Vaillant boiler, no error on display but not working properly

[armyash]

On unvented system. Not sure what model boiler. (not helpful I know)

A tenant where I do some work has been in touch, he said the boiler was not working.

I asked him to check the usual, thermostat turned up, gas on (light the hob). Can't check the pressure because it means taking the case off to see the gauge (I assume, he said he cant see a gauge anywhere) low pressure would display a fault anyway. Trvs open.

I told him to switch off at the mains and back on. He did and he said it fired up. Half an hour later i got another text to say the rads warmed up but it's now gone off again.

He has sent me pictures of the boiler and there is definitely no faults being displayed. Waiting for him to tell me if the boiler works ok if hw is turned on.

Any ideas? First thing i'm going to do is put new batteries in the thermostat.

 

[Chalked]

External controls.
Could even be thermostat set too low.

 

[armyash]

First thing I asked, he said it was on 30.

 

[armyash]

Had a look. surprise, surprise it was all working fine.

Tenant told me they had changed some times on the programmer so maybe they had switched the heating off by mistake.

Another thing, its an ideal not a Vaillant. doh.

Boiler display shows '0' then 'c' when there is a demand. Sometimes 'c' will show on display but the burner light doesn't come on.

Wired honeywell thermostat working fine when on site.

Any intermittent faults that are common that anyone knows of?