P0301 OBD2 Code Guide
P0301 is a engine diagnostic trouble code. Use this hub to understand the plain meaning, the first checks to run, and which vehicle-specific guides may be more useful before replacing parts.
Cylinder 1 Misfire Detected
Code P0301 signifies that the Powertrain Control Module (PCM) has identified a specific misfire event occurring in cylinder #1. This means that the PCM has detected that cylinder #1 is not contributing its expected power output to the engine, or that the crankshaft speed has decelerated excessively during the power stroke of cylinder #1. This condition is monitored by the PCM using crankshaft position sensor data and can be caused by issues related to ignition, fuel delivery, or engine mechanical integrity for that specific cylinder. A persistent misfire can lead to increased emissions and potential damage to the catalytic converter.
Code verification standard
P0301 is published with SAE J2012 / ISO 15031-6 code structure plus OBD2 P0301 diagnostic context.
First checks
- - Read all stored and pending codes before clearing anything.
- - Save freeze-frame data so you know speed, load and temperature when the code set.
- - Inspect wiring, connectors, fuses, hoses and obvious leaks tied to the named system.
- - Compare live data against realistic values before buying sensors or modules.
- - Clear the code only after repair and complete a drive cycle to confirm it does not return.
P0301 diagnostic path
- 11. Verify Code and Check for Other Codes: Connect an OBD-II scanner to confirm P0301 and check for any other related codes (e.g., P0351 for ignition coil primary/secondary circuit, P0201 for injector circuit). Clear codes and test drive to see if P0301 returns immediately. Note any pending or history codes.
- 22. Inspect Ignition System for Cylinder 1: With the engine off, visually inspect the ignition coil, spark plug wire (if applicable), and spark plug for cylinder #1. Look for cracks, corrosion, oil fouling, or signs of arcing. If possible, swap the ignition coil and spark plug from cylinder #1 with a known good cylinder (e.g., cylinder #2) and retest. If the misfire moves to the new cylinder (e.g., P0302), the swapped component is faulty.
- 33. Test Fuel Delivery for Cylinder 1: Using a noid light, check for injector pulse at cylinder #1's fuel injector connector. If pulse is present, use a fuel pressure gauge to confirm adequate fuel pressure. Then, perform an injector balance test or use a stethoscope to listen for the injector clicking. If possible, swap the fuel injector from cylinder #1 with another cylinder and retest. If the misfire moves, the injector is faulty.
- 44. Perform Compression Test on Cylinder 1: If ignition and fuel systems check out, perform a dry and wet compression test on cylinder #1. Compare the readings to specifications and to other cylinders. Low compression indicates an internal engine mechanical issue (e.g., worn piston rings, burnt valve, head gasket leak).
- 55. Check for Vacuum Leaks and PCM Issues: As a final step, check for vacuum leaks near cylinder #1's intake runner using a smoke machine or by carefully spraying carb cleaner around the intake manifold gasket. If all components test good and no other issues are found, consider a potential PCM issue, though this is rare and should be a last resort.
Verify before repair
- - Replace the spark plug for cylinder #1.
- - Replace the ignition coil (and spark plug wire, if applicable) for cylinder #1.
- - Replace the fuel injector for cylinder #1.
- - Repair internal engine mechanical issues (e.g., valve job, piston rings, head gasket replacement) if low compression is detected.
Vehicle-specific pages to check next
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