Setting the max temperature with M143 helped and I am up and running.
Thanks for the help, and thanks for a great platform.
--bill

@dc42 They are very stable after 75C but at room temp they flop around to almost +/-15C.

The TValues were provided by the manufacturer.

I have only had the patience to get all three of them up to 300C, and that took almost an hour of sitting by the console and resetting the faults. I have made a couple test prints at 250 and the sensors are stable.

@Phaedrux
The heaters are 110VAC controlled by 3 DC-AC SSR relays.
They are 450,250, and 100 watts, respectively.

If I start from room temp, we get the "temperature not reached" error.

Thank you for the information!
My M143 is back in.
Here is my config.g
The heaters and thermistors are custom from my extruder vendor.

config-0804.g

I have a pellet extruder with 3 heaters. Each heater and thermistor is rated up to 500C.

The sensors are not stable at room temperature, so I have been running the heater tuning by starting at a higher temperature (75-100C). I have been able to complete tuning up to 250 (M303 h2 S250) but any higher and the tuning will time out before completing.
I cannot set an active temperature for any of these heaters without a half hour of heating fault->reset fault->set heater active cycles.
The tuning parameters are not right, because I get overshoots when trying to go from 240-250, for example.

We no longer have a simple command to tell RRF that the max temp for this sensor/heater is 480C (M143).
Can anyone suggest some configuration or tuning tricks to get these heaters tuned properly?

Running a Duet 2 ethernet board, RRF 3.1.1.
Thanks in advance.

Things are working.
I added ferrite cores on the motor output lines of the external stepper drivers, to help reduce the noise. I made sure all the cable shields, power supplies, and drivers are on the same ground. And finally I wrapped the TC in a layer of kapton tape to make sure it wasn't grounding against the nozzle housing.

I failed to mention the basics:
Firmware Version: 2.02(RTOS) (2018-12-24b1)
Web Interface Version: 1.22.6
This is an IDEX sort of mechanism with 2 independent X drives on a moving Y gantry.
There are external stepper drivers Wantai DQM542 and the motors are big Nema23's with part number 23HS45-4204S.
I replaced the Arduino/ramps hardware with the Duet board, and replaced the overstuffed direct-wired electronics cabinet with a larger industrial enclosure.

UPDATE: No chance to apply a 'scope to look at the signals, so I am using lower-tech methods. I have reverted back to the thermocouple daughterboard and my previous sensors. I was able to get a good solid temperature reading from the Duet, but only if I disabled all my external stepper drivers by disconnecting their power. As soon as I connected power to even one driver, the temperature returns to 2000C, and the fault LED on the daughter board flashes about once per 1-2 seconds.
I will continue to seek the interference source(s) that are preventing me from running and will let you know the results.

The cap(s) didn't help. We are going to try getting a scope on the cable this weekend and see what is happening.

I made some more tests. Replaced the CAT5 with some 22gauge shielded cable with stranded vs solid conductors. Resistance of my cable with my multimeter seems to be 7-8 Ohms on each individual conductor. I have the 2-wire PT100 wired up in 4-wire configuration with the jumpers removed on the daughterboard. I still get a solid 2000C temperature, even with the included resistor instead of the sensor. If I shorten the cable, the sensor reads correctly.
I am unable to shorten the cable enough to make a difference with my machine geometry.
Is there a way to introduce a cable length compensation for temperature sensors?
Other ideas?