1999 Nissan Sentra P0138 and P0325 Error Codes: How to Diagnose Without Replacing O2 or Knock Sensor
Hello! Just a day or two ago, the 'Service Engine Soon' light came on in my 1999 Nissan Sentra 2.0L engine. After taking it to a mechanic, I was given error codes P0138 and P032
Assuming this is a SR series engine. For P0138: First verify that the heater circuit (wire BR/Y) has 12V at key-on engine-off (KOEO). Then test the oxygen sensor's two outer terminals (not center): resistance should be between 2.3 and 4.3 ohms. For P0325: Confirm there are no engine knocks, tapping sounds, or exhaust leaks. Check that the knock sensor shield is properly grounded—ensure the connector at the sensor is tight and free of corrosion. The next step would be using an OBD-II scanner to check the actual knock sensor voltage being read by the PCM.