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DTC codes / P04xx · Auxiliary emission controls / P0418

P0418Secondary Air Injection System Control 'A' Circuit

P0418 means the powertrain control module (PCM) has detected a fault in the electrical control circuit for the secondary air injection (AIR) system's 'A' control valve or pump relay. The computer commands the AIR system on during cold starts to reduce exhaust emissions, but the circuit's voltage or current feedback does not match what the PCM expects, so it flags the control circuit as faulty.

SystemPowertrain
Code typeGeneric (all makes)
SeverityLow
Fix difficultyIntermediate
AreaAuxiliary emission controls

Symptoms you may notice

  • Illumination of the check engine light, often with no other noticeable symptoms.
  • A slight hesitation or rough idle during the first minute after a cold start.
  • A brief, audible whine or rush of air from the engine bay when the AIR pump should run, followed by silence.
  • Failed emissions test due to the check engine light being on.
  • In rare cases, a noticeable exhaust smell or poor cold-start driveability if the AIR system remains stuck on or off.

Common causes, most likely first

  1. Failed secondary air injection pump or control valve solenoid.
  2. Damaged, corroded, or disconnected wiring or connector at the AIR pump, valve, or relay.
  3. Blown fuse or faulty relay in the secondary air injection circuit.
  4. Open or shorted control circuit between the PCM and the AIR pump or valve.
  5. Sticking or seized AIR control valve or combi-valve, causing excessive current draw.
  6. Internal PCM driver failure (less common).

How to diagnose and fix it

  1. Check the fuse and relay for the secondary air injection system and replace any that are blown or faulty.
  2. Inspect the wiring harness and connectors at the AIR pump, control valve, and PCM for corrosion, chafing, or loose pins.
  3. Use a scan tool to command the AIR pump on and off while listening for operation and checking for voltage and ground signals at the pump connector.
  4. Measure resistance of the AIR pump and control valve solenoid to compare against manufacturer specifications.
  5. Test the control circuit for continuity and short-to-ground or short-to-power between the PCM and the AIR components.
  6. If electrical tests pass, remove and inspect the AIR control valve for carbon buildup or mechanical binding.
  7. Replace the faulty component (pump, valve, relay, or wiring) and clear the code, then verify the repair with a cold-start drive cycle.

What P0418 really tells you about the AIR circuit

  • P0418 specifically points to the control circuit for the 'A' side of the secondary air injection system, which on many engines is the pump relay or the control valve solenoid, not the pump motor itself.
  • A common misdiagnosis is replacing the AIR pump when the actual fault is a corroded connector or a failed relay that supplies power to the pump.
  • The PCM may set P0418 even if the AIR system mechanically works, because the code is triggered by an electrical feedback mismatch rather than a performance problem.
  • On some vehicles, a weak battery or charging system can cause a false P0418 because the PCM sees low voltage on the control circuit during the cold-start test.
  • Always check for related codes like P0410 or P0411 first, as they can indicate whether the fault is electrical or a flow problem.

Can I keep driving with P0418?

You can generally continue driving with P0418 because the secondary air injection system only operates briefly during cold starts and does not affect engine safety or performance. However, the check engine light will remain on, and you should avoid long-term driving without diagnosis to prevent potential emissions test failure or secondary damage to the catalytic converter.

Tools that help

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Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.

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