1998 Sienna o2 oxygen sensor locations???

P0155 indicated a problem with the heater circuit of O2 sensor #1 for Bank

2. O2 sensor #1 is located in the exhaust manifold; O2 sensor #2 is located after the catalytic converter.

Bank 1 is always the cylinder bank with cylinder #1 so Bank 2 is the opposite bank.

So, in the case of your Sienna, O2 sensor #1 for Bank 2 is located in the exhaust manifold closest to the front of the car, away from the passenger compartment. To access O2 sensor #1 for Bank 2, open the hood and look down at the front exhaust manifold for a cylindrical object with wires coming out the end. To change it, unplug the wires, unscrew the sensor, reverse the process.