P159F Code: Meaning, Causes, and Symptoms | OBD2 Codes

P159F – Fuel Economy Mode Switch Circuit Low Voltage

 

P159F Possible Symptoms

 

Engine Light ON (or Service Engine Soon Warning Light)

 

P159F Possible Causes

 

Faulty Fuel Economy Mode Switch,Fuel Economy Mode Switch harness is open or shorted,Fuel Economy Mode Switch circuit poor electrical connection

 

P159F Description

 

The Engine Control Module (ECM) supplies 5 V reference voltage, through the 5 V reference voltage circuit, to the S87 fuel economy mode switch. The 5 V reference voltage causes current to flow through a series of resistors in the switch to the switch ground circuit. The ECM monitors the reference voltage through a discrete fuel economy mode switch signal circuit. When the switch button is momentarily pressed, the switch contacts allow the 5 V reference voltage to by-pass one of the resistors. The ECM detects the resulting voltage drop as a fuel economy mode request and sends a GMLAN message to the Transmission Control Module (TCM) indicating the request. When the TCM receives the request, throttle progressions and transmission shift points are modified to optimize fuel efficiency. The Instrument Panel Cluster (IPC) also receives the fuel economy mode request message from the ECM and illuminates the fuel economy mode indicator lamp. The fuel economy mode switch must be pressed for between 0.094-1.5 s to activate the fuel economy mode.