P1399 – Random misfire (Honda)

Bojan Popic
By Bojan Popic (Contact Me)
Last Updated 2022-09-27
Master Mechanical Engineer
CodeFault LocationProbable Cause
P1399 Random misfire
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Fuel/ignition system, MAP   sensor, lAC valve

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Table of Contents

  1. What Does Code P1399 Mean?
  2. Where is the P1399 sensor located?
  3. What are the common causes of code P1399?
  4. Get Help with P1399

What Does Code P1399 Mean?

OBD-II fault code P1399 is the manufacturer-specific code, which Honda refers to as Random misfire.

As many of you already know, a misfire is when the air-fuel mixture fails to ignite during a combustion process. This causes the engine to shake and shudder while accelerating and, if severe enough, might cause a significant loss of power. Still, sometimes the misfire will occur only randomly, with sizeable time gaps in between. Their symptoms are not that prominent, so the driver might fail to notice them. Because of that, Honda has this dedicated P1399 error code. It should warn the owner about the developing problem before it gets worse.

Misfires on their own are often challenging to diagnose and repair. But those that only happen at random are especially troublesome. So, here, we’ll find out why this happens and what are the most common causes.

The meaning of the P1399 is basically the same as the generic P0300 misfire code. And both of them are triggered when the PCM detects variations in engine speed based on readings from the crankshaft position sensor. The difference between the two is that the P1399 code is triggered at far lower speed variations than P0300. In other words, P1399 is more sensitive than P0300. Or, to illustrate, the PCM will come up with a P1399 code when it suspects a misfire, and a P0300 when it’s sure there is one.

Be aware other car makers also use this code, with different definitions between different makes. This guide applies primarily to Honda vehicles.

Where is the P1399 sensor located?

Most of us will first associate the misfire with spark plugs and the ignition system. This is no surprise, as these are the components responsible for igniting the compressed air-fuel mixture during the combustion process.

And in many cases, faulty spark plugs or other ignition system components will cause a misfire. But in reality, they are only a part of the combustion equation. Or one-third of it, to be completely precise.

The engine has to receive the right amount of air and fuel mixed up in a homogenous mixture to fire up its cylinders. Theoretically, the ratio between them should be 14.7:1 in favor of air.

This ratio, however, changes slightly one way or another depending on the running conditions. For instance, when accelerating, the PCM will add more fuel and make the mixture rich. As opposed, the mixture will be lean, with more air in it, while coasting on the highway. But if the ratio between fuel and air is way off, there will be a misfire.


What are the common causes of code P1399?

A wide range of failures may cause a misfire, even those that only happen at random. This may be either a bad spark plug or a broken ignition coil. On the other hand, the injector may be clogged or leaking, or there might be intake leaks offsetting the mixture. But all these are, in essence, Mechanics 101, and we won’t be covering them here. Instead, we’ll focus on those issues specific to Honda and their vehicles that may be causing this code. These are also the first things to check if you’re getting the P1399 on its own, without any other misfire-related codes (P0300, P0301, and so on).

Incorrect valve clearance

While most engines use hydraulic valve lifters filled with oil, most Hondas have solid ones. Besides being more robust, this configuration works better at high engine speeds. But unlike maintenance-free hydraulic lifters, solid ones need to be regularly adjusted. If the owner fails to do so, the gaps inside the valve-lifting mechanism may become too excessive. This causes the valve not to open as much as it should, meaning less air will enter the cylinder during the intake phase.

Stuck EGR valve

Modern engines have an emission control device called EGR, or Exhaust Gas Recirculation system. It returns a small portion of exhaust gases back into the intake, where they are being burnt once again. The flow through the system is controlled by the EGR valve that ensures exhaust gasses are only passing in appropriate situations. But over the years, this valve may get stuck in a wide open or closed position. This usually happens because of carbon buildups, allowing exhaust gases to flow uncontrolled towards the engine. Effectively acting as an intake leak, this unmetered flow will offset the air-fuel mixture and make it too lean to ignite.

Lose fuel injector wire plugs

Wire plugs that connect the fuel injector to the corresponding wiring may become loose. While such connectors will still be in place, they might cause interruptions in the electrical signal. This usually only happens sporadically and results in a random misfire. One way of checking this is by wiggling the connector while the engine is running. If one of them is loose, the corresponding cylinder may start misfiring.

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