Carrier 58MVP -sometimes gives code 42
Hello I have 100k BTU Carrier MVP natural gas two stage furnace installed in 2005. I have just single stage thermostat. After few initial setup issues were fixed by installer my furnace worked without problems up to this year. But now I am noticing that furnace fails from time to time. When furnace fails, I see error code 42. Usually furnace fails for 20min or more and after that time it starts working again by itself. I am not 100% sure, but I have a feel that furnace tends to fail much more after it runs on second stage heat. That is normally thermostat calls for heat, furnace starts in low power and satisfies thermostat. But when it is cooler outside, after about 14min in low heat furnace switches to high heat and I just have a feeling that failures happen more in such case (but I could be wrong).
Any idea what could cause problems? I checked intake/out pipes and they do not appear to be clogged in any way. I do not hear any water sounds out of my "OUT" pipe - I know that sound as originally pipes were installed without enough slope and that had to be re-installed same year.
I already had contractor come once, but at the time furnace was running OK. Also we did a test with contractor - poored bottle of 375ml water into OUT pipe and water appear to go down through furnace into drainage, but at that time furnace stopped with error 32. After watter was completely out, furnace worked again. Contractor said that is all normal behaviour.
Frequency of failure - I had it working OK for 5 days, then 2 days in the row I noticed it failing at least once each day. But after that furnace works OK again.
Is there anything else I can check? Any idea what could cause my problem.
Failure happens when furnace tries to start motor starts increasing speed, there is still no gas start or anything, then in about 10-15second it stops and reports error 42. After multiple attempts like that furnace will eventually start working.
I also uploaded youtube video with failure: http://youtu.be/w-vBHaBh7BY