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P2099 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.


P2099 is an OBD-II Code that refers to Post Catalyst Fuel Trim System Too Rich Bank 2


A P2099 code is a generic diagnostic trouble code that occurs when the PCM/ECM detects a signal voltage input from the downstream oxygen sensor for engine bank 1 that indicates a problem with the air-fuel mixture. Engine exhaust gases are pushed through the exhaust manifold, into the exhaust pipe, and through the catalytic converter as spent engine Exhaust Gases. 

The ECU then goes on to incorporate the downstream O2 sensor into the equation. When the bank 2 downstream O2 sensor circuit input readings are too few oxygen molecules, a P2099 code will be stored and the indicator lamp for this fault may be illuminated.


Common causes for this code include:

  • The MAF sensor’s circuitry could be damaged
  • The mass air flow sensor could be malfunctioning
  • The air intake tubing could be disconnected from the intake manifold and have a vacuum leak, allowing unmetered air to enter the engine.
  • The O2 sensor or circuit could be damaged
  • Anything that could cause a rich condition, such as a faulty fuel pressure regulator or MAP sensor
  • The PCM could be malfunctioning


The symptoms of a P2099 code are:

  • The Check Engine Light will come on.
  • It may not accelerate properly
  • The engine may not have enough power.
  • May emit black smoke upon acceleration or start up
  • Poor fuel economy
  • The engine may stall


To diagnose a P2099 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 air intake system.
  3. Check the mass air flow sensor. 
  4. Inspect the fuel and exhaust systems.
  5. Examine the fuel pressure regulator and the oxygen sensors for proper operation.
  6. Inspect the manifold absolute pressure sensor 
  7. Use a high-tech scan tool to test the powertrain control module using the manufacturer’s recommended process.

Common mistakes

The following are some of the most common mistakes that a technician may make when diagnosing a P2099 code:

  • Not thoroughly inspecting the air intake system for leaks.
  • Not properly checking the mass air flow sensor.
  • Not cleaning the MAF sensor.
  • Not checking the fuel pressure regulator.
  • Not properly testing the oxygen sensors.
  • Failing to properly diagnose the PCM.

How serious is this?

A P2099 code is considered to be a worrisome code and should be addressed as soon as possible as it can cause a variety of problems with the engine if neglected too long such as reduced engine power, poor fuel economy, black smoke from the exhaust pipe and eventually even engine stalling.

What repairs can fix the code?

The following are solutions that may fix this problem:

  • Repair or replace any faulty parts such as the mass air flow sensor, the fuel pressure regulator, the MAP sensor or the oxygen sensors.
  • Inspect and clean the MAF sensor if it is dirty.
  • Check for vacuum leaks in the intake system and fix them.
  • Ensure that all connectors are properly connected and secured.
  • Reset the ECM/PCM.

Related codes

A P2099 is related to and may be accompanied by the following codes:

P2196 – Post Catalytic Converter Fuel Trim System Too Lean Bank 1

P0401 – EGR System Malfunction

P0456 – Evaporative Emission Control System Leak Detected (Small Leak)


In conclusion, the P2099 code is a diagnostic trouble code that refers to a problem with the air-fuel mixture on engine bank 1. There are a variety of potential causes for this code, and the most common ones should be inspected and repaired as soon as possible to avoid further damage to the engine.