P0399
Stop drivingEngine Misfire or Ignition System Fault
Code P0399 means an engine misfire or ignition system fault. Common causes: Ignition coils (35%), spark plugs (30%), fuel injectors (15%). Drivability: No, stop driving. Repair cost: $50-$200 DIY, $150-$500 professional.
Can I Drive With P0399?
No, stop driving
Immediate repair needed
Stop driving immediately if: Flashing Check Engine Light, loud engine noises, burning smell, rotten egg smell from exhaust, significant power loss
Symptoms of P0399
- 1Check Engine Light is on or flashing
- 2Engine runs rough or idles poorly
- 3Noticeable loss of engine power
- 4Engine hesitation or stumbling during acceleration
- 5Poor fuel economy
- 6Engine vibrations
Common Causes of P0399
Parts most likely causing this code, ranked by probability
A faulty ignition coil fails to provide adequate spark to a cylinder, causing a misfire.
35% of P0399 cases are fixed by replacing ignition coils
Worn, fouled, or incorrectly gapped spark plugs prevent proper ignition of the fuel-air mixture.
30% of P0399 cases are fixed by replacing spark plugs
Clogged or faulty fuel injectors deliver too little or too much fuel, disrupting combustion.
15% of P0399 cases are fixed by replacing fuel injectors
Unmetered air entering the engine through a vacuum leak leans out the fuel mixture, causing misfires.
10% of P0399 cases are fixed by repairing vacuum leaks
Insufficient fuel pressure prevents proper fuel delivery to the injectors, leading to misfires.
5% of P0399 cases are fixed by addressing fuel pressure issues
Damaged wiring or loose connectors to ignition components or injectors can interrupt signals, causing misfires.
5% of P0399 cases are fixed by repairing wiring or connectors
Check These First (Before Buying Parts)
- Visually inspect all spark plug wires and ignition coil connectors for looseness or damage.
- Listen for any hissing sounds around the engine intake manifold or vacuum lines, indicating a vacuum leak.
How to Diagnose P0399
- 1Check for additional misfire codes (P030X) or fuel trim codes (P0171, P0174).
- 2Visually inspect spark plugs, ignition coils, and their connectors for damage or corrosion.
- 3Perform a spark plug and ignition coil swap test to isolate a misfiring cylinder.
- 4Check for vacuum leaks using a smoke machine or by listening for hissing sounds.
- 5Test fuel pressure at the fuel rail to ensure it meets manufacturer specifications.
- 6Perform a fuel injector balance test to identify faulty injectors.
- 7Conduct a compression test to rule out internal engine mechanical issues.
P0399 on Popular Vehicles
Toyota Camry
2007-2024While P0399 is less common than P030X codes, when it appears on a Camry, it often relates to failing ignition coils or worn spark plugs. Fuel injector issues are also a known cause for misfires in these engines.
65% fixed by replacing ignition coils or spark plugs
Honda Accord
2003-2024Honda Accords experiencing P0399 typically have issues with ignition coils or spark plugs, particularly if they are past their service interval. Valve adjustments, if neglected, can also contribute to misfires.
60% fixed by replacing ignition coils or spark plugs
Ford F-150
2004-2024For Ford F-150 trucks, P0399 often points to common misfire causes like failing ignition coils (coil-on-plug design) or worn spark plugs. Vacuum leaks, especially around the intake manifold or PCV valve, are also frequent contributors to misfires.
70% fixed by replacing ignition coils, spark plugs, or repairing vacuum leaks
Frequently Asked Questions
What does P0399 mean?
P0399 indicates a general engine misfire or a fault within the ignition system that the vehicle's computer has detected. This code is often manufacturer-specific, pointing to an issue that doesn't fit the standard P030X misfire codes.
Can I drive with P0399?
No, stop driving. A flashing Check Engine Light with P0399 means the misfire is severe enough to cause immediate damage to your catalytic converter. Driving with a severe misfire will lead to expensive repairs.
Will P0399 clear itself?
No, P0399 will not clear itself. Misfires are serious engine problems that require diagnosis and repair. The code will remain until the underlying issue is fixed and the system monitors complete their checks.
How do I fix P0399 myself?
