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P0491 – 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.


P0491 is an OBD-II Code that refers to Secondary Air Injection System Insufficient Flow Bank 1


A P0491 code is a general diagnostic trouble code that occurs when the ECM/PCM detects a problem with the air injection system. The air injection system is designed to inject fresh air into the exhaust stream in order to improve emissions. 

When the ECM/PCM detects and determines that the secondary air flow in bank 1 is too low, it will set a P0491 code and illuminate the Check Engine Light.


Common causes for this code include:

  • Damaged wiring or connectors, or loose sensor connections
  • Bad one-way check valve in the air inlet
  • Bad secondary air pressure sensor
  • Faulty system relay
  • Faulty air injection pump or fuse
  • Significant vacuum leak
  • Clogged secondary air injection ports
  • Bad PCM/ECM (rare)


The symptoms of a P0491 code are:


To diagnose a P0491 DTC code, a technician would:

  1. Scan for codes in the ECM and look at the freeze frame data for failure.
  2. Visually inspect for damaged wiring and/or connectors
  3. Perform a system check for proper operation of the air injection system
  4. Check for any vacuum leaks that could be affecting performance
  5. Test the secondary air pressure sensor for proper function
  6. Check the secondary air injection ports to make sure they are not clogged 
  7. Check the system relay for proper function
  8. Check the air injection pump for proper function
  9. If all else fails, the PCM/ECM may need to be replaced

Common mistakes

The following are some of the most common mistakes that a technician may make when diagnosing a P0491 code:

  • Not thoroughly inspecting all the wiring and connectors for damage
  • Failing to check for vacuum leaks
  • Not checking the system relay for proper function
  • Not testing the secondary air pressure sensor for proper function 
  • Replacing parts without first checking for proper operation

How serious is this?

A P0491 code is a moderately serious code because it can cause the vehicle to run poorly and fail emissions tests. In some cases, it can also cause the engine to stall while idling or at start-up.

What repairs can fix the code?

The following are solutions that may fix this problem:

  • Broken or faulty wirings must be repaired or replaced
  • Replacing damaged one-way check valve pump
  • Replacing secondary air injection pump
  • Replacing pump fuses
  • Replacing damaged vacuum hoses or fittings that causes leaks
  • Replacing damaged connectors, wiring, and other electrical components
  • Cleaning the cylinder head

Related codes

A P0491 is related to and may be accompanied by the following codes:

P0410 Secondary Air Injection System

P2420 Secondary Air Injection System Switching Valve Stuck Open Bank 1

P2421 Secondary Air Injection System Switching Valve Stuck Closed Bank 1


In conclusion, the P0491 code is a diagnostic trouble code that refers to a problem with the air injection system. The code is caused by a variety of factors, including damaged wiring, faulty sensors, and vacuum leaks. In some cases, the code can also be caused by a bad PCM/ECM. 

If you have this code, it is important to have it diagnosed and repaired as soon as possible to avoid further damage to your vehicle.