Check Bosch web site for pictures of damaged sensors. Rich Fuel is indicated as a potential cause of sensor damage. Also, coolant (due to silicon) and oil can damage sensors.

http://www.boschusa.com/AutoParts/TechnicalResourceLibrary/

Also, from the Bosch site here's some information from their FAQ section relative to sensor damage causes:

Q. What will damage my oxygen sensor?

A. An oxygen sensor can fail prematurely if it becomes contaminated with phosphorus from excessive oil consumption, silicone from internal coolant leaks, using silicone sealant in the engine, and some over-the-counter fuel additives. Even a small amount of poorly refined gasoline can kill an oxygen sensor. Environmental factors such as road splash, salt, oil, and dirt can also cause a sensor to fail, as can thermal shock, mechanical stress, or mishandling. However as required by vehicle manufacturers, Bosch sensors are designed and tested with these extremes in mind.

I believe that there are a number of people that utilize Techron for fuel injection cleaning.
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dnvolvo '89 765T 200K - '91 245 100K