I have a honeywell smartvalve sv950. I have replaced the igniter and the burners light and then go out. Also the fan is always on. What can cause this?

If the burners come on and then go right out the flame sensor might be to blame. They do get dirty buy since you have a smart valve and probably a smart valve flame sensor they could be difficult to clean with sand paper.
This set up is delicate so do not get around the ignitor part just clean the sensor part. Post back with results.