[tblake]

P0131 = O2 Sensor Circuit Low Voltage (Bank 1 Sensor 1)

I would have a look at the Pre-cat 02 sensor thats mounted in your rear exhaust manifold. If it checks out, I myself would probably just replace the sensor. They do go bad from time to time and after years and milage tend to get "lazy" where the switching time becomes longer and longer. If you do replace it, you may need to rent or borrow an O2 sensor socket, and sometimes removing it with the exhaust manifold hot makes it come out a little easier. Make sure you stick with a stock delco brand.