P0043

P0043 sets when the ECU detects a 'circuit low' fault on the bank-1 sensor-3 (the second downstream O2 sensor on vehicles with three O2 sensors per bank, such as some Toyota, Lexus, and certain heavy-duty applications) heater control circuit - the heater driver is reading a stuck-low control voltage or unable to verify heater current draw. Sensor 3 is rare in light-duty service but appears on vehicles with multiple catalysts in series (close-coupled + underbody) where additional monitoring is needed for emissions compliance. Cheapest-first ladder: (1) Confirm the vehicle actually has a sensor 3 (not all platforms wire one even if the connector is provisioned) and locate it - usually the rearmost O2 sensor in the exhaust path. (2) Check the heater fuse and the connector at the sensor for melt damage from heat shield loss. (3) DMM the heater resistance pin-to-pin - typical 3-15 ohms; OL means the heater element is dead. (4) DMM the harness control wire for short-to-ground with the sensor unplugged. Expensive-misdiagnosis trap: replacing the closer downstream O2 sensor (sensor 2) thinking the numbering follows the dash-display convention - sensor 3 is its own physical component.

Vehicle-specific patterns: 2007-2013 Toyota Tundra / Sequoia 5.7L 3UR-FE throws P0043 from a melted bank-1 third-O2 sensor connector aft of the underbody cat. 2008-2014 Lexus LX570 with the 5.7L throws P0043 from genuine heater element failure on the rearmost bank-1 sensor after long service. 2010-2016 Toyota Sienna 3.5L 2GR-FE throws P0043 from chafed wiring at a body-mount clip. 2011-2017 Toyota Camry / Highlander with the 2GR-FE V6 (when equipped with the 3-sensor configuration for CARB markets) throws P0043 after exhaust work where the rear sensor harness gets dragged. Estimated repair: $50 to $350