oxygen heater circut malfunction on 2003 Toyota Sienna

code po135 can not locate problem have replaced both front and back oxygen sensors any advice?

Asked by for the 2003 Toyota Sienna
P0135 indicates a fault in the cylinder bank #1, O2 sensor #1 heater circuit. Check for 12 volts and ground when the engine's running cold. the heater won't be on all the time so you need to see it on sometimes. If not, look for a fault in the wiring or the connector at Bank 1 Senosr 1 circuiut.