Noritz NR501-OD Service Manual page 54

How to reset the lock of the unit (for error code 908 and 99)
1. Disconect electrical power.
2. Wait 10 seconds and reconnect electrical power.
3. Press and hold both the Mode and Maximum Manifold Pressure Set Button simultaneously for more than 5 seconds.
(This procedure has to be perfomed within 10 minutes after reconnecting the power.)
Mode Button
Maximum Manifold
Pressure Set Button
54