The Oxygen Sensor (O2) is equipped with a heating element, which reduces the time for the sensor to enter a closed loop. This diagnostic code refers to the front Oxygen Sensor (O2) in the bank number 2. When the O2 sensor heating element reaches operating temperature, the O2 sensor responds by switching according to the surrounding exhaust oxygen content.
The Engine Control Module (ECM) monitors the time taken by the O2 sensor to switch. Setting the fault code P0155 OBD2 means that the Oxygen Sensor (O2) heating element is exceeding the time limit set for it to heat up.