Ford Repair: 97 T-bird codes P0131 & P1131, autolite plugs, coolant level

Question
Hi, I have a ’97 Thunderbird, 3.8L that recently began sputtering periodically. It mostly runs fine, but feels like it’s misfiring randomly. Still getting normal gas mileage. Initially I feared it was the trans. Then the torque converter. Then the engine light came on and I had codes pulled. P0131 and P1131. The two upstream O2 sensors were replaced 3 years ago when I was getting P0171 and P0174. Ended up being the MAF, though. Cleaned it and has been fine for 18 months. I just cleaned it again last weekend, so I'm confident it isn't the problem.  Based on some of what I’ve read, I feel I should focus most on a vacuum leak or misfiring. Any tips? I’m not familiar with testing either at this point. Not sure the best way to go about it or what tools I should have beyond a scan tool. Thanks!

So far, I have pulled codes, changed air filter, cleaned MAF and all air ducts to intake.  Changed spark plugs.  Checked spark on 4, 5 & 6 wires (1, 2 & 3 are hard to get to, but I’ll go for those next).  #4 wire boot had some rubber chewed, but spark was strong all the way to the plug.  #6 wire seemed to have a weaker spark, but maybe the tester just isn’t consistent?  It had spark, though.  Just not as fast as the other one.  A few of the wires had some white “corrosion” inside, so I cleaned them with sandpaper the best I could.  I have another set of wires I can put on it, but I felt like this wasn’t my issue.  They’re the original wires which I took off at 47K, but the issue wasn’t the wires…it was that I used Autolite plugs.  The car apparently is finicky and needed Motorcraft.

Answer
your codes are for lack of o2 switching lean bank 1. the side #1,2,3, cylinders are on. your going to be looking for a air leak at the intake manifold. if you have a general air leak ie..  a vacuum line- you would have codes for both sides. these codes are isolated to one bank. question though. how is your coolant level ? if you find your losing alot of coolant. it is entirely possible you have a head gasket problem on that side causing those codes as well as a miss firing cylinder will cause lean codes as the sensors see to much oxygen exiting the engine. you could run the engine and spray carb cleaner along the base of the intake and see if this changes how the car runs. if it does, you know you have an intake problem