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· Junior Charter Member
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O2 sensors basically send a signal that the ratio is either low or high. After the element warms up, it transitions from one to the other according to the fuel/air ratio. When they "go bad," it's because they don't change rapidly enough - literally worn out at the molecular level. Bosch claims they should only last 80,000 miles.

Diagnosing them requires getting them to operating temps, which is why so many now have heater elements to speed things up for the EPA cycle. Secondly, they have to have input voltage to put out a signal, and third, they need to switch states rapidly enough that the computer doesn't flag them for slow response.

This is why more pro touring and competition cars have a running display of the EFI program on the dashboard to modify and diagnose things. It's really just one big multipurpose gauge displaying all the inputs.
 

· Junior Charter Member
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1,146 Posts
Oh, yeah, O2 symptoms - generally, as they age, they slow down, and the system goes rich, taking about 2mpg off the top. Don't know why it goes rich, I suspect that's better than lean and holing a piston.
 
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