Poor mileage and codes P0135, P0141, P0155 and P0161

2003 FORD ESCAPE
190,000 MILES • 3.0L • V6 • 2WD • AUTOMATIC
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I am working on my GF's car and getting codes (P0135, P0141, P0155 + P0161) indicating faults of all four sensors (bank 1 + 2, upstream and downstream), corresponding to an abnormally high fuel consumption.

When actually inspecting the exhaust system, I notice that the catalytic converter has been replaced (with a smaller/undersized unit) and there is no downstream O2 sensor.

Also, the motor is mounted across so the exhaust banks seem to merge right by the firewall, and I can't even find the upstream sensor.

So my questions is, how many O2 sensors are there supposed to be and where are they located (could some moronic previous owner have ripped them off?).

My Torque app is indicating there are four live sensors, but all are giving me 0,000V. What is the prescribed voltage range?

Thanks in the advance!
Dec 31, 2019 at 12:22 PM
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JIS001
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There should be 4 sensors. The first thing you should do is check your fuse #5 in the engine compartment. It is a 15 amp fuse that supplies power to all 4 sensor. See images below. Let us know how the fuse is. Replace it if it looks questionable and reset the codes and also verify you have power going through the fuse.
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Dec 31, 2019 at 8:33 PM
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Thank you for the swift reply! I checked the fuse and (unfortunately) it was intact and looked fine.

Are you sure there are supposed to be four oxygen sensor? There are none downstream of the cat so I suppose there are two behind each exhaust manifold before the collector? My OBD app also made me aware that the ECU seem to be "stuck" in open loop status regardless of operating temperature and idle time (which is why I guess I am getting 0V across all sensors).

What could be the cause of this? Is the coolant temp sensor the trigger for the ECU to switch loop status?

Is it correct that voltage on upstream sensors should be around 2.5V to yield a 14.7:1 A/F ratio?
Jan 5, 2020 at 11:42 AM
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JIS001
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The reason it will not go close loop is because the A/F sensors are not responding. Here is a YouTube link so you can see the exact locations of the 4 sensors. If the catalytic converters were stolen causing a replacement to be installed then most likely they cut the sensor wires during the removal. The catalytic converters are part of the manifolds.

https://youtu.be/Xz8dxd61cz4
Jan 5, 2020 at 1:52 PM
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Thanks a lot! So I guess I was looking at a second silencer and not the catalyst (i.e. cat is not stolen, just really tucked up into the engine bay). Can it really be that all four O2 sensors have flatlined and are completely dead, or is it more likely that the culprit lie somewhere else? Will it go into close loop if both upstream sensors are functioning (i.e. start replacing them)?
Jan 6, 2020 at 12:21 PM
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JIS001
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First verify you have 12 volts to all the heater circuits on pin #1 on all 4 connectors. The wire color should be light blue with an orange strip. All 4 connectors will have that one power wire. Measure the voltages and let us know what you get.
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Jan 7, 2020 at 10:34 PM