Fuel and air

P004C — Turbo/Supercharger Boost Control B Circuit Low

Severity: medium Drive with care, to a shop

What P004C means

The ECU monitors boost control circuit B, which drives a boost solenoid or an electric wastegate or variable-geometry actuator. P004C sets when the voltage or signal in that circuit is lower than expected. The ECU can then no longer regulate boost accurately, so engine power drops.

Symptoms

  • Check engine light on
  • Reduced power and sluggish acceleration
  • Insufficient boost pressure
  • Poor fuel economy
  • Limp mode with limited power
  • Unusual turbo noise

Possible causes

  1. Faulty boost control solenoid B
  2. Damaged wiring or poor connector contact (open circuit or short to ground)
  3. Faulty wastegate actuator or actuator position sensor
  4. Sticking variable geometry mechanism
  5. Vacuum leak or charge-air leak
  6. Mechanical turbocharger failure
  7. ECM fault (rare)

How to diagnose

  1. Scan for codes and freeze frame data, and look for other boost-related codes
  2. Inspect the harness and connector at the solenoid or actuator for chafing, corrosion or oil contamination
  3. Check vacuum hoses and charge-air pipes for cracks, leaks and loose clamps
  4. Compare specified and actual boost in live data
  5. Check power, ground and solenoid coil resistance at the connector
  6. Check actuator movement across its full range
  7. Command the solenoid with a scan tool and confirm it responds
  8. If all is fine, evaluate the ECM and the turbo condition

Common fixes

  • Replace the boost control solenoid
  • Repair or replace damaged wiring and connectors
  • Fix vacuum and air leaks by replacing hoses, pipes or clamps
  • Repair or replace the wastegate actuator or turbocharger
  • Clear the codes and road test to confirm boost is normal

Questions and answers

What does P004C mean?

The ECU detected a low signal in boost control circuit B. The usual culprits are the solenoid, wiring or a connector, and less often a vacuum leak or the turbo itself.

Can I drive with P004C?

For a short time, yes, but power will be limited. Get it diagnosed soon to avoid damaging the turbo and engine.

How do I fix P004C?

Start by inspecting the wiring, connector and vacuum hoses, then test the solenoid and actuator. Often replacing the solenoid or repairing the harness solves it.

This is general information about the code. The meaning can differ between makes and models: a scan tool and the repair manual for your car will show the exact cause.