Ignition and misfires code
P0300: Random or multiple cylinder misfires
The car computer watches how smoothly the crankshaft spins. It saw brief slowdowns that jump around between cylinders, or that it could not pin on one cylinder. Each slowdown is a cylinder that failed to burn its fuel, which wastes fuel and sends raw gas into the exhaust.
- Safe to drive?
- Short trips only
- Most common cause
- Worn spark plugs
Drive gently only if the light is steady; if it flashes, stop and get a tow because raw fuel can destroy the catalytic converter.
How this range is built
| Part of the job | Low | High |
|---|---|---|
| Parts | $20 | $120 |
| Labor: 0.6 to 2.5 hours × $125 to $175 an hour | $75 | $438 |
| Fees: shop supplies, disposal, diagnostics | $0 | $15 |
| Total, rounded | $95 | $580 |
The hourly rate is the BLS Occupational Employment and Wage Statistics, May 2025 average pay for this trade in Boston, $31.54 an hour, times 4 to 5.5 to cover what a shop bills (overhead, tools, insurance, profit). Parts are a band from budget aftermarket to the manufacturer part. Full method.
Not a repair shop, estimates only. Typical range, estimate. Not a quote. Your car, parts and shop change the number.
This is the range for the most common fix, spark plug replacement. Which repair your car needs depends on a proper diagnosis.
What you may notice
- Shaking or rough idle
- Flashing check-engine light
- Loss of power
- Smell of fuel from the exhaust
Common causes, most likely first
- Worn spark plugs
- Weak ignition coils
- Vacuum leak or lean mix
- Low fuel pressure or clogged injectors
- Low compression or timing trouble
Check this first
Check when the spark plugs were last changed. Old plugs are the cheapest and most common cause.
Other repairs that sometimes fix P0300
| Repair | Typical range, Boston |
|---|---|
| Ignition coil replacement (one coil) | $80 to $350 |
| Vacuum leak repair | $100 to $530 |
| Fuel injector replacement (one injector) | $180 to $910 |
Before you pay
- Ask the shop what test showed this part is the cause, not just that the code appeared.
- Ask for parts and labor listed separately, and for the hours.
- Ask for a written OK before any extra work. Massachusetts rules require written authorization before repairs.
Got a quote already? Check it against the range. Want to see if your car has a recall tied to this problem? Check recalls by VIN.
Need to read a code yourself? OBD-II scanners on eBay (affiliate link, disclosure).
P0300 questions
What is the difference between P0300 and P0301?
P0300 means the misses are spread across several cylinders or hop around. P0301 through P0308 name one specific cylinder that is missing most. A random code points to something shared, like old plugs, fuel supply or a vacuum leak.
Why does the light flash?
A flashing light means the misfire is strong enough to overheat the catalytic converter right now. Ease off the gas, avoid hills and towing, and get to a shop soon rather than driving across town.
Are spark plugs expensive to replace?
On most four-cylinder engines it is a modest job because the plugs sit on top. Some V6 engines bury the rear plugs under the intake, which adds labor, so ask for a price that covers all of them.
How much does it cost to fix P0300?
The most common fix, spark plug replacement, has a typical range of $95 to $580 in Boston. It is an estimate built from parts prices and local labor wages, not a quote. Your car and shop change it.
P0300 repair cost by city
Related codes
- P0301: Cylinder 1 misfire
- P0302: Cylinder 2 misfire
- P0171: Engine getting too much air or too little fuel (bank 1)
- P0420: Catalytic converter not working well (bank 1)
More ignition and misfires codes
Code meaning written in our own words. Some codes mean different things on different makes, so check your scan tool or manual. Not a repair shop, estimates only.