P0300: Random/Multiple Misfire — Causes and Severity
Key Takeaway
P0300 means multiple cylinders are misfiring. Here's what causes it and how serious it is.
What P0300 Means
P0300 indicates the ECM detected misfires across multiple cylinders — not limited to just one. This is often more complex to diagnose than a single-cylinder misfire.
Common Causes (ordered by likelihood)
| Cause | Fix Cost | Notes |
|---|---|---|
| Worn spark plugs | $100 – $300 | Most common — check gap and condition |
| Bad fuel | $50 – $100 | Bad gas batch; run a fuel treatment |
| Vacuum leak | $100 – $400 | Unmetered air affects all cylinders |
| Low fuel pressure | $200 – $800 | Weak pump starves all cylinders |
| Dirty fuel injectors | $100 – $300 | Cleaning or replacement |
| Worn ignition coils | $150 – $400 | If multiple coils are aging |
| Clogged catalytic converter | $1,000 – $2,500 | Restricts exhaust, causes backpressure |
| EGR valve stuck open | $200 – $500 | Dilutes intake charge |
Severity
| CEL Behavior | Severity | Action |
|---|---|---|
| Steady | Medium | Diagnose within days |
| Flashing | Critical | Stop driving |
ProHow Steer Helps
Steer detects P0300, tells you the severity, and recommends the most likely cause based on your vehicle data.
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