Honda P1399 Code: Random Cylinder Misfire Detected
An older Honda's PCM counted misfires that jump between cylinders instead of staying in one.
At a glance
Severity: Moderate
- Meaning
- Random Cylinder Misfire Detected
- System
- Ignition & Misfire · Powertrain
- Safe to drive?
- With caution
- Typical repair cost
- $50–$650
- DIY difficulty
- Moderate
P1399 is the random misfire code Honda used on many of its earlier OBD-II engines, roughly the 1996 to early-2000s Civic, Accord, CR-V, Prelude and Odyssey, before the generic P0300 took over. The PCM watches crankshaft speed for the small slowdowns a misfire causes, and when the misfires wander from cylinder to cylinder rather than repeating in one, it stores P1399. On these engines the list of suspects is a little different from a modern coil-on-plug car: valve clearance that has closed up since the last adjustment, a worn distributor cap and rotor, tired plug wires, and on some models carbon-choked EGR passages.
Can I keep driving? Drive gently and get it checked soon. If the check engine light starts flashing, reduce load right away and stop driving when you can, because heavy misfiring overheats the catalytic converter.
How to read P1399
- P Powertrain
- 1 Manufacturer-specific
- 3 Ignition / misfire
- 99 Specific fault
Symptoms
- Rough or shaky idle, sometimes worse when cold
- Hesitation or bucking under load, especially going uphill
- Check engine light on, flashing during severe misfire
- Slightly lower fuel economy and a faint raw-fuel exhaust smell
- Idle that smooths out after a valve adjustment or tune-up
Possible causes
Ranked by how often each cause is behind this code (editorial estimate).
- Valve clearance too tight, especially on the exhaust valves
- Worn spark plugs, plug wires, distributor cap or rotor
- Weak ignition coil or ignitor inside the distributor
- Vacuum leak at a cracked hose, PCV hose or intake gasket
- Carbon-clogged EGR passages feeding the cylinders unevenly
- Dirty injectors or low fuel pressure causing a lean mixture
- Low compression from worn rings or burned valves
Diagnosis step by step
Check the misfire pattern
Look at the freeze frame and, if your scan tool supports it, the per-cylinder misfire counters. Misfires mainly at idle and cold point to valve clearance or vacuum leaks, while misfires under load point to ignition parts.
Tools: OBD-II scanner
Inspect plugs, wires, cap and rotor
Pull the plugs and read them: black soot means rich, white and blistered means lean or hot. Check the cap for carbon tracks and a worn center button, the rotor tip for erosion, and each wire with an ohmmeter against spec.
Tools: Spark plug socket, Multimeter, Flashlight
Check and adjust the valve clearance
With the engine fully cold, set each cylinder to top dead center on its compression stroke and measure intake and exhaust clearance with a feeler gauge against the spec on the valve cover label or service manual. Tight clearance can't be heard, which is why it's often skipped.
Tools: Screwdriver, Torque wrench
Smoke test for vacuum leaks
Feed smoke into the intake with the engine off and watch vacuum hoses, the PCV hose, the IAC valve gasket and intake manifold gaskets. A leak makes the mixture lean and the misfire random.
Tools: Smoke machine
Check EGR passages and fuel delivery
On engines known for EGR port clogging, remove the EGR valve or plenum access and check that the ports aren't packed with carbon. Then check fuel pressure against spec and consider injector cleaning if the plugs look lean.
Tools: Fuel pressure gauge, Flashlight
Confirm with a compression test
If misfires persist after ignition and valve work, run a dry and wet compression test. Cylinders should be within roughly 10 to 15 percent of each other; one that stays low with oil added points to a valve problem.
Tools: Compression tester
Where the parts are
- 1Ignition coil
- 2Spark plug
- 3Fuel injector
- 4Camshaft sensor
- 5Crankshaft sensor
- 6Engine computer (ECM)
- 7Intake manifold
Repair options & costs
| Repair | DIY? | Estimated cost |
|---|---|---|
| Tune-up: plugs, wires, cap and rotor | Yes | $120–$350 |
| Valve adjustment with valve cover gasket | Yes | $150–$400 |
| Vacuum leak repair | Yes | $50–$300 |
| EGR passage cleaning | No | $150–$450 |
| Distributor or ignition coil and ignitor replacement | Yes | $250–$650 |
Rough US estimates for parts and labor; shop rates vary by region. Not a quote.
Emissions / smog test impact
Will fail
An active P1399 fails an OBD-based smog check because the check engine light is on, and the misfire monitor itself must also pass. Fix the cause, clear the code and drive several days so the monitors report ready before retesting.
Frequently asked questions
Is P1399 the same as P0300?
It describes the same kind of fault, random misfires across cylinders, but it's Honda's own number from its earlier OBD-II systems. Later Hondas use the generic P0300 with cylinder-specific codes like P0301 and P0302.
Why do so many people say a valve adjustment fixes P1399?
Many older Honda engines have screw-type adjusters that need periodic checks, and exhaust valves tend to tighten over time. A valve that doesn't close fully leaks compression and causes a misfire that moves around, especially at idle.
Can I clear P1399 and keep driving?
It will come back if the cause is still there. Short-term driving is fine as long as the light stays steady, but a flashing light or strong fuel smell means the catalytic converter is at risk.
How often should the valves be adjusted?
Honda's schedules for these engines typically call for an inspection at long intervals or when the valves become noisy. Check the maintenance schedule for your model, and do it sooner if P1399 appears.
Sources
- Manufacturer public service information (cross-checked)
- Methodology: How We Build Code Pages · Data Sources We Use · Editorial Policy