Oxygen Sensor Codes Keep Coming Back

So you tested the oxygen sensor and they tested good or you have already replace one or both and yet the PCM keeps accusing them as being bad (which also means that the check engine light is back on).

When this happens, it's usually due to either a Rich condition or Lean condition that's being caused by some other component on the engine.

What sucks about this, is that whatever is causing the Rich or Lean condition is not present all of the time (specially when you're conducting the tests). This happens quite a bit. The strategy here is to wait a few days for the condition (that's causing the O2 sensor issues) to get worse. When this occurs, you'll be able to test it and solve it.

Symptoms Of A Bad Oxygen Sensor

The effects of a bad oxygen sensor can be very subtle since they usually do not cause serious drive-ability problems. Here are the most common symptoms:

  1. The check engine light (CEL) will be illuminated on your instrument cluster.
  2. The diagnostic trouble codes lighting up the CEL usually are:
    1. P0131 Upstream Heated Oxygen Sensor (HO2S11) Circuit Out Of Range Low Voltage (Bank 1).
    2. P0133 Upstream Heated Oxygen Sensor (HO2S11) Circuit Slow Response (Bank 1).
    3. P0151 Upstream Heated Oxygen Sensor (HO2S21) Circuit Out Of Range Low Voltage (Bank 2).
    4. P0153 Upstream Heated Oxygen Sensor (HO2S21) Circuit Slow Response (Bank 2).
  3. Really bad gas mileage.
  4. Won't pass state mandated emission testing.

More Test Articles

I've written quite a few Ford 3.0L, 3.8L 'how to' tutorials that may help you troubleshoot the issues on your Ford car, pick up or SUV. You can find the complete list at: Ford 3.0L, 3.8L Index Of Articles.

Here's a sample of the Ford 3.0L, 3.8L V6 articles you'll find:

  1. How To Test The Fuel Injectors (Ford 3.0L, 3.8L).
  2. How To Do And Interpret An Engine Compression Test (Ford 3.0L, 3.8L).
  3. How To Diagnose Misfire Codes (Ford 3.0L, 3.8L).
  4. How To Test The Coil Pack (at: easyautodiagnostics.com).
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