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What Does Code P0155 Mean?

Code P0155 is triggered when the Oxygen Sensor signal that is sent to the Powertrain Control Module remains at or near the .10 voltage point.

Related OBD-II Codes and Information

  • P0135 - Oxygen Sensor Heater Circuit Malfunction (Bank 1, Sensor 1)

The purpose of codes P0135 and P0155 is to track the amount of time that the Oxygen Sensor or Air Fuel Ratio Sensor takes to go from indicating that the fuel control system is in open loop to rapidly switching above and below the 450 millivolt set point, thus indicating that the fuel control system is in closed loop. This time measurement begins after the Engine Coolant Temperature Sensor indicates that the engine has warmed up sufficiently. If the signal lingers or is set below the lean-to-rich cross-count point—generally around 450 millivolts for an Oxygen Sensor—then the code P0135 or P0155 is triggered.

The switching time of an Air Fuel Ratio Sensor can also be observed using a scanner, though this data is only an approximation created by the Powertrain Control Module for diagnostic purposes. In order to set, this Oxygen Sensor/Air Fuel Ratio Sensor code usually requires malfunctions on two different drive cycles by the vehicle; however, if the problem is severe enough, the code can set in less than fifteen minutes on an initial test drive, after the clearing of all codes. In other words, the code setting criteria varies from vehicle to vehicle.

P0155 Symptoms

  • Check Engine Light will illuminate
  • In many cases, no abnormal symptoms may be noticed
  • In some cases, the vehicle will fail an Emissions Test
  • In many cases, the NOx emissions will be elevated

Common Problems That Trigger the P0155 Code

  • Defective Oxygen Sensor 
  • Faulty Oxygen Sensor wiring or connections

Common Misdiagnoses

  • Front Catalytic Converter is replaced when problem is a defective Oxygen Sensor
  • Oxygen Sensor is replaced when the problem is faulty sensor wiring or connection

P0135 and P0155 Diagnostic Theory for Shops and Technicians: Oxygen Sensor Heater Circuit

When the code P0135 or P0155 is set, record the freeze frame data in fine detail. Next, duplicate the code setting conditions on a test drive, paying particular attention to load, MPH, and RPM. The best tool to use on this test drive is a data streaming scan tool that has factory quality and dedicated live data. Be sure to verify the code conditions before you advance to the next set of tests.

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