George, it could be bad fuel (Silicon content) causing a bad o2 sensor... but tesco fuel also has a high ethanol content, which can cause acellorated deteriation of rubber components....
In particular, the 944 S2 can be prone to fuel pressure regulator failure, either causing high fuel pressure, or a leaking diaphragm causing raw fuel to creep up the vacuum hose into the intake... if your diaphragm was on the way out anyway, the higher content of ethanol could have caused it to fail a little earlier.
Take off the vacuum hose from the fuel pressure regulator and the fuel dampener and smell the nipples and hoses to see if they smell of fuel. if so, you probably have a ruptured diaphragm