Hello, it's my 2002 Sienna CE with 90,000 miles.

shortly after State Inspection last month, the check engine light went on.

With multimeter, I found the front A/F sensor resistor too low, just 1.5 Ohm. My obd-ii scanner showed me three faults:

P0420 -
P1135 - A/F sendor heater circuit response (bank 1 sensor 1)
P1155 - A/F sendor heater circuit (bank 1 sensor 2)

So I replaced the bank 1 sensor 1 that is located at the front, easy to replace, and cleared the error code. But the engine light went back on after a short while. A recheck now gives only one code:

P1135 - A/F sendor heater circuit response (bank 1 sensor 1)

So I'm puzzled. bank 1 sensor 1 is brand new. How come the other two error codes dissappeared while the p1135, which is about the sensor I just replace, is still there.

It is highly unlikely the newly purchased Toyota part is defective.

Can any more experienced please give me some tip over how to isolate the problem?

it cost over $200 a piece. so I don't want to waste the money to replace the bank 1, sensor 2 which is not even indicated to be bad.

many thanks for any tip.

Problem solved. The Bank1 sensor one is not the one at the front of the vehicle, rather the one at the firewall end of the engine.

Lesson learned:

never try to loosen the A/F sensor with an open wrench, it will damage the hex surface and you end up having to bring it to the dealer, who may inturn replace the whole exhaust manifold, at your money!!!, which I almost did.

At one point, I decided I've got to the get the right tool, so went to AutoZone and get a 22mm Oxygen Sensor Removal socket. It then took me 20 minutes to remove the old sensor, and 10 more minutes to install the new one.

Big saving, at least 200 labor, plus potential new exhaust manifold $300

Happy Repair!