P0301 is a diagnostic trouble code that sets when the vehicle’s computer detects a misfire in cylinder #1. Even though this is a generic trouble code that can set on all vehicles, the repair steps can vary depending on your make and model.
Never ignore a misfiring cylinder! P0301 is set because cylinder #1 is misfiring. Symptoms include a stumble or hesitation in your engine, and a hard to start car. It’s common for other codes to be present, especially if a failing sensor is causing the misfire.
There are a dozen things that can cause P0301 to show up on an OBD reader, and not all of them are easy to recognize.
A faulty spark plug or wire is often the culprit, especially if you haven’t had a tune-up in a while. Some vehicles have coil packs instead of spark plug wires. If that’s how your vehicle is set up, then check the coil pack on cylinder 1 and check the wiring attached to it.
Here are some other common causes:
- Faulty oxygen sensor
- Failing catalytic converter
- Not enough compression
- Bad ECU
- Burned exhaust valve
- Low on fuel – unlikely, but possible.
P0301 can be a tricky one if you don’t know what you’re doing or looking for. If you don’t notice any of the symptoms mentioned above, then your best bet is to clear the code with a scanner and then drive the car. If the code comes back, you have a problem.
However, if the engine is stumbling and running rough, you know there is a problem related to cylinder 1 and need to find it. Start by checking the wiring to cylinder 1. If everything is connected properly, then pull the spark plug. It’s best to replace all your spark plugs at once, and some vehicles may need spark plug wires, a distributor cap and possibly a new rotor.
If you have coil packs, then check the connection to the coil pack on cylinder 1. Sometimes, the connector gets brittle and starts to break. When that happens, you can buy new connectors. Coil packs are known to go bad on cars with high mileage, but they’re more expensive to replace and this should be a last resort.
Before you go out and buy new coil packs, make sure it’s not your exhaust causing a problem. Do you notice the smell of rotten eggs coming from your exhaust? A failing or broken catalytic converter can cause P0301 to set, so check your catalytic converter as well.
This article will update with more information on P0301, such as more causes, solutions and guides to troubleshooting the trouble code.