Hi everyone,

Newbies here. 

We recently purchased a lightly used TFB29LZB (Costco version of a Tailgater).  The original owner thought the RTD needed replacement because the grill temperature quickly and uncontrollably rises from the start up smoke to 400+ degrees in a matter of seconds, no matter how you set the controller.  We've replaced the RTD but the problem continues. We're now thinking we may have a bad controller but we're reaching out for your thoughts.  Anyone have the same problem and if so, what needs replacing?  Thx.

That is not a problem that I've run across, call Traeger support they may be able to help you. 

Does it do it when the lid is left open on start up?

If it is going from outside ambient temp to 400 in seconds, I doubt the 400° is real.
More likely a bad temp sensor or controller.
Try to find an external thermometer and see what the real internal temp is after the controller readout says 400°.

 If it comes down to the controller, the Tailgater controller is different from its larger brethren. Not in looks but operation. There are plenty of fact similar controllers in the marketplace that will fit but not

function correctly in the Tailgater, causing major temp swings ..You may wish to confirm that w/ Traeger..

Their are just 2 things I am aware of that could cause your almost instant 400º readings:  1/ The RTD circuit. This was eliminated by replacing the RTD. Both the original and replacement RTDs were in use while

 this problem was present. I am assuming the complete RTD assembly: RTD w/ the wires was changed? The wires terminate @ the controller, 2 small screws in junction blocks on the controller attach the wires.
 
 If the wires were just spliced in, that could be the problem.

 2/ The controller itself.

 Finally: WELCOME! That's diving right in on your first post.

Welcome!  Nothing like jumping into the deep end from the get go.  Make sure the RTD has a good ground.  Mine was loose because the guys at the road show where we got it had put it together with everything just finger tight.  I had the same issue.  You might want to scrape the paint off a little where it mounts.  And I bet an Ortech controller would fit it.  That was a game changer on mine.

Thanks for all your responses. 
Brushpopper - We'll double check that our new RTD is not loose and is properly grounded.
Canadian Joe - yes, we replaced the entire RTD assembly.
W6YJ -We'll test true temp with thermometer.   - stand by for results!
Bentley - On smoke, it seems to work/heat up fine. As soon as we close the lid, the temp skyrockets and the unit feels very hot.
We'll also give Traeger a call see what they think might be the issue - will post our results.
Thanks again.

My Traeger takes about 1/2 hour to get even close to 400.  Unless the flame from the firepot is in direct line with the RTD, then the temp is not accurate.