QuestionI have a 1995 Previa Alltrac S/C with almost 110K. Both O2 sensors were changed with universal sensors which are wired correctly. The check engine light comes on and ODBII readout is PO135 Heated oxygen sensor heater circuit malfunction Bank 1 Sensor 1. The #1 sensor is a Bosch, #2 Denso. The Bosch sensor was wired wrong initially, but wireing was corrected after less than 200 miles.
Prior to changing the O2 sensors there was one more code on the analyzer - PO125 Insufficient coolant temp for closed loop fuel control, but this code did not come back after installing O2 sensors and resetting the EFI fuse.
I am told that this model has numerous problems with various sensors and that the PO125 code comes on because the coating inside the radiator flakes off and clogs the coolant hose that has the temp sensor by the intake manifold. The ECU reads coolant temp too low, resulting in very poor gas mileage (I now get appr. 13 mpg around town and 16 on the hway). Cleaning the hose will not help since the flakes will make it clog again. However, it appears that my van has a new radiator (got it used at 109K). Would appreciate your advice how to diagnose and correct the check engine light, and how to improve mileage.
AnswerI have never heard of the radiator flaking and clogging the sensor, the PO 125 code is a direct result of the heater circuit malfunction of the sensor, the sensor heater doesn't either come up to the temperature that the computer needs to see to make the system go into closed loop operation or the temperature comes up to slow and it takes too long for closed loop operation.
I kind of think that the sensor was damaged by the incorrect wiring procedure, my advice is to replace the #1 bank #1 sensor with a factory part.
