P05BC: Stop-Start Mode Switch Circuit Open

What it means

The driver-controlled switch that disables stop-start has an open control circuit - the ECU cannot read the driver preference and falls back to the default mode.

At a glance

• Usually lights the check-engine light

How it works

The stop-start mode switch lets the driver request a change to the system’s operating mode. The control module monitors the switch circuit, and P05BC sets when it detects an open circuit and cannot read the driver’s selection.

Common causes

Most likely first.

1. Open in switch or harness
2. Pin pushed back out of connector body
3. Cracked solder joint inside the module or sensor

Symptoms

• Check-engine light
• Stop-start mode may not respond to the switch
• Stop-start may remain enabled or unavailable

How to diagnose it

1. Use a scan tool to check the stop-start switch status and freeze-frame data, confirming whether the input stays unavailable when the switch is pressed.
2. Inspect the switch and its connector for looseness, corrosion, damage, or a terminal pushed back into the connector body.
3. Check the switch harness for breaks or damaged insulation, especially where it bends or passes through trim.
4. With the circuit isolated as specified by the manufacturer, check continuity through the switch and wiring to locate the open.
5. If the wiring and switch test correctly, inspect the connected module or sensor for a cracked solder joint or internal circuit fault.

Common fixes

• Reconnect or repair a loose switch connector or pushed-back terminal.
• Repair an open wire in the switch harness.
• Replace a faulty stop-start mode switch.
• Repair or replace the module if an internal circuit fault is confirmed.

Can I keep driving?

The fault usually affects the stop-start preference rather than basic engine operation, but the system may remain enabled or unavailable regardless of your selection. Avoid relying on the switch to disable stop-start until it is repaired.