I have a three-year old Burnham gas burner running my house's (steam)
heat in my unfinished basement. It had been working fine up until
today, when it stopped running. The pilot light was still on and the
water level was fine, so I had a handy-man friend take a look at it. He
determined that the issue was an electronic sensor called a "spill
switch" and got the burner working by temporarily circumventing the
device.
I took the sensor with me to Home Depot, and learned that the store
does not carry such parts. A plumber I ran into while shopping
suggested that the problem was more likely due to inadequate carbon
monoxide ventilation, and removing the switch is a safety hazard. He
suggested I clean my chimney which has not been done in three years
(though it is capped).

I have a carbon monoxide detector within five feet of my gas burner,
and luckily it's alarm has not gone off. I also brought a
battery-operated carbon monoxide detector up close to the gas burner as
well, but doing so did not trigger an alarm.

I am looking for suggestions as to the next logical step. Should I
replace the sensor? Get the chimney cleaned? Have the burner tuned-up?

Thanks in advance.
-Ray