On-Board Diagnostics (OBD)](https://www.epa.gov/state-and-local-transportation/vehicle-emissions-board-diagnostics-obd) ## Other ignition and misfire codes - [P0300 Random/Multiple Cylinder Misfire Detected](https://diagxcar.com/codes/p0300) - [P0302 Cylinder 2 Misfire Detected](https://diagxcar.com/codes/p0302) - [P0303 Cylinder 3 Misfire Detected](https://diagxcar.com/codes/p0303) - [P0304 Cylinder 4 Misfire Detected](https://diagxcar.com/codes/p0304) - [P0305 Cylinder 5 Misfire Detected](https://diagxcar.com/codes/p0305) - [P0306 Cylinder 6 Misfire Detected](https://diagxcar.com/codes/p0306) - [P0307 Cylinder 7 Misfire Detected](https://diagxcar.com/codes/p0307) - [P0308 Cylinder 8 Misfire Detected](https://diagxcar.com/codes/p0308) - [P0316 Engine Misfire Detected on Startup (First 1000 Revolutions)](https://diagxcar.com/codes/p0316) - [P0320 Ignition/Distributor Engine Speed Input Circuit](https://diagxcar.com/codes/p0320) - [P0201 Injector Circuit/Open - Cylinder 1](https://diagxcar.com/codes/p0201) - [P0261 Cylinder 1 Injector "A" Circuit Low](https://diagxcar.com/codes/p0261) - [P0351 Ignition Coil "A" Primary/Secondary Circuit](https://diagxcar.com/codes/p0351) ## Symptoms - [Why is my car idling rough?](https://diagxcar.com/symptoms/rough-idle) - [Why does my car hesitate when I accelerate?](https://diagxcar.com/symptoms/car-hesitates-when-accelerating) - [Why does my car jerk when I accelerate?](https://diagxcar.com/symptoms/car-jerks-when-accelerating) - [Why does my car shake at idle?](https://diagxcar.com/symptoms/car-shakes-at-idle) - [Why does my car shake when I accelerate?](https://diagxcar.com/symptoms/car-shakes-when-accelerating) ## Repair costs - [Ignition coil replacement cost](https://diagxcar.com/cost/ignition-coil-replacement): $150-$400 - [Spark plug replacement cost](https://diagxcar.com/cost/spark-plug-replacement): $120-$450 - [Catalytic converter replacement cost](https://diagxcar.com/cost/catalytic-converter-replacement): $1,000-$3,000 - [Fuel injector replacement cost](https://diagxcar.com/cost/fuel-injector-replacement) - [Head gasket replacement cost](https://diagxcar.com/cost/head-gasket-replacement) ## Warning lights linked to this - [Check engine light](https://diagxcar.com/warning-lights/check-engine) ## Terms explained - [What is Misfire?](https://diagxcar.com/glossary/misfire) --- Source page: https://diagxcar.com/codes/p0301 Updated: 2026-11-03 General information, not a substitute for a qualified mechanic.