Honda Misfire Codes and What Causes Them

What a misfire code actually tells you

A misfire means a cylinder did not burn its air and fuel on that cycle. The PCM watches crank speed, feels the small drop when a cylinder fails to fire, and counts it. One dead cylinder runs rough and shakes at idle. A random misfire that moves around feels like a general stumble. The code names the cylinder, not the cause. Spark, fuel and compression can each produce the same code, so the code opens the diagnosis instead of finishing it.

The misfire and ignition codes this site covers

P1300 is the random misfire, the one that does not stay on a single cylinder. P1201 through P1206 are the single-cylinder misfire codes, one per cylinder, so P1204 points at cylinder four. The ignition side carries P1310 for the ignition control module, plus P1311 and P1312 for its coil secondary signals. The P1381 and P1382 cylinder position sensor codes turn up when the PCM cannot place the misfire in the firing order. Generic misfire codes like P0300 and P0301 to P0306 are on OBD Codes List.

First checks: plugs and coils

Most misfires are ignition. A plug that is overdue, fouled or cracked will miss before anything else does. On an engine with coil-on-plug ignition, each coil sits on its own spark plug, and heat, moisture and a tired plug all shorten coil life. The cheap test is to swap parts between cylinders and watch the misfire counter. Move the coil from the misfiring cylinder to a known-good one. If the miss follows the coil, the coil is bad. Move the plug and see if the miss follows the plug. This costs nothing and tells you which side of the ignition to trust. While a coil is out, look into the boot for carbon tracking or moisture inside it.

Valve clearance and tight exhaust valves

When spark and fuel both check out, look at the valves. A valve that does not fully close cannot seal the cylinder, so compression leaks past it and that cylinder misfires. On an overhead-cam engine the exhaust valve is the one to suspect. It runs hottest and wears into its seat, and the clearance slowly closes up until the valve is held slightly open when it should be shut. The engine can then miss when cold or at idle and still feel fine at speed, which is why this fault hides. This is normal wear, not a defect. Set the clearance to the figure in the service manual and recheck it. Do not guess at the number.

Carbon, EGR and lean misfires

Not every misfire is ignition. A lean cylinder misfires because there is not enough fuel in the mix to light. Common causes are a clogged injector, a vacuum leak, low fuel pressure, or an EGR valve that does not close and lets exhaust leak into the intake. Carbon is the usual villain behind the EGR case. Buildup around the valve seat or in the passage can stop it sealing. The tell is a rough idle and a random misfire that arrives with an EGR code such as P1406 or P1498. Clean the EGR passages and the throttle body and smoke test the intake before replacing sensors.

The order to work through it

  1. Read every code and the freeze frame. Misfire plus an EGR code points at carbon. Misfire plus an injector code points at fuel.
  2. Check the misfire counters in live data. If the scan tool shows Mode $06 data, use it to see which cylinder is really counting, even before a code sets.
  3. Replace overdue plugs. Swap coils and plugs to see whether the miss follows the part.
  4. Check valve clearance and set it to spec.
  5. Run a compression test. A wet test that raises a low reading points at rings. No change points at valves or a head gasket.
  6. If spark, compression and clearance are good, test fuel: injector pulse and spray, fuel pressure, and a smoke test for intake leaks.

When a flashing check engine light means stop

A steady check engine light says get it looked at. A flashing one says stop now. The PCM flashes the light for a severe misfire because unburned fuel and high hydrocarbons are being pushed into the hot catalytic converter, where they can overheat and damage it. Driving on a flashing light can turn a cheap repair into an expensive one. Pull over, shut the engine off, and tow the car or have it checked before you drive it again. If the light has gone back to steady, drive gently to a shop. Do not keep pushing it to get home.