The A01 error occurs when the boiler is blocked due to a failure in the automatic ignition system. It is likely to happen due to unstable voltage. In this case, a stabilizer can be a solution. Also, the issue may occur due to reverse polarity.
There may be a problem with the flame ionization sensor. Make sure it is not blocked by debris, deformed, broken, and the lead between the sensor and the main control board is not corroded. Clean or replace the lead.