P0125 and P1135 OBD2 Diagnostic Issues on 2000 Toyota 4Runner V6 Engine
I recently brought my 2000 Toyota 4Runner (4WD, automatic transmission, V6 engine) to a dealership for check engine light diagnosis. The scan revealed two OBD2 fault codes: P0125 and P1135. The technician explained that P1135 indicates a failure of the Air/Fuel (A/F) Sensor (Bank 1, Sensor 1), which may require replacement. I'm trying to locate this sensor and understand whether it's feasible to replace it myself. Regarding P0125, the dealer suggested a coolant flush as a potential fix—however, I'm unsure if this is effective. I've heard that a thermostat replacement might be more appropriate for P0125, especially if the engine isn't reaching proper operating temperature. The dealer's quoted repair cost is $650, which seems high. Can anyone confirm whether these codes are commonly resolved by simple maintenance or require part replacements? Any insights on correct sensor types and installation would be greatly appreciated.
A coolant flush is unlikely to resolve a P0125 code. The A/F Sensor (Bank 1, Sensor 1) is similar in function to an oxygen sensor and is mounted in the exhaust system near the manifold. However, ensure you purchase the correct part for your 2000 4Runner—some model years use different sensor types. Using the wrong sensor can result in continued failure or even damage. Other Toyota owners have reported this issue and confirm that OEM parts are essential.