Service engine light, Can you help please?

2004 Toyota Forerunner, SR5 with V-6. The service engine light is on. It was diagnosed at one of the local chain parts stores as having a defective
O-2 sensor. I learned this vehicle has two sensors. The first is before the catalytic converter (the engine side). The second is after the catalytic converter (tail pipe side).

When I was under the car I saw the front sensor had evidence of minor leakage around the threads. It appeared to be a fluid stain. ??? Is there a way to determine which sensor is defective and needs replacement. I hate to replace both at $75 per sensor.

I'm a retired auto shop teacher with extensive trade experience (pre computer). Is this a straight forward R & R. Should thread sealent be used? Ant pitfalls?

The owner is an elderly neighbor and she is on social security.

What code is stored by the ECM? That will tell you which sensor is bad, if it is a sensor.

If it is a sensor, it is a staightforward R&R.

Thank You Brian,
I'll go to the auto chain store with the vehicle owner and watch the process. The last time she went there on her own. Hopefully the parking lot scan will provide an answer. I wasn't particularly anxious to start installing parts based on someone elses diagnosis. The key words in your response was "If it is the sensor".

Before you do anything, post the trouble code and someone can help you identify the problem better.

BTW, any bolts or threads on the exhaust system should be covered in anti-seize compound during assembly. The reason I said "if it is a sensor..." is that many trouble codes don't distinguish between the testing sensor and the function being tested, such as the cat converter efficiency.