UPDATE: As correctly pointed out by a commentator, you should call a pro when there is an issue such as this. At the time, I was a NATE certified technician with 25 years of experience and intended this as a technician job only. If you're qualified to work on furnaces, this may be just the ticket as I've worked on a few of these only to have a callback because the grommet to the ignitor wire didn't seal. If you are a homeowner and want to try and tackle this yourself, you need the proper tools as well as the knowledge to do these safely. Do not replace any part that is not intended for your furnace, or you could snuff out you and your family. That would not be good. This is for entertainment only, do this and any other fix or repair at your own risk.
Sorry, I didn't upload pictures yet again, but this my answer to a problem that in reality started two years ago. Anyone who has a Coleman mobile home furnace, or anyone who had the seeming misfortune of trying to diagnose an apparent problem with the combustion air (pressure) switch. This nearly always happens after replacing the hot surface ignitor or flame sensor. Putting the burner back in, along with the gas valve will inevitably bring this code up when starting the furnace.
You can put a manometer on the combustion air switch and test the pressure (not on the plastic part) and you'll get anywhere from -0.30 to -0.16 inches of water column, but the switch is set for -0.15. This isn't the problem; now what.
The board may look a little discolored around a couple resistors, but this isn't necessarily an issue by itself either. Replacing it won't solve it and the combustion air code will come back. This is very frustrating and expensive. Taking the vent and intake apart won't reveal much either, unless these are clogged, but again, probably not.
If you're a heating technician, this is frustrating, but completely fixable without charging the customer more money. Try this tip before calling tech support (which will tell you the thermostat or control board is bad because the furnace is losing its call for heat. This is if you can even get through. Remember that black grommet you removed when you replaced the ignitor or flame rod. Well do you? No doubt this is burned or not sealing properly. That combustion air switch is more than a bit finicky and from my experience, a manometer, properly zeroed in will still throw it off. Remove the manometer and put the hoses back where they belong on the switch and intake. Use a high temperature (red) silicone caulk or sealant around that grommet and seal up where the wires go in. This could be the cure to the problem. Let it dry a few minutes and turn the furnace on, cycling it naturally a few times. This means waiting about 15 to 20 minutes, but the furnace should run normally now. Maranatha!