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In case anyone has this code, Lack Of HO2S21 Switch - Sensor Indicates Lean, this was my experience...

This would lead you to believe it the O2 sensor, as indicated by the code and several posts on here, but it could be several things.

I didn't want to throw parts at it right away because the sensor, while easy to get to, was pretty froze up in my manifold so I would certainly need to take the manifold off to remove it.

  1. First I checked the leads to the O2 sensor and made sure they were clean, dry, and corrosion free...still had the code.
  2. Next I checked all my vacuum hoses as I had just had the plenum off, and I also checked leads and hoses on the EGR system
  3. Next I did a PCV check, and that was good
  4. Then I checked for a short in the wire harnes from the PCM to the O2 sensor connector...all was good there.

I finally decided that it probably was the O2 sensor so I changed it. So far, after 3 warmup cycles it hasn't thrown out the code yet. Looks like it was the sensor after all.:rolleyes2:
 
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