They are screwed into the exhaust system at various points. Some people say that there are four oxygen sensors, but technically,
that is not correct. Only the front two are considered oxygen sensors. They are screwed into the exhaust pipe, one on each bank,
after the exhaust manifold, but before the catalytic converter. Sometimes they are screwed directly into the exhaust manifold
itself. The other two are actually called catalyst monitors. They are oxygen sensors, but they serve a different purpose.
They monitor the activities of the catalytic converters, thus the name, catalyst monitors. They are screwed into the exhaust
one on each bank,after, or downstream, of the catalytic converters.
A 7/8" wrench is the size needed to turn the O2 sensors. They
just unscrew out normally. The connectors on the front O2's will be a
challenge. They are way up behind the engine in a really stupid place
to get access and unplug/plug in.
There is actually 4 o2 sensors all located in the exhaust, one on each manifold or just after the manifold and then there is one before the catalytic converter and one after it
SOURCE: Oxygen Sensor Location 2001 Ford Expedition Eddie Bauer
There are four o2 sensors on this truck. one on each side just below the exhaust manifold, and one on each side behind the second catalytic converters. (just about even with the back of the front doors.
SOURCE: location of O2 sensor in bank 2, 99 5.4 expedition, running lean
I have a ford 2003 windstar that was running very rough when cold and almost stalling. engine light came on. went to autozone and they read the codes for free and told me it was running lean on Bank 2 also. i came home searched for the O2 front sensors online using the info autozone gave me. there are 2 front sensors on my car that need to be replaced at the same time. look for ones that have a direct plug in and don't need wire splicing. the ones without the wiring harness are cheaper but more of a pain, i think.
they were fairly easy to replace. getting access with the wrench was the worst. reset the light by disconnecting the battery for a few minutes. seems fine now and the light hasn't come back on.
SOURCE: getting a code po171 on a '98 ford expedition...O2
CODE PO171 -SYSTEM ADAPTIVE FUEL TOO LEAN. POSSIBLE CAUSE.AIR LEAKS AFTER THE MAF SENSOR OR LEAKS IN THE PVC SYSTEM. EXHAUST LEAKS BEFORE OR NEAR WHERE THE HO2S IS MOUNTED. FUEL INJECTORS RESTRICTED OR NOT SUPPLYING ENOUGH FUEL. FUEL PUMP WEAK. LEAKING EGR VALVE GASKET OR DIAPHRAGM.MAF SENSOR DIRTY CAUSES PCM TO UNDERESTIMATE AIR FLOW / NOW THE BANK 1 SENSOR ITS MOUNTED IN THE EXHAUST SYSTEM BEFORE THE CATALYTIC CONVERTER.THE BANK 2 SENSOR IS IN EXHAUST AFTER THE CATALYTIC CONVERTER.
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