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P1421 – What Does It Mean and 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.


P1421 is an OBD-II Code that refers to Cold Start Control


A P1421 code is a manufacturer specific diagnostic trouble code related to Nissan and Infiniti vehicles that occurs when the PCM/ECM wasn’t able to monitor the engine’s idle speed and ignition timing when the engine is started with pre-warming up condition.

P1421 OBDII is a fault code stored by the engine control module when it can’t properly control ignition timing and idle speed when the engine is started up under warm-up conditions on Nissan and Infiniti vehicles.


Common causes for this code include:

  • Lack of intake air volume (Blocked air filter)
  • Fuel injection system (Restricted fuel injector or bad fuel pump)
  • Bad Engine Control Module or needs reprogramming (ECM)


The symptoms of a P1421 code are:

  • Illuminated Check Engine Light
  • Engine starting difficulties


To diagnose a P1421 DTC code, a technician would:

  1. Scan for codes in the ECM and look at the freeze frame data for failure.
  2. Visually inspect the throttle body and intake manifold for any vacuum leaks.
  3. Check the engine idle speed and ignition timing with a scan tool.
  4. Check the fuel injection system for restrictions.
  5. Check the wiring harness for damage.
  6. Check the ECM for any programming issues.

Common mistakes

The following are some of the most common mistakes that a technician may make when diagnosing a P1421 code:

  • Not thoroughly inspecting all the wiring and connectors for damage.
  • Not checking the intake manifold vacuum.
  • Not checking the ignition timing.
  • Not properly diagnosing the root cause of the code.

How serious is this?

A P1421 code is a moderate severity code and should be repaired as soon as possible.

What repairs can fix the code?

The following are solutions that may fix this problem:

  • Repair or replace any wiring that is damaged
  • Replace or clean the air filter
  • Check for and repair any vacuum leaks
  • Check the fuel injection and pump system for restrictions
  • Check the engine control module for programming issues

Related codes

A P1421 is related to and may be accompanied by the following codes:

P0171 – System too Lean

P0300 – Random/Multiple Cylinder Misfire

P0505 – Idle Control System Malfunction

P1516 – Cold Start Injector Circuit Malfunction


In conclusion, the P1421 code is a diagnostic trouble code that refers to a problem with the engine’s idle speed and ignition timing when the engine is started with pre-warming up condition. There are many potential causes for this code, so it is important to diagnose and repair it as soon as possible. Some of the most common repairs include repairing any damaged wiring, replacing or cleaning the air filter, checking for and repairing any vacuum leaks, and checking the engine control module for programming issues.