P0504 Volvo Brake Switch Circuit A / B Correlation

Possible causes

– Faulty stop lamp switch
– Blown stop lamp bulb
– Brake switch harness open or shorted
– Brake switch circuit poor electrical connection

Symptoms

– Engine Light ON (or Service Engine Soon Warning Light)

P0504 Volvo Description

In addition to turning on the stop lamp, the stop lamp switch signals are used for a variety of engine, transmission, and suspension functions as well as being an input for diagnostic checks. It is important that the switch operates properly, therefore this switch is designed with two complementary signal outputs: STP and ST1.

The Engine Control Module (ECM) analyzes these signal outputs to detect malfunctions in the stop lamp switch.