I have a code 151-11 on a cat c7 engine.what is a soot trap?

Limited help in the link, but the tech at Just Answer says they need a good ground.

https://www.google.com/search?q=2000+Sterling+water+truck+with+a+c7+Cat.+Starter+wiring+diagram

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There definitely seems to be something wrong here with the engine if he is plugging into the engine electronics to get it to start. Get 2 or 3 other mechanics to look at it for some second opinions and to see if they can figure out the problem. These CAT engines should NOT need a 'Code' every time you need to start it.
The mechanic you are currently using either can't or won't fix the issue and is happy just taking your money.

code refers to "particulate trap" which must be the catalytic convertor put some fuel addative in the fuel tank, get one that specifically states the cat con as well as valves/cylinders etc follow the products directions

You need to have the carbon soot trap serviced by the dealer.

No it may also have a bad or blocked cat though. It sounds like the alternator needs to be replaced or that the belt may be stretched or broken therefore not doing anything.

sounds like your cat converter has broken up and is now plugging you exaust.turn off engine crawl under truck and bang on cat converter with piece of wood if it rattles cat is bad. but dont just replace it because something caused it to go bad,number one reason of broken cat is a hard missfire so if runs rough fix that first.number two reason is to rich fuel mixture is it burns the eyes smells bad or has black soot or smoke coming out the tailpipe fix that first also.

carb automatic choke sticking closed or your float setting too high causing flooding engine.

That's a generic O2 sensor circuit overvoltage warning. Did they find any additional codes? Also, did they clear the code for you? Your car is an OBD2-emissions car, meaning you'll have an O2 sensor before and after each catalytic converter. Is your engine the V8 or the inline 6? If it's the V8, you'll probably have four (although it could be two or three) O2 sensors. If it's the inline 6, you likely only have two. I'm asking about additional codes because that one doesn't help narrow down which sensor is bad. Have you noticed your gas mileage dropping at all? Or a buildup of black carbon soot in the tailpipes? Maybe less power than usual? If so, it's probably a primary (pre-cat) O2 sensor. Those are the ones that determine how much fuel the engine will be fed. If you don't have any discernable difference at all, it's probably a secondary sensor (post-cat). The issue I have is, either of those should throw at least one additional code (either to indicate which sensor is going bad, or in the case of a secondary sensor, you'll usually see a "catalytic converter below threshold" or similar code, since the secondary sensors are used by the computer to see how well the cats are working).

Any additional codes in there, or symptoms you can describe?

The plenum problem should not affect compression tests. It allows oil to get sucked into the intake and will cause carbon buildup, plug fouling, and pinging. If you have zero compression on a cylinder, it is usually a valve problem. Even a burned piston will make some compression, unless it has a big hole in the middle.
Sorry, but if your compression results are to be trusted (and it seems they are), then you have to tear a head off to find the problem, or at least get a borescope in it.

Injectors from the same ccm engine are most probably sounds same but to be on the safe side check not only the shape but code on the top of injector too. Because they are piezo effect injectors and set by the manufacture for a specific pressure for specific engine codes.