Aqua-Hot issue

Had my annual service done mid December and all was fine for about 3 weeks then problems. The unit will start, go through the purge cycle then it ignites but within seconds shuts down. Have replaced numerous Aqua-Hot recommended parts and the problem persists. The last suggestion from A-H was to separate the exhaust pipe to see if there’s some blockage there. Tried but the joints are tight and am unable to separate them. Am considering cutting but before I do thought I might see if anyone else has experienced a similar situation. Am in central Ohio and it’s going to get very cold this weekend and so far the electric portion is doing fine but not sure how well it’ll work at 0 degrees. Any help would be greatly appreciated. Can be reached at 740-318-1773 or [email protected]