Hello, I'm Chris, a current Ford Lincoln Mercury employee.
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DTC 1131 means the the upstram O2 sensor is not switching. This means it is not able to detect rich or lean conditions. In more detail it really means it sees to much oxygen in the exhaust which means you are sucking air into the engine from anohter location other than the air filter or your not getting enough fuel.
Checks here would be looking for vacuum or PCV hose leaks or intake gasket leaks. Ether sprayed on suspect areas while idleing will increase the idle if a leak is hit. Fuel pressure checks are also needed.
Here is the pinpoint test for this code straight from ford, the same thing all ford techs view to help us.
Diagnostic Trouble Codes (DTCs) P1131 bank 1 (Cylinder 1) and P1151 bank 2 indicate the fuel/air ratio is correcting rich for an overly lean condition. The HO2S voltage is less than 0.45 volt. DTCs P1132 bank 1 (Cylinder 1) and P1152 bank 2 indicate the fuel/air ratio is correcting lean for an overly rich condition. The HO2S voltage is greater than 0.45 volt.
DTCs P1130 and P1150 indicate the fuel control system has reached maximum compensation for a lean or rich condition and the HO2S is not switching.
DTC/HO2S Reference List
Are any of the above concerns present?
Diagnostic Trouble Codes (DTCs) P0171 bank 1 (Cylinder 1) and DTC P0174 bank 2 indicate the fuel/air ratio is too lean. The fuel trim system is at the rich correction limit.
DTC P0172 bank 1 and DTC P0175 bank 2 indicate the fuel/air ratio is too rich. The fuel trim system is at the lean correction limit.
DTCs HO2S Reference list:
Are any HO2S DTCs P1127, P1128 P1129, P1131, P1132, P1151 or P1152 present?
Note: If a spark occurs, remove jumper and go to Pinpoint Test Step H47 (check fuse in the heater circuit).
Is the reading 1.30 volts or greater?
Is the resistance reading less than 5.0 ohms?
Is each resistance greater than 10,000 ohms?
Is resistance greater than 10,000 ohms?
Is the reading 0.2 volt or less?
Is the resistance greater than 10,000 ohms?
Note: HO2S displayed as O2S on Scan Tool.
Is the HO2S voltage greater than 0.45 volt?
WARNING: THE FUEL SYSTEM IS PRESSURIZED WHEN THE ENGINE IS NOT RUNNING. TO PREVENT INJURY OR FIRE, USE CAUTION WHEN WORKING ON THE FUEL SYSTEM. BECOME FAMILIAR WITH THE WARNING CAUTION AND NOTE IN PINPOINT TEST HC BEFORE SERVICING.
Is the fuel pressure between 310-415 kPa (45-60 psi) for (2.5L) Contour/Mystique or 207-310 kPa (30-45 psi) for all others?
Does the fuel pressure remain within 34 kPa (5 psi) of the highest reading after one minute?
Does the fuel pressure remain within 34 kPa (5 psi) of the highest reading after 10 seconds?
Was there a pressure drop greater than 34 kPa (5 psi) while cranking the engine?
Note: This erases Continuous Memory DTCs.
Note: If misfire DTCs are displayed with the Fuel Control DTCs, use the misfire DTCs to determine the fuel injector circuits requiring testing.
Is the resistance between 11.0-18.0 ohms?
Is each resistance less than 5.0 ohms?
This test requires a standard 12 volt test lamp.
Note: Properly operating system will show a dim glow at idle on the test lamp.
Does test lamp have a dim glow while cranking or running engine?
After reading your answer I'm wondering if it could be just old age and too many miles.
It's got 180,000 on it. It's on it's third timing belt also. It seems to run fine and this only happens occasionally and always when putting around town. Last road trip was 1600 miles round trip (almost all interstate) and never once did the light come on.
I've checked for vacuum leaks myself and two dealerships did the same. She's not as zippy as she use to be and the compression ain't what it use to be either.
Old age can be a factor. As you can see in the long test. Its not a simple fix or find. Personally I would not bother with it since its so rare in occuring. Since you have checked for vacuum leaks it sure wouldnt hurt to check the fuel pressure. Here is a fuel system tuneup we do often to aid in getting some mpg and performance back.
I'll try the fuel system tune up and recheck the fuel pressure. (dealership said that it was within limits)
I'm thinking now that as long as it's only occasionally and It passes state inspections I may just run it till it drops so to speak!