Q: oxygen heater circut malfunction on 2003 Toyota Sienna

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code is po135 oxygen heater circut malfunction. I replaced both front and rear oxygen sensors but that did not help. someone said they thought there was 4 oxygen sensors on this year vehicle ? any sugestions ?
(1) Answer
The code P0135 is for the heater circuit on the Air Fuel Ratio Sensor, Bank 1 Sensor 1, ( this is on the exhaust manifold near the firewall NOT the radiator ) which is a different type of part than an Oxygen Sensor, it is also much more expensive, sorry to say. It is some what like 2 oxygen sensors combined into 1 specialized unit. You have to MATCH the part number of the old part with the new part and it HAS TO BE A FACTORY PART, no exceptions. No universal or after market parts, they will just re-set the code. Also, see if the new part needs a software update. I know that this sounds complicated, but I do Emissions Diagnosis in California and this is what I deal with with every day.

Then you must clear the code and do a drive cycle test drive.

You must also have a battery in very good condition or this too can set the heaters codes.
You must also be SURE that the wiring is all ok to the sensor and that the 12 volt source and ground are working properly to the sensor and at the correct times. Hope that this helps.
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