P004C: Turbocharger/Supercharger Boost Control B Circuit Low

What it means

The turbocharger or supercharger boost control B circuit is reporting voltage below 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 circuit and actuator used to regulate turbocharger or supercharger boost. The engine computer detects voltage below the expected range on this circuit, which can happen when its wiring is grounded, damaged, or making poor contact.

Common causes

Most likely first.

1. Wiring shorted to ground
2. Connector pins corroded - high contact resistance
3. Harness chafing through insulation

Symptoms

• Check engine light on
• Reduced engine performance

How to diagnose it

1. Save freeze-frame data and check whether boost control B voltage or commanded boost changes when the fault sets.
2. Inspect the control B connector and harness for corrosion, loose pins, and insulation rubbed through to ground.
3. With the wiring diagram, check the control B circuit for a short to ground and inspect continuity through suspect sections.
4. Check connector terminal condition and circuit connections for excessive resistance using the manufacturer's test procedure.
5. Test the actuator and control circuit against the manufacturer's specification before replacing any component.

Common fixes

• Clean and secure corroded or loose control B connector terminals.
• Repair insulation chafed through to ground and secure the harness away from rubbing points.
• Repair an open or high-resistance connection in the control B circuit.
• Replace the boost control B actuator only if circuit checks pass and the actuator fails testing.

Can I keep driving?

Drive gently and avoid high boost demand, towing, and hard acceleration because reduced power or limp mode may occur. Stop if power loss makes it unsafe to keep pace with traffic.