Start by checking for other misfire codes to pinpoint a cylinder. Inspect spark plugs and ignition coils for damage. Test for vacuum leaks. If those are good, you might need to test fuel pressure and injectors. Replacing faulty spark plugs or ignition coils is a common DIY fix.
How much does it cost to fix P0399?
DIY costs range from $50-$200 for parts like spark plugs or a single ignition coil. Professional repair costs typically range from $150-$500, depending on the cause and labor rates. This includes diagnosis and parts replacement.
P0399 keeps coming back - why?
P0399 keeps coming back because the root cause of the misfire has not been fully resolved. This often happens if only one component was replaced when multiple were failing, or if the initial diagnosis missed a less obvious issue like a vacuum leak, fuel pressure problem, or a faulty fuel injector.
Will P0399 fail emissions/smog test?
Yes, P0399 will cause your vehicle to fail an emissions or smog test. A misfiring engine produces excessive pollutants, and the active Check Engine Light indicates a fault in the emissions control system.
What's the difference between P0399 and P0300?
P0300 indicates a 'Random/Multiple Cylinder Misfire Detected,' meaning the computer cannot pinpoint a specific cylinder. P0399 is a 'Manufacturer Controlled Ignition System or Misfire' code, which is often a broader, manufacturer-specific code for a misfire or ignition system fault that might not fit the P030X pattern, or it could indicate a more severe, undefined misfire.
P0399 on Ford F-150?
On a Ford F-150, P0399, like other misfire codes, commonly points to faulty ignition coils or spark plugs, especially on higher mileage vehicles. Vacuum leaks in the intake manifold or PCV system are also common culprits for misfires on these trucks.
Questions to Ask Your Mechanic
Did you check for other specific misfire codes (P030X) to narrow down the cylinder?
Good answer: Yes, I scanned for all codes and found P0399 along with P030X for cylinder X. This helps focus the diagnosis.
What were the results of the spark plug and ignition coil inspection/testing?
Good answer: I pulled the spark plugs and found cylinder X's plug was fouled. I also tested the coil on that cylinder and it showed weak spark.
Did you check for vacuum leaks or fuel pressure issues?
Good answer: Yes, I performed a smoke test and found a small vacuum leak on the intake manifold gasket. Fuel pressure was also checked and was within spec.
Repair Cost Estimate
DIY Repair
Parts only
$50-$200
Shop Repair
Parts + labor
$150-$500
Save $100-$300 by doing it yourself
Difficulty
intermediate
Time
1-3 hours
Intermediate - requires diagnostic steps beyond simple parts replacement, including potentially using a scan tool for live data and specific testing.
Cost of Waiting
Continued misfires send unburned fuel into your exhaust, rapidly overheating and destroying your catalytic converter. A $50-$200 spark plug or ignition coil job now prevents a $800-$2,500 catalytic converter replacement later.
While You're In There...
If replacing spark plugs or coils, inspect and replace old or cracked wires/boots to ensure proper electrical connection and prevent future misfires.
Apply dielectric grease to spark plug boots and coil connections to prevent moisture intrusion and ensure good electrical contact, extending component life.
Apply anti-seize to spark plug threads to prevent them from seizing in the cylinder head, making future replacements easier.
Tools You'll Need
- OBD-II Scan Tool
- Spark Plug Socket Set
- Torque Wrench
- Multimeter (for coil testing)
- Fuel Pressure Gauge (if testing fuel system)
- Vacuum Gauge or Smoke Machine (for vacuum leaks)
- Basic Hand Tools (ratchets, extensions, screwdrivers)
P0399 indicates a misfire, which is a common engine problem. Most misfire causes are straightforward to diagnose and fix, and your engine will run smoothly again once repaired.
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Your Next Steps
Step 1: Connect an OBD-II scanner to check for any additional misfire codes (P030X) that might pinpoint a specific cylinder. Note any other related codes.
Step 2: Visually inspect the spark plugs and ignition coils for the affected cylinder(s). Look for signs of fouling, damage, or corrosion. If no specific cylinder is indicated, inspect all.
Step 3: If visual inspection doesn't reveal the issue, test for vacuum leaks using a smoke machine or by carefully spraying carb cleaner around vacuum lines and intake manifold gaskets while the engine is running. Listen for changes in engine RPM.