Can anybody advise why Flexicom 35CX goes into state S53 when in DHW mode?

The boiler works fine for central heating and DHW for sink taps and shower, but when bath tap is used the boiler first starts at a high rate then goes into low rate mode,waiting time mode S53, so the bath tap water starts at normal temperature then runs cold as boiler goes into low rate. The book says due to difference flow/return sensor to high so have checked thermistors, connections and refit them to pipework using heat conductive contact grease. But to no effect. I would appreciate any advice/guidance.