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P1399 Code – What Does It Mean & How To Fix It

This is one of the most frequent OBD2 trouble codes. Read the full article below to know what it means, how to fix it, and what other codes may show related to it.


P1399 is an OBD-II Code that refers to Random Cylinder Misfire Detected


A P1399 code is caused by an ECM (Engine Control Module) receiving a signal from the PCM (Powertrain Control Module) or from the camshaft position sensor, signifying a misfire in one of the engine’s cylinders. The ECM will activate the check engine light and store a code in its memory that identifies which cylinder is misfiring.


Common causes for this code include:

  • A valve that is not opening, or is stuck open
  • A faulty spark plug
  • Low fuel pressure
  • Blocked exhaust pipe


The symptoms of a P1399 code are:

  • The Check Engine Light comes on
  • Hard starting, which may be accompanied by engine misfires
  • Loss of power and fuel economy


To diagnose a P1399 DTC code, a technician would:

  1. Scan for codes in the ECM and look at the freeze frame data for failure.
  2. Check the compression of the misfiring cylinder (s), using an appropriate device
  3. Check for vacuum or PCV faults
  4. Check for low fuel pressure or blocked fuel injectors, as well as engine mechanical failure.
  5. Look at the ignition timing to see if it is within specifications. A timing problem will cause a misfire.
  6. Replace spark plugs if needed (consult car’s maintenance manual first)
  7. Clear codes and test-drive the car to see if code returns. If it does return, then there are further problems that need to be addressed.

Common mistakes

The following are some of the most common mistakes that a technician may make when diagnosing a P1399 code:

  • Not checking the freeze frame data to see if the misfire was present at the time of failure
  • Assuming that a misfiring engine will always have no compression or vacuum. Only checking fuel injectors for flow but not verifying pump operation
  • Using an inadequate scan tool, which does not provide enough detail about the vehicle’s computer system.
  • Not referring to manufacturer-specified spark plug gap and heat ranges – code will indicate which is required
  • Not checking all ignition components involved in firing a specific cylinder
  • Not giving enough time for a test drive – car may stall or run roughly due to a bad fuel injector  (fuel pressure regulator) or water contamination from changing spark plugs on flooded engines

How serious is this?

A P1399 code is not an emergency, but it is a signal that the vehicle has a substantial problem. If this code appears, it should be addressed right away. If left unattended, it will likely cause additional problems with the vehicle which may lead to costly repairs.

What repairs can fix the code?

The following are solutions that may fix this problem:

  • Check spark plugs and clean, replace if necessary
  • Check fuel injectors for clogging or damage; replace as needed
  • Replace spark plug wires (check for pinched wires near spark plugs)
  • Clean exhaust manifold (if applicable) and check it for leaks. Repair as needed.
  • Repair intake manifold cracks (if applicable) (this can also cause misfires)
  • Verify that fuel pressure is at specifications
  • Reset the ECM/PCM code using a scan tool (rare)

Related codes

A P1399 is related to and may be accompanied by the following codes:

P0171 – System Too Lean (Bank 1)

P0174 – System Too Lean (Bank 2)

P0420 – Catalyst System Efficiency Below Threshold; Bank 1; Sensor Indicates Rich Condition

P1369 – Ignition Coil Primary/Secondary Circuit Malfunction


In conclusion, the P1399 code is a generic trouble code that means the computer has detected an issue with the engine’s firing system. This code signals that there is a misfire present in one of the cylinders. There are many reasons for this code to be triggered, including spark plugs, fuel injectors or even timing issues. It is up to the technician to use their best judgment when diagnosing this problem and determine which component needs replacing to correct it.