This article will help you to diagnose Diagnostic Trouble Codes: P0135, P0141, P0155, P0161 on your GM 2.2L, 2.4L, 3.1L, 3.4L, 3.8L, 4.3L, 5.0L, 5.7L. All of these Trouble Codes have to do with a malfunction in the Oxygen Sensor's Heater or it's circuit. For the GM 4.8L, 5.3L, and 6.0L Oxygen Sensor Heater Performance Diagnostics, go here: Testing P0135, P0141, P0155, P0161 O2 Heater Performance Problem (GM 4.8L, 5.3L, 6.0L). This article is divided into several headings and they are: Important Suggestions and Tips Symptoms of a BAD Oxygen Sensor Oxygen Sensor Trouble Code Basics O2 Sensor Heater Tests Where Are The O2 Sensors Located?