Whenever the temperature knob is turned up higher than half way, the boiler shuts down, flashing L and 3. Have to reset it and turn temp back down. It will run OK if the temperature is set lower, but the radiators are not warm enough on cold days.
The hot water is the same, can't set it higher than half way either.

Difference in temperature between flow/return pipes is less than 30 degC, the return pipe is a little bit less warm than flow pipe. Aluminium main heat exchanger. Plate second heat exchanger.

Water seems to be flowing around system, but it's difficult to tell as the pipes are not transparent enough to see inside. If the flow was bad, then wouldn't the flow and return pipes be different more than 30? Has a new pump head fitted anyway, no difference. The manifold the ports and both heat exchangers are all free flowing. The water is a pale grey colour but not black.

I allowed heat exchanger temperature to equalise and measured the 3 sensors, they all read the same value. All the connections are tight, including the pump.

All the air is bled out the system and radiators, including the air bleed valve. The pressure is just over 1 bar.

I don't think it's a PCB because it is less than 2 years old.

The only thing to me that does not look right is main heat exchanger gets very hot at the top of the water jacket (where the 'no-flow' sensor is). As I said, there was plenty of water flow through this heat exchanger, a hose pipe had ran through it at full speed and water coming out as fast as it was going in. The pump spins round fine and the water can be heard flowing through all the radiators. It's just that they don't get hot enough before the boiler shuts down with 'L3' flashing.

MArcus, February 2013

I’ve had five engineers out to this issue on L3, he exchanges changed all three senses changed Diverter cartridge and still have the same issue. Engineer should be coming out today to look at something else. I guess been going on for two weeks now heating will run in very low boiler temperature will run for two hours, showering 10 minutes L3 fault code on

Nige, December 2023

I've had this problem and can confirm that the thermistor needs replacing. Two engineers replaced the diverter but the last engineer said it was a sensor error. Had this fault for two years.

Kim, July 2021

Has to be a blockage somewhere, I'd start with disconnected flow pipe from top of Lh manifold, get some water down there check div cartridge is activating. Move on to plate hex check that.i had 2 this week on L3 pump was ceased and could not free one of them up, good luck.

Richy, April 2021

def plate heat ex blocked with scale
changed many of them

dave p, April 2016

L3 is a thermistor lock out problem , you need to change it , your looking around £20 to £30 for the part from ideal

Its not mentioned in the book , as i had to phone Ideal for it

Good luck

Richard Brearley, October 2013

does top of main heat excahanger get very hot compared to middle and bottom? it could be top section of the heat exchanger blocked up and water not flowing at top where sensor is. that would explain why shuts down

jo, February 2013