Toyota Repair: 98 Toyota 4Runner - High NOx, toyota 4runner, service mechanic

Question
Background
1998 Toyota SR5 3.4L V6 EFI DOHC 24V 4WD
No EGR?

Given recent diagnostic testing:
Catalytic converter changed
O2 sensor checked
MAF filter checked
Scoped on treadmill and highway

A non-Toyota service mechanic is suspecting leak in exhaust pipe, internal exhaust valve seating, or computer problem

I am now planning to take in to Toyota service for diagnostic review.

Other items to consider:
1 Air/fuel sensor; where is this located?
2 Hot engine; how to check - coolant? or thermostat?

Any thoughts on troubleshooting this problem?  

Answer
In the EFI engines. oxygen sensor is the main part sensing unburned fuel going through the exhaust and  EGR valve is the main part controlling the NO2,if the sensor is faulty. The fuel consumption will also increase significantly. For hottness  you better check the thermostat valve. Some time it stucks when clogged or if any rust is there. you can  simply check the water circulaion by removing the radiator cap. but becareful when if its hot. Increase throttle and see if the water circulation is fast then there is  no blockage. You must  also consider other  things for exaple ignition timing etc. but  it will be a good idea to check your vehicle by qualified mechanic.