Check engine light came on a few months ago, but car was running fine so I put off dealing with it until recently. Reading the light blinks, I got:
0409
0413

This translates to:
P0133
P0154

It appears both my oxygen sensors are not working correctly. However, after listening for air leaks, checking the fuse, checking for a good ground, measuring the resistance of the sensor and even monitoring the sensors while working, they seem to be within the specifications described in the fsm. One of them is kind of close to the edge of the tolerances, but the other is noticeably better. Is there something else I should check to verify that these are the problem, or should I just replace them? I don't like to shotgun-replace parts like a sloppy mechanic. How exactly does the computer determine that they are bad?

 

When you checked the fuse, did you make sure that it is bright and shiny? As I mentioned in another post, both sensors use the one fuse. So you have 2 oxy sensors which use the same fuse.
What do they have in common? The fuse. Please either sand or replace the fuse.

 

I looked at the fuse.I know what good contacts look like. They were nice and shiny. but I wiped it down cleaned it with IPA anyway. It's pretty clear that it's not the problem.

 

Ok. I had this particular issue myself, and it was the fuse.
Wanted to save you some frustration and money.