P0300 – Random/Multiple Cylinder Misfire Detected (OBD2)
Fault code P0300 means "Random/Multiple Cylinder Misfire Detected". Below are the real-world causes, step-by-step tests and OEM part numbers.
Last updated
P0300 quick summary
| Fault code | P0300 |
|---|---|
| Meaning | Random/Multiple Cylinder Misfire Detected |
| System | Powertrain (Engine & Transmission) |
| Vehicles | Generic OBD2 |
| Severity | High |
| Can I drive? | Only gently and for short distances. If the Check Engine light is flashing, stop – unburnt fuel can overheat and destroy the catalytic converter. |
| Repair cost | €30 – €800 |
Symptoms of P0300
- Engine shakes or runs rough, especially at idle or under load
- Check Engine light on or flashing
- Loss of power and hesitation when accelerating
- Higher fuel consumption and smell of fuel from the exhaust
- Engine may stall or be hard to start
Most common causes of P0300 (most likely first)
Mechanic's Pro Tip
P0300 means the misfire moves between cylinders – so look for something that affects ALL cylinders: a vacuum leak, low fuel pressure, a stuck-open EGR valve or bad fuel. If you also get a cylinder-specific code (P0301–P0308), start with the ignition coil and spark plug of that cylinder instead.
1.Worn or fouled spark plugs (petrol engines)
Most likely2.Weak ignition coil(s) or damaged coil boots
3.Vacuum / intake air leak (intake gasket, hoses, PCV valve, brake booster line)
4.Low fuel pressure (weak pump, clogged filter, faulty regulator) or dirty injectors
5.EGR valve stuck open, letting exhaust gas into the intake
6.Low compression (burnt valve, head gasket, worn rings) or poor-quality fuel
Step-by-step diagnostic guide
- 1
Read the misfire counters per cylinder in live data (or Mode 06). If the misfires concentrate on one or two cylinders, swap the ignition coil with a neighbouring cylinder and see whether the misfire moves with it.
- 2
Remove and inspect the spark plugs: oily or carbon-fouled plugs point to engine wear; worn electrodes mean the plugs are simply due.
- 3
Check fuel trims at idle and at 2,500 rpm: high positive trims at idle that fall at higher rpm point to a vacuum leak; trims that rise at load point to low fuel delivery. Confirm leaks with a smoke test.
- 4
Measure fuel pressure against the manufacturer's specification and check the injectors (resistance, balance or pressure-drop test).
- 5
Command the EGR valve in output tests and make sure it closes fully at idle.
- 6
If the cause is still not found, do a compression test: all cylinders should be within about 10 % of each other. A low cylinder that improves with a little oil points to rings; no change points to valves or head gasket.
Frequently asked questions
What does a flashing Check Engine light mean with P0300?
A flashing light means the misfire is severe enough to damage the catalytic converter. Reduce load immediately and have the car repaired before driving further.
How much does it cost to fix P0300?
A set of spark plugs costs roughly 20–50 € in parts, an ignition coil 25–200 € depending on the engine, plus labour. Vacuum leaks are often cheap to fix. Mechanical faults such as low compression are much more expensive.
Related fault codes
This guide is for information only. Working on vehicles can be dangerous — if you are unsure, have the car checked by a qualified mechanic. Values can differ between model years; always check the manufacturer's repair information.