The expansion tank. Next to the pressure cap is a coolant level switch (improperly referee as 'a probe'). See examples @ BusDepot though they don't list the switch. After I had it replaced the problem was solved.

Expansion tank:
http://www.busdepot.com/details.jsp?partnumber=025121403A

Pressure cap:
http://www.busdepot.com/details.jsp?partnumber=025121321B

> What tank has a temp probe???
> 
> Larry A.
> 
> 
> On Sun, Jan 8, 2012 at 9:02 AM, Tom Torrella <ttorrella4@comcast.net> wrote:
>> My 1.9 had same intermittent issue. The probe was replace solving the problem.
>> 
>> Possibly two issues:
>> Air in reserve tank off the water pump.
>> Defective temp probe in same tank.
>> Also check condition of pressure cap - tight?  leaking?
>> 
>> TomT