how much to replace o2 sensor and how can they know both are bad?

ouch, that sounds a little steep. try to get them from a local parts store, then call around to see how much to get them put in. if you are handy with a wrench, you'll need a 7/8 to do it yourself. a very simple job.

Call us at 410-661-3900, $850.00 is high for 2 oxygen sensors. The only way to find out if it needs 1 or 2 is to run a computer diagnostics. Joe Melvin