Another combi boiler hot water problem

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I was wondering if anyone had a suggestion or idea that may help. We have a Baxi Duo Tec 33 erp, which is about 5 years old, and working absolutely fine until last week. The central heating is working perfectly, however, when you turn the hot water on, after a few minutes (long enough to fill a washing up bowl) the temperature gauge rises to just under 3 bar, a fan (I assume the venturi) speeds up, it makes kettling noise and then will either shutdown with error 125, or if the hot tap is turned off soon enough, will reset itself.
Having read a number of threads about this, I thought it was a blockage / lack of water on the hot water feed, so following the advice of others the following has been done:
1: Diverter Valve has been tested, by firstly touching the flow pipe to check if getting hot, it wasn't. Stripping the valve down and cleaning the cartridge and manually testing the plunger. Removing the motor and verifying that it is working correctly.
2: Next, removed the pHex and left it soaking in x800 for a couple of hours. Flushed it through with mains tap pressure. Spoke with a plumber who suggested I needed to use a stronger chemical substance to clean it, so purchased brick acid and left it soaking for 30 mins, then flushed it out, using mains pressure again. Each time I flush it, pressing a hosepipe connection on one end of the pHex, I get a 3inch fountain the other end, on both sides.
3: Stripped out the filters and made sure they were clean.
4: Checked Hall effect sensor is working, both visually and electrically.
5: Tested the resistance of the DHW thermistor, which seemed to be giving the correct reading, as I can't verify temp of water in the diverter valve, I assume it's a few degrees less than the gauge.
6: Haven't tested the Water pressure sensor, as I can't find any information on what readings to expect.
7: Spoke to Baxi approved engineer who insists that it is the pHex that's the problem.

So I was wondering, from the above information, if anyone who has more experience than I do, had any other idea before I go down the route of replacing the pHex.

Thank you in advance for any help that's given.
 
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Download the boiler manual and have a look at the fault finding charts contained within. They may give you some clues.
 
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