A P0300 engine code will read: “cylinder misfire detected”. It can sometimes be accompanied by another code that will point to a specific cylinder misfiring, but the 300 code indicated it is either random cylinders, or multiple misfiring at a time.
In this guide we’re going to provide some diagnostic and troubleshooting steps to help you solve any P0300 codes you are getting for cylinder misfires.
- Check engine light
- P0301-P0312 codes might be present
- Flashing check engine light
- Rough idle
- Stuttering/hesitation accelerating
- Power loss
- Poor overall performance
This engine code indicates a cylinder misfire. Misfires often result in rough idle, stuttering, power loss, and poor overall performance. While the symptoms are easy to identify, there are dozens of potential problems that can cause a misfire, so we need to dig in deeper to understand what underlying problem is actually causing it.
Causes of P0300 Fault Codes
If you would rather consume this content via a video, check out our How to Diagnose and Fix an Engine Misfire video below:
- Spark plugs
- Ignition coils/distributor
- Fuel injectors
- Fuel delivery issues
- Vacuum leaks
- Cam or crank sensors
Single Cylinder vs. Multiple Cylinder Misfires
Single cylinder misfires will usually give you an engine code ranging from P0301 to P0312, indicating which cylinder is the one misfiring. Misfires on a single cylinder are almost always a single spark plug, ignition coil, or fuel injector.
However, a more generic P0300 code usually indicates multiple misfires. This opens up the number of underlying problems that could cause it. Multiple cylinder misfires usually points to the fuel pump, rail, cam sensor failure, and potentially some even more serious issues.
Less Frequent Causes
- Bad fuel
- Ignition timing off
- Low compression
Running into bad fuel quality doesn’t happen too often, but it’s possible, especially with diesel fuel. Poor fuel might not ignite properly and lead to misfires (usually on all or most cylinders). Ignition timing off usually means the timing belt or chain snapped or jumped a few teeth. It’s a serious issue when this occurs and you’ll likely notice very severe symptoms and drivability issues.
Otherwise, as engines age they can begin losing compression (or if a failure occurs). Loss of compression is a serious internal engine problem. It’s not something you want to run into, but it’s probably the least common cause of diagnostic trouble code P0300.
Below is a way to diagnose code P0300 assuming other single cylinder misfire codes are present:
- Confirm code P0300 and individual misfires codes are present
- Delete codes with scanner and test drive. If no codes are present then repeat this cycle to confirm. Otherwise, if the codes come back then it’s time to track down the actual issue
- Double check to confirm the individual cylinders that are misfiring
- Visually inspect ignition coil wires and other easily visible sensors, hoses, lines, etc
- If nothing seems out of place then it’s time to move onto some dirty work that’s pretty straight-forward for even novice car owners
- Swap the ignition coils from the misfiring cylinders to cylinders that are not misfiring
- If the misfires follow to the new cylinders then you have your issue
- Otherwise, swap the spark plugs from the misfiring cylinders to other good cylinders
- If none of these then fuel injectors are a possibility, but they can be complex to swap
Again, the above steps work well if you have cylinder specific misfire codes alongside P0300. It’s easy to swap ignition coils and on most engines should only take about 10-30 minutes to swap a few coils around. Spark plugs are also easy, but they lie below the ignition coils so it’s best to start from the top.
When Other Codes Are Present
- Essentially same first steps from above. Confirm which codes are present. Delete the codes and proceed on a few test drives. If the codes come back then it’s time to diagnose the problem(s) at hand
- Double check the basics like ignition coil wires, vacuum hoses, etc. Ensure there are not any loose connections, broken wires, or cracked/dirty sensors that may cause code P0300
- Check fuel pressure if any codes are present indicating low fuel pressure. The fuel pump and fuel pump sensor are two common issues
- Consider camshaft and crankshaft sensor issues. These will often throw a code suggesting something is wrong with the respective sensor
- Perform compression and leakdown tests. This will confirm whether any cylinders are losing compression that may cause misfires
The most important thing is using the internet and some critical thinking. Find other codes present with P0300. Determine the meaning of the other code and use that info to diagnose the problems.
- Spark plugs: $25-500 (assuming all are replaced)
- Ignition coils: $100-600 (assuming all are replaced)
- Fuel injectors: $400-1,500+
- Fuel pump: $100-1,000+
- Sensors: $40-300+ (fuel pump sensor, crank sensor, cam sensor)
- Vacuum hoses: $0-200