P004D: Turbocharger/Supercharger Boost Control B Circuit High

What it means

The turbocharger or supercharger boost control B circuit is reporting voltage above the expected range.

At a glance

• Usually lights the check-engine light
• Affects emissions; the car can fail an emissions test
• The engine or gearbox may go into limp mode

How it works

Boost control B is the electrical control path and actuator that regulates turbocharger or supercharger boost. This code sets when the computer detects circuit voltage above the expected range, often because the wire is shorted to battery power or a connection is faulty.

Common causes

Most likely first.

1. Wiring shorted to battery
2. Connector pins corroded - high contact resistance
3. Supply wire pinched against B+ source

Symptoms

• Check engine light on
• Reduced engine performance

How to diagnose it

1. Record freeze-frame data and inspect control B circuit voltage and boost behavior when the fault is present.
2. Inspect the control B harness for pinching or insulation damage where it could touch a battery-positive feed.
3. Check the connector for corrosion, displaced pins, and poor contact that could distort the circuit reading.
4. Use the wiring diagram to test the control B circuit for a short to supply and check continuity as specified by the manufacturer.
5. Test the boost control B actuator and circuit against the manufacturer's specification before replacing parts.

Common fixes

• Repair a pinched control B wire that is contacting a power source.
• Clean, repair, or secure damaged control B connector terminals.
• Repair harness insulation and route it away from power feeds and hot or moving parts.
• Replace a failed boost control B actuator only after confirming its circuit is intact.

Can I keep driving?

Avoid hard acceleration, towing, and sustained high-load driving because boost control may be limited and the vehicle may enter limp mode. If power loss is severe, pull over safely before continuing.