P0155 code brings up MIL and VSC and TRAC OFF

2003 Avalon: Last week the Check Engine, VSC and TRAC OFF lamps light following almost 2 years of trouble free motoring (84000 miles, but only the last 18K is mine).

The ODB scanner shows a P0155 code (O2 sensor, heater circuit, Bank 2 Sensor 1). A search in this forum shows it as the upstream one at the front, right near the oil filter.



RockAuto has it for $109 + $8 shipping and it arrives the next day. The Denso sensor is a perfect match and it comes with a tube of "never sieze" grease. The electrical fitting slips right thru the 22mm closed end socket, so it really was a 15 minute job.



Reset the MIL code, and the VSC lamp disappears as well.



I checked the old 4wire sensor (2 wires for the air/fuel element, and 2 wires for the small heater filiment) and sure enough the heater showed open on the ohmmeter. It's amazing how specific the new ODB systems are.



Now for the mystery: Does anyone know why Toyota would make the VSC traction control system shut down for an O2 sensor failure ???



Happy Motoring.