Code P0300: misfires on multiple cylinders

Original definition (SAE J2012): Random/Multiple Cylinder Misfire Detected^[1][2]

• P0300Random or multiple-cylinder misfire detected.
• P0301Misfire detected in cylinder 1.^[4][6][7]
• P0302Misfire detected in cylinder 2.^[4][8][9]
• P0303Misfire detected in cylinder 3.^[4][10][11]
• P0304Misfire detected in cylinder 4.^[4][12][13]

1. 1Worn, dirty or damaged spark plugs (and their leads, on cars that have them). Carista describes them, in its P0301 entry, as the most common cause and the first thing to check.^{[1][2][3][4][7]}
2. 2Faulty ignition coils. Samarins points to them as the most frequent cause of P030X misfire codes on modern cars with a coil on each spark plug.^[1][2][3][4]
3. 3Intake air or vacuum leaks: cracked or disconnected hoses, intake manifold gasket, broken intake pipe or faulty PCV valve.^{[1][2][3][4][14]}
4. 4Blocked, leaking or faulty injectors.^[1][2][3][4]
5. 5Low fuel pressure: weak pump, blocked filter or faulty pressure regulator.^{[1][2][3][14]}
6. 6Mass air flow (MAF) sensor, lambda sensors or other sensors (crankshaft, camshaft, throttle position) giving wrong data and upsetting the mixture. Samarins highlights the MAF sensor: if it fails, the mixture goes lean and can cause random misfires.^[1][3][14]
7. 7Mechanical problems: insufficient compression due to burnt or worn valves, piston rings or a leaking head gasket, or worn valvetrain parts.^{[1][3][4][14]}
8. 8EGR valve or purge valve (solenoid) stuck open, or blocked EGR passages.^[1][14]
9. 9Blocked or faulty catalytic converter.^[1][3][14]
10. 10Carbon build-up on the intake valves and injectors, common on direct-injection engines.^[4][14]
11. 11Incorrectly set timing (for example, after changing the belt or chain) or a stretched timing chain.^[3][4][14]
12. 12Engine control unit failure.^[1][3][4]

• Engine warning light on. It may flash, especially with the engine under load, when the misfire is severe.^{[1][2][4][15]}Engine warning light: faults that cause it
• The engine shakes and the idle is rough; it is more noticeable at idle or at high revs depending on the cause.^[2][4][14]Vibrations: faults that cause it
• Hesitation or jerking when accelerating.^[1][2]Jerking: faults that cause it
• Loss of power, with weak acceleration.^[1][2][3][4]Less power: faults that cause it
• Increased fuel consumption.^[1][3][7]Uses more fuel: faults that cause it
• Starting problems.^[1][3]Hard to start: faults that cause it
• The engine stalls when stationary.^[1][6]Engine stalls: faults that cause it
• Smell of unburnt petrol from the exhaust.^[6][7]

1. 1Read the codes with a scan tool and see whether, alongside the P0300, there are cylinder codes (P0301 to P0308) or others, such as P0171 (lean mixture), P0101 and P0102 (MAF sensor) or P0401 (EGR). According to Samarins, those accompanying codes must be diagnosed first, because the random misfire is often a consequence of them.^[2][3][14]
2. 2If the car is running normally at the moment, the fault may have been a one-off. MechanicBase suggests noting the code, clearing it and seeing whether it comes back: if it does not reappear after a week of use, it treats it as temporary; if it does, you need to diagnose it.^[3]
3. 3If there is a cylinder code (P0300 plus P0301, P0302…), start with the spark plug and coil of that cylinder. To find out which of the two is failing, swap the coil (or the plug) with that of another cylinder, clear the code and drive for a while: if the fault moves with the part, that part is faulty; if it stays on the same cylinder, suspect the injector or the compression. The same swap can be used to rule out a faulty injector.^[3][4][9]
4. 4If the P0300 appears on its own, without a cylinder code, suspect the mixture first rather than a specific spark plug: look for air and vacuum leaks (with a smoke machine, for example) and check the fuel pressure before replacing parts.^[2][3]
5. 5With the scan tool, look at the short-term and long-term fuel trims to see whether the mixture is lean, the MAF sensor and lambda sensor data, and the idle misfire counter, with the engine warm.^[3][14]
6. 6Check the injectors (supply, earth and click with the engine running) and compare them between cylinders if necessary; also check the filter, pump and regulator if the fuel pressure is low.^[3][7]
7. 7If the fault only happens at idle, check whether the EGR valve or the purge valve is staying open.^[14]
8. 8If the ignition and fuel supply are fine, do a compression or cylinder leak-down test and check the timing (especially if the problem began after changing the belt or chain), the valvetrain and, if the catalytic converter is suspected, the exhaust back pressure.^{[3][4][7][14]}
9. 9Because the code does not indicate the cylinder, Autodoc Club recommends working with the manufacturer's service manual and an advanced scan tool, and MechanicBase warns that replacing parts without diagnosing usually means spending money without fixing anything.^[1][3]

Faults in which this code appears

• Medium severityOccasionalFrequency: Occasional1.0 EcoBoostMisfires and engine warning light caused by the wiring harnessFord documented in a technical service bulletin that the engine wiring harness can rub against the exhaust heat shield on the 2018-2019 automatic EcoSport, which turns on the engine warning light with misfire fault codes. Spark plugs and coils are also common wear items on this turbocharged three-cylinder.