Burner only running a few seconds

[mats_li]

Seems like my furnace has developed a problem...
It's a Rheem RGDJ-07EAUER gas hot air furnace - close to 10 years old.

It's not responding properly to the thermostat; even though the temperature is lower than the desired temperature setting, it's not starting. If I turn off the thermostat, and then on again, I can get the blower engine started.

After a few seconds the igniter lights up, the gas valve opens and the burner starts firing - but only for a few seconds before it goes out again. This cycle repeats several times while the blower keeps blowing cool air, until the blower also stops.

This happened a couple of times in the past, but it self-corrected and started working again after some time. Today, no such luck!

If I read the troubleshooting flow-chart right in the manual, I get to the "Main burner remains powered and lit" -> No. It then goes on to say to check polarity, continuity of ground wire etc. And also that there is supposed to be an LED to check if the flame sensor has good current. I can't however see where this LED is supposed to be on the furnace...

Any thoughts? Does it sound like this could be a problem with the flame sensor - or something else?
thanks

[HayZee518]

flame sense sounds good.thermocouple, millivolt systems or a cad eye are typical flame sense circuits. thermocouple relies on actual flame detect. millivolt depends on voltage developed when a flame impinges on the thermopile, cad cell depends on the infrared spectrum of a flame.