Janitrol GDT 090 4A will ignite on first cycle, then burn 1-5 minutes, then cycle to blower for several minutes, then try to ignite again, sometimes it gets a minute of burn time, but mostly won't even get to ignite. Eventually cycles until I reset, then starts the whole process over again.

Sometimes, it will actually cycle properly for a few hours.

LED initially indicated open pressure switch and open limit. Replaced both (including aux limit) as well as new flame sensor. Now I still get both signals, along with open flame sensor, but only intermittently. Mostly it just cycles.

Check the ground wire to the ignition control/circuit board. If the connection is poor it will produce the same fault code as a dirty sensor. If you are still getting the high limit fault code, then you still haven't corrected the condition that is making the furnace overheat. More than likely, you have an airflow restriction somewhere in the system. You can replace the limit control every day and it won't correct the problem.

Check the ground wire to the ignition control/circuit board. If the connection is poor it will produce the same fault code as a dirty sensor. If you are still getting the high limit fault code, then you still haven't corrected the condition that is making the furnace overheat. More than likely, you have an airflow restriction somewhere in the system. You can replace the limit control every day and it won't correct the problem.

Thanks, but the system isn't getting hot enough to over heat, after the first burn, and I get the same open limit code even when it is cold after running for minutes or hours without ignition.

I have replaced the filter and cleaned the AC coil in the bottom of the unit. I even let it run with the upper access open and no filter, and the bottom access open, so I could make sure it wasn't an air restriction issue.

I'll recheck the ground, but everything there is clean and appears to be all right.

Odd, to get so many fault codes at the same time... check the multi-connector plugs at the circuit board-unplug and reconnect them several times to "scrub off" the pins and sockets. Also check board for evidence of moisture exposure and/or oxide bridging of the circuit paths.

I'll do that, but right now it seems to be running like it should, and I hate to provoke it with the temps in the low teens.

I did look at the board prior, and although I saw some dark areas on the back of the circuit board, there was no real sign of damage.

I considered that this might be the problem (considering the lack of an obvious cause), but at more than $100, I'd rather cover my bases first.

Thanks for your help

Dark areas on the board? A sign that you may be headed toward a dark place... if the problem returns take a closer look.