said by Joe :I do see flames come on, but sometimes they do not stay on and it will display the r again.
That observation helps to narrow down the possible causes. The error code 11 means failing to ignite, but since you can actually see the flame the most likely problem is the flame sensor.
Per troubleshooting section 11.F:
- check wiring and connections of the flame sensor
- check whether flame sensor is properly mounted in the path of the flames and grounded
- check polarity of the voltage and measure the current through the flame sensor
- clean the flame sensor rod
Not mentioned in that section, but if all else fails replace the flame sensor with a new one.