Auto Air Conditioning & Heater Repair: 2001 Toyota Avalon heating problem, toyota avalon, intermittent fault

Question
I have a Toyota Avalon 2001 model.  This past year we have experienced a heating problem.  The heater will work normally for about 30 minutes then starts blowing cold air.  We have had our Toyota dealer look at the problem several times. Each time they reported not finding any problems because it would not fail in the shop.  Each time when we drove it out of the shop it would fail on the way home.  On our last visit to the shop after putting preasure on them to locate the problem they agreed to replace the two new abient temerature sensors.  They road tested the vehicle outside for 20 minutes and the temperature stayed hot.  When we drove the vehicle home from the shop it failed again.  We have schedule another visit to the shop to try something else.  Could you help us locate this problem.  Thanks

Answer
Since this IS an intermittent fault problem, They can be tricky to find.

What I used to do was get written permission from the customer to drive the vehicle for a few days or even over the weekend so I could have a better chance of having the problem occur.

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