I replaced the O2 sensor, but "chk engine" light continues to show up. New readout shows above
SOURCE: P0155 & P0135
O2 sensors are tricky. sometimes like anything else they fail. many times though the exhaust mixture they encounter is out of their range and that shows up as a failure.since you seem to have no other problems , I would go ahead and change them, but don't be disappointed if the code shows up again (at least you know it's not the sensors.) I don't like using the process of elimination when doing repairs but sometimes you must.
SOURCE: more 99 cougar problems
there is a fuse for the o2 heaters if one of the sensors blew the fuse they will all be out
A code P0135 and P0155 may mean that one or more of the following has happened:
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Cud b bad connection or sensor is toast
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