A few different model numbers came out on that boiler over the years, Here is the trouble shooting manual I have however, tell me if it looks like your unit or not. Error codes start on page 12. Your 9176 code is not shown, if you can give me the full model number of the boiler I might be able to pull that manual.
Here is a note about myself:
I am 74 years old, I owned my own mechanical contracting company from 1970 to 1985, P Scott, Contract Services in Los Angeles, with 27 employees at peak. We did industrial systems design build projects for several aerospace firms such as Northrup Aircraft, General Dynamics and Standard oil among others.
I sold the company and began consulting nationally in 1985, clients included several fortune 500 companies, IBM, Texas instruments. Borden foods and many others, my specialty is trouble shooting problem projects.
What I found in myself and others, in all cases where there is a problem, is that I had made a mistake even though I was 100% dead sure that I was right and had made no mistakes.
In actual fact I had made a mistake or overlooked something. I have found that to be true with every project that I had been called in to sort out someone had overlooked something. Once I get a look at the job I can normally spot the error in fairly short order.
If this boiler worked properly before you did fit the mixing valve, but is not operating now, there is an error of sorts. Maybe not where you think it is, the job might be piped correctly but merely be vapor locked because air got in the system and was not vented out etc. That is most likely the case here, since the boiler operated properly before you installed the mixing valve.
Regarding the error codes... they are very useful in about 50% of the cases, the other 50% of the time one error code can be entirely bogus.... or cover many different possibilities, I suspect that the error code here is a high temperature limit cut out, or insufficient water flow error....probably air in the system, trapped in a radiator, or inverted U shape in the piping. That is why I asked for a few photo's.
Let me know what you can do, we can go from there on my own time and expense, without any time limits unless you choose to rate my input positively. In that case the company pays me enough for a cup of coffee and a sandwich at Starbucks. and that's fine, I am retired now. I do this to say active and help where I can.
The more data you can give me the better and faster work I can do for you. I need the model number of the boiler, and I need to know where the error code showed up, photo's will speed things immensely. along with a sketch of the system.
Or you can tell me a bit about what sort of air bleed measures you have taken and the type of radiators you have, under floor pex tube or standing radiators etc... I can very often get a system debugged in seconds that way, The chances that this is a water flow problem are in the 99% range, the chances that there is an air lock causing it are in the 90% range, and the chances that the manufacturer of the valve gave vague piping instructions or that you missed a dust plug that was fit too deep into the valve to be seen cover the other 10% of possibilities.