Toyota Repair: Diagnostic code, emissions test, oxygen content

Question
Ted, I have a 2003 Toyota Sienna. The check engine light came on and using a diagnostic tool, I came up with code P0135, which I learned means a 'heated oxygen sensor' is not working. How relevant is that to the reliability of the vehicle? The van has about 150,000 miles on it. I've heard it would cost about $1000 to repair. I don't want to spend that kind of money on a vehicle we might not keep much longer. I realize it probably won't pass an emissions test, but is it still reliable or is there a greater chance it will break down unexpectedly? Thanks for your time.

Answer
The O2 sensor sends a voltage signal to the computer by measuring the oxygen content in the exhaust gas, the computer makes small adjustments to the fuel mixture, a bad O2 sensor does not affect driveability to a large degree, mostly unnoticeable. I don't know where the $1000 figure comes from but the sensor can be bought online or at most autoparts stores for about $100 plus about $80 installation.