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 (O2S11) Circuit Out Of Range Low Voltage (Bank 1).
    2. P0133 Upstream Heated Oxygen Sensor (O2S11) Circuit Slow Response (Bank 1).
  3. Really bad gas mileage.
  4. Won't pass state mandated emission testing.

More Test Articles

I've written quite a few 1.9L, 2.0L 'how to' tutorials that may help you troubleshoot the issues on your Ford Escort, Mercury Tracer or Ford Focus. You can find the complete list at:

  1. Ford 1.9L, 2.0L Index Of Articles.

Here's a sample of the Ford 1.9L, 2.0L articles you'll find:

  1. Crankshaft Position (CKP) Sensor Test.
  2. How To Test Engine Compression.
  3. Ignition Coil Pack Test (this info is found at:
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Ford Vehicles:

  • Escort
    • 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002
  • Focus
    • 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009

Mercury Vehicles:

  • Tracer
    • 1995, 1996, 1997, 1998, 1999