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@rbroberts,

Welcome to CL.

With your circuit as posted at 130830, looking at the current in R21 (the heater), it most certainly is current limiting at about 1.5A.

If you set R20 = 100M to stop Q1 having any significant effect then I(R21) reaches almost 18A.

Have you modified the schematic since you first posted your question? Trouble is, if you do that then the circuit in the original posting updates too so it causes chaos in trying to understand the original question.

To avoid such confusion, it's a good idea to leave the posted circuit alone, save a copy and then continue working on that copy.

:)

I have not modified the circuit since posting.

But I think I see where I confused myself. The spice simulation does show current still flowing through M3. I just failed to put probes in the right place see it.