A P1155 error code in a Toyota Sienna indicates a faulty rear air/fuel (A/F) sensor. The A/F sensor is one of the types of oxygen sensors used by the electronic control module (ECM) to regulate the amount of oxygen to fuel rations received by the engine to manage performance. Toyota Siennas have several oxygen sensors in different positions along the exhaust system to monitor the engine's efficiency. To install a rear A/F/sensor P1155 in a Toyota Sienna requires no special tools and provided the minivan is completely cooled down, will take 20 minutes to complete the job.
Allow your Toyota minivan to cool down until the exhaust pipes are cool to the touch.
Locate the rear A/F sensor on the exhaust system. The sensor is located on the top side of the exhaust pipe, towards the rear of the vehicle, just after the welded joint where the header pipes join the exhaust pipe.
Disconnect the sensor by pressing the two tabs on the side of the connector at the end of the sensor wire in while pulling the plug -- attached to the vehicle's wiring harness -- out of the connector.
Unscrew and remove the old sensor from the exhaust system using a Crescent wrench to turn the sensor counterclockwise until it is loose enough to be pulled free. Make sure to attach the wrench to the lock nut at the base of the sensor and not the body of the sensor or it may break.
Insert the threaded end of the new sensor into the sensor receptacle and turn the sensor clockwise until it is hand tight. Use a Crescent wrench to complete seating the sensor by turning it clockwise 1/4 turn past hand tight.
Connect the sensor wire to the vehicle's wiring harness by pushing the plug end of the vehicle's wire into the connector on the end of the sensor wire.
Start the Sienna and let it run until it reaches operating temperature to clear the error code P1155.
