System: Nordyne/Miller Model #G5RC-100
Propane/DirectVent Furnace

Problem: Getting 4 blinks on the Controller Board every once in a while. Fault Condition is "Ignition Failure".

What I have noticed on start-up:
The system commences it's sequence start 1.exh. blower starts, 2.ignitor lights, 3.gas valve opens, 4.burners ignite.

But on some starts all the burns don't ignite, especially the ones near the temp. sensor, so it shuts down & re-starts after a few seconds. I'm assuming after a few of these failures the trouble codes on the C.B. goes off.

What I have tested:
1. Ignitor, tested then just replaced w/ a spare I had.
2. Temp. sensor, cleaned w/ light sandpaper.
3. Fresh Air Intake, Checked exterior opening for blockage, disassembled near furnace & blew-out w/ pressurized air.
4. rechecked all sensor connections & vacuum lines for problems.

This has still not solved my problem, what else can I check?
Do I need to replace the burner unit or have it cleaned (flames are a solid blue when it is running).