I have a 96 Previa with 245,000 miles. The engine light came on and I had it checked out. The previa takes awhile to shift into overdrive and the gas milage performance has dropped by 100 hundred miles. One place said it was the cat converter. I had a 2nd place check it. The thermosat was reading 150 degrees. The second place said to replace the both oxygen sensors, the thermosat and the mass air flow sensor. Any thoughts.