2.0 Rough Idle code 1131 testing

[Telegrapher]

1996 Contour 2.0 4-cyl. 75k miles

Complaint:
Very rough idle. Okay above +/- 1,500 RPM. Engine has plenty of power and accelerates fine. Occasionally the Check Engine Lite will come on with code 1131 (Lack of Upstream Heated oxygen sensor switch; sensor indicates lean - Bank 1).

Parts Replaced:

• Spark plugs
• Plug wires
• Coil pack
• PCV
• Air filter
• TPS

Testing:

• Put vacuum on EGR valve while idling and engine dies.
• Cleaned throttle body and sprayed port to IAC.
• Idle comes up when cold and settles down after warm up.
• Vacuum is about 16 inches at idle.
• Middle two cylinders drop less rpm when shorted than outer two cylinders.
• Checked rubber "t" piece and it is not cracked.
• Checked for vacuum leaks and used carb cleaner near intake manifold to cyl. head - none found.
• Cleaned MAF sensor.

Scanner Readings:

• Bank # 1, 1 - 3 to 67%
• Bank # 1, 2 - 75 to 80% steady
• MAF - 0.41

Any assistance would be appreciated.

 

[Tony2005]

Welcome. Change the Upper (before catalytic converter) O2 sensor.

 

[Telegrapher]

Change the Upper Oxygen Sensor

Change the Upper Oxygen Sensor

Welcome. Change the Upper (before catalytic converter) O2 sensor.

Thank you. Why would this oxygen sensor only affect two cylinders? It's mounted in the middle of the exhaust manifold for all four cylinders.

 

[Tony2005]

...Why would this oxygen sensor only affect two cylinders? ...

I don't know. The "two cylinder issue" could be unrelated to your O2 issue.