I personally think you have an air or vacuum leak because I had the HEGO 41-91 errors too when I had an air leak. I think 41 is left and 91 is right so it is unlikely that both of your 02 sensors happen to be bad at the same time.
Checking for vacuum leaks: Buy a can of sensor-safe carb cleaner (about $5 bucks). With the motor running, give a small spritz of carb cleaner around the lower intake seams, upper intake seams, air tube seams after the air cleaner, and vacuum lines. Basically, any seam in the air intake system. The engine will rev higher if it sucks in carb cleaner. This way you can find the leak instantly. Keep the carb cleaner away from the air cleaner so you don't suck in carb cleaner through the normal channels. You can block off areas with rags or tape when you start narrowing it down. Good luck.
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