Toyota Repair: toyota tundra 2000 4.7 v8 exhaust, check eng, side exhaust manifold, tundra 2000

Question
Vehicle has 147K, no joy with extended warranty claim.  Now I'll try and be more concise.  If the O2 sensor has not failed again, would an exhaust manifold leak cause a false reading/code?  Is this more likely than another bad OEM O2 sensor?  I think the sensor has gone bad again and that "if" I have a leak between the manifold and the cylinder head that would not be enough problem to cause a code from this sensor.  Supposedly there is another leak on the passenger side from that exhaust manifold.  That sensor has not coded on us yet.  The dealership said that a manifold leak would not effect the sensors readings.  What do you think?

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Followup To
Question -
Last Februrary check engine light came on.  Code was for a failed O2 sensor, drivers side exhaust manifold top sensor.  Had it replaced with OEM sensor by private shop.  July back came the same code on the same sensor.  Guarantee says free replacement for one year, parts and labor.  Shop says the sensor is throwing this code because of a leak in exhaust manifold (non-specified area of leak).  Is this likely for I don't hear or see any areas of leakage?  I think it's a failed sensor (again).  I know Tundras have problems with exhaust manifolds BUT that is usually for stuck sensors and possible damage during sensor extractions.  Should I demand a new Toyota sensor?  Do these manifolds warp and leak?
Answer -
I think you should take the tundra to your toyota dealer and have it checked because there is an enhanced warranty in effect from the factory for the O2 sensors, depending on the mileage either one or both O2 sensors are replaced with updated sensors, without looking it up, I believe the warranty has been extended to something like 8yrs/90,000 miles so check with your dealer, if the sensor gets stuck in the manifold the manifolds and the sensors are covered under the warranty, even though there have been cases of warped manifolds they have been rare. So do yourself a big favor and contact the toyota dealer about this, if they act like they don't know what you are talking about I can give you the factory warranty bulletin number and more detailed information.  

Answer
An exhaust leak upstream of the O2 sensor will most definetely cause an error code because of the extra oxygen that the sensor sees and tries to correct by sending a signal to the computer to make the air/fuel mixture more rich, the computer sees this as a lean condition and tries to compensate up to a point within it's ROM parameters, if it can't correct the condition it will give up and turn the check engine light on.