I have a trane xe 90, or ex 90, (gas) I forget, and the problem is this. The unit will cycle off and not come back on. The blower keeps running but I can turn it off at the thermostat. I get a "4 flash code on the bite light which is high temp limit. Cahnged out the hi temp swtch and the same thing. Eventually changed the pc board and now I get 6 flashes on the bite light which is reverse polarity or weak/bad ground. reversed the polarity and added a ground, which I did not have, but still get the same thing only witht the "new" PC board, I cannot turn off the blower,which runs all the time, at thermostat, I have to pull the breaker to get it to stop. I have better luck with the old PC board than the new one. All the limit swtchs appear to check out good. The trane man said that prolonged use with no ground can eat up the PC board. Any help will be nice AND warm... Thanks, MOnty

If you have a volt meter check the voltage wires coming in to the furnace. Check voltage from each wire to ground Black 115V and above, white 0 volts. If black has the 115volts set it up that way and try switching sec1 and sec2 on the board you may have got them crossed when you changed it out.

What is sec1 and sec2? I don't see anything like that on the PC board.
Thanks, Monty

Did you adjust the dip switches to the factory settings on the new board? Look at the wiring diagram on the back of the door panel and it will give you these settings. There are a number of limit sensors on these models, one is actually on the inducer blower that you may be overlooking. Follow the wiring diagram to each one and test it.

Dip swtchs are the same on both boards and I checked all 5 limit swtchs. I did run a ground to the h2o pipe and with the old board back in, the unit has been running for a week or so now w/no problem. Go figure...