Problem with testing WB (heater circuit)

[Brian Renegar]

Started testing my WB circuit yesterday, and have run into a 
problem.  First a run down of the specs....

US PCB
Parts kit from Steve
Power source from battery on running car (13.50V)

The problem is, the sensor heats up and appears to be working, but the LED 
never comes on.  I have been monitoring VOut, and it never deviates from 
2.50V.  The current to the sensor started at about 1.33A.  Reading through 
previous posts, I see it should be just over 1.4A.  So I added several 
resistors to R4, one by one.  Eventually I got the current up to about 
1.5A, which I thought was more than adequate to properly heat it.  However, 
within a minute of connecting the power, the current slowly goes down to 
about 1.23A.  Is this supposed to happen?  I'm afraid to add any more 
resistors.  I don't want the current to go too high on initial turn-on.

Can someone please give me some ideas as to what's going on?  I don't know 
where to begin testing.  BTW, I built two boards, and they are both 
exhibiting the same behavior.

Thanks,
Brian

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