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DTC codes / BMW codes / P1420

P1420 · BMWSecondary Air Valve Control Circuit Electrical

P1420 on BMW vehicles indicates that the engine control module (DME) has detected an electrical fault in the control circuit for the secondary air injection valve, which is responsible for routing fresh air into the exhaust during cold starts to reduce emissions. The computer monitors the circuit for expected voltage or current and flags the code when the signal is out of range, indicating an open, short, or excessive resistance in the wiring or valve solenoid.

SystemPowertrain
Code typeBMW specific
SeverityLow
Fix difficultyIntermediate
AreaAuxiliary emission controls

Symptoms you may notice

  • Illumination of the check engine light, often with no other noticeable symptoms
  • Rough idle or stumbling during the first few minutes after a cold start
  • A slight hissing or rushing air noise from the engine bay during cold start due to a stuck-open valve
  • Delayed or failed secondary air pump operation, which may be audible as the pump not running
  • Increased tailpipe emissions during warm-up, potentially leading to a failed emissions test

Common causes, most likely first

  1. Failed secondary air injection valve solenoid (electrical coil open or shorted)
  2. Corroded, loose, or damaged electrical connector at the secondary air valve
  3. Broken or chafed wiring in the valve control circuit, often near the exhaust manifold or engine mount
  4. Faulty secondary air pump relay or fuse supplying power to the valve circuit
  5. Internal fault in the engine control module (DME) driver for the valve circuit, though rare
  6. Water intrusion into the valve connector or wiring harness causing intermittent shorts

How to diagnose and fix it

  1. Perform a visual inspection of the secondary air valve connector and wiring for corrosion, damage, or loose pins
  2. Check the fuse and relay for the secondary air injection system and replace if blown or faulty
  3. Measure resistance across the secondary air valve solenoid and compare to BMW specifications; replace valve if out of range
  4. Backprobe the valve control circuit at the DME connector and check for voltage or ground signal during cold start activation
  5. Inspect the wiring harness for continuity and shorts to ground or power, repairing any damaged sections
  6. Clear the fault memory and perform a cold-start drive cycle to verify the repair and ensure the code does not return
  7. If all electrical checks pass, suspect the DME and perform further diagnostics before replacement

Common Misdiagnosis Traps for P1420

  • A stuck-open secondary air valve can cause a lean exhaust condition during cold start, leading to misfire codes that may be misdiagnosed as fuel delivery issues.
  • The secondary air valve is often replaced unnecessarily when the actual fault is a corroded connector pin or a chafed wire hidden behind the engine block.
  • BMW DMEs are robust, but a shorted valve solenoid can damage the driver circuit; always check solenoid resistance before installing a new valve to avoid repeat failure.
  • In some cases, a failing secondary air pump can draw excessive current and set a P1420 code indirectly by affecting the valve control circuit voltage.
  • Cold weather can exacerbate intermittent electrical faults in the secondary air system, so a code that appears only in winter may point to a marginal connection rather than a failed component.

Can I keep driving with P1420?

The vehicle remains safe to drive under normal conditions, but prolonged driving with a faulty secondary air system can increase cold-start emissions and may eventually clog the catalytic converter. Avoid extended idling during cold starts and schedule diagnosis soon to prevent secondary damage.

Tools that help

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

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