It would help if I knew the model of heater because there are 2 different technologies that are used in Reddy Heaters. Having more information could affect my answer.
Generally, if a heater runs with the lid off but shuts down with the lid on, its a sign that either the air pump pressure is not to specs or the nozzle is restricted or worn out. Desa International, the manufacturer, recommends annual replacement of the nozzle, but this is arguable, depending on the age of the heater.
I am pasting a document that I've had around for awhile, that may help:
"A worn nozzle creates a spray
pattern, where a good nozzle creates a fog
. With a coarse spray pattern poor combustion results in an off color flame, . The photocell doesn’t like the color, sends a signal to
the computer which shuts everything down. That’s the job of the electronics. On HSI heaters it is all about correct flame color
, not just flame.
The part where you write "If I remove the cover again it runs fine" is a symptom of a bad, worn, or restricted nozzle. Remember- the ignitor only stays red for 5 seconds, then the flame keeps itself lit. The nozzle's pattern is a cone. Lift the lid & the pattern widens (aerodynamics) - replace the lid and the cone narrows & possibly distorts then the flame goes out.
This nozzle rule will only
apply if everything is correct as far as fuel delivery applies. The pump pressure must be correct- not almost correct. Set the pressure with a low-pressure gauge that registers12-15 psi max. A gauge with a higher scale just will not read close enough to satisfy the system. If the pressure is not correct, the above rule will not apply.
Instructions for setting the pressure can be viewed at http://reddyparts.com/pressure.htm
In addition, a fan can cause the same problem if damaged or mis-matched to your specific model.
I must mention that any air leaks on the pressure side (or vacuum leaks on the fuel line, for that matter) between the pump and the nozzle will create a rich/lean mixture at the combustion chamber, creating an off-color flame."
Good luck, I hope this helps.