The following is from the Ford TSP. In short, it doesn't seem that the O2s are bad, but rather something in common with all of them: the harness, battery voltage, or PCM:
P0135=Upstream Heated Oxygen Sensor Heater (HTR 11) circuit malfunction (Bank #1)
P0141=Downstream Heated Oxygen Sensor Heater (HTR 12) circuit malfunction (Bank #1)
P0155=Upstream Heated Oxygen Sensor Heater (HTR 21) circuit malfunction (Bank #2)
P0161=Downstream Heated Oxygen Sensor Heater (HTR 22) circuit malfunction (Bank #2)
Diagnostic Trouble Codes (DTCs) P0135, P0141, P0155 and P0161 indicate a short to ground, open, or short to VPWR in the HO2S heater circuit.
DTC/HO2S Reference List:
DTC P0135 = HO2S HTR-11
DTC P0141 = HO2S HTR-12
DTC P0155 = HO2S HTR-21
DTC P0161 = HO2S HTR-22
Possible causes:
-- Shorts to B+ in harness or HO2S.
-- Water in connectors.
-- Cut or pulled wires.
-- Disconnected wiring.
-- Open VPWR circuit.
-- Open GND circuit.
-- Low battery voltage.
-- Corrosion or poor mating terminals.
-- Damaged PCM.
* Visually inspect the HO2S circuit for exposed wiring, contamination, corrosion and proper assembly.
NOTE:
On some applications, a vehicle hoist may be required to access the HO2S harness.
96 Contour SE
Duratec V6 24-valve 2.5L ATX
108,000 mi.
Replaced: crankshaft w/ DMD, EGR valve (clogged), EVR, DPFE, PCV valve, evap emissions hose & tube (cracked), window regulator (broken), LH & RH PCV tubes, UIM gasket (leak), ignition coil
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