Number one -- Do not change antthing until you test the item and determine that the item is bad or has failed. A mechanic who cannot perform tests to determine this is not a mechanic.
Oxygen sensors usually run between 60 to 80 bucks a piece. Depending on your exhaust system there are probably two O2 sensors -- possibly three -- located just before and after the catalytic converter - or - converters.
I have no opinion on changing a sensor until the sensor in question is tested and I have the results of the test. Is your check engine light on?
$625.00 is not a crazy amount for changing the intake manifold gaskets. I question the wisdom behind changing them in the first place. A failing intake gasket is not the normal course of action for solving a problem like you described. The first thing you should do is have the compression tested on all the cylinders of your engine. This will determine the condition of the cylinder walls and piston rings inside of your engine. There should be no more than a 10 percent difference in the compression numbers. If there is then your engine must be rebuilt. This test is easy and cheap -- have this done or do it yourself. Keep me posted---
Tuesday, November 4th, 2008 AT 10:52 PM