We have no hot water!!!! We have a powerstar #45-557701 for a year now the water would get hot then cold then warm well now we have no hot water at all... How do I even try to fix this problem?

Water leaking into elements. Bad PC board. Undersized breakers. Undersized main panel.
Bosch Powerstar is a computer that happens to heat water.
If your 5-year-old laptop computer was malfunctioning, you would not be surprised.
But for some reason (false marketing by manufacturers?), folks believe a tankless computer is going to last 20 years.
Moreover folks also believe tankless will provide endless hot water while saving money.
The whole idea of tankless saving money is a load of sales-BS that defies physics and common sense.
There are no service manuals... except bosch website has troubleshoot section in product manual.
Call a bosch-qualified service tech.
Go to Lowes and buy ordinary tank heater, and fix your heater today for only $250.
Tank-type electric water heater can be easily repaired and maintained by homeowner to last forever... that is a fact
http://waterheatertimer.org/How-to-maintain-water-heater.html

http://waterheatertimer.org/How-to-wire-Tankless-electric-water-heater.html

Gene

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

Problem might be crossover from failed single-handle faucet cartridge:
http://waterheatertimer.org/Crossover.html

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

Any suggestion could include a list of possible problems from bad PC board, burned out element, bad sensor, loose wire, or combination or these.
More information is needed to make a guess.
Test elements for continuity.
Check wires for loose and burned connections.
Open TP valve and test if hot water is available at machine.
If water at TP valve is hot, then problem might be plumbing-related, for example crossover.
http://waterheatertimer.org/Crossover.html

I don't have specific suggestion for tankless tripping breaker or if that is a circuit breaker problem or a tankless problem.

1) Move wires to another same-size breaker and see if problem persists.

2) Compare amp draw on tankless label with wire size and breaker size amp rating.
For example 30 amp breaker using 10ga wire is rated for 80% of listed amps or 24 amps.

3) If you are drawing full 30 amps, then 30 amp breaker is getting hot. Breaker trips with heat. Older breakers can trip more easily.
http://waterheatertimer.org/How-to-replace-circuit-breaker.html

4) Before calling service technician, open following link for Bosch troubleshooting steps.
http://waterheatertimer.org/Troubleshoot-Bosch-Tankless-water-heater.html

5) Be sure to read manual for yearly maintenance requirements.

6) Add a tempering tank for incoming cold water problem.
http://waterheatertimer.org/Tempering-tank.html

7) All gas tankless brands can be repaired by homeowner for $330.
http://waterheatertimer.org/How-to-install-gas-water-heater.html

Tankless heater are a little frustrating sometimes but this is generally an issue of waterflow I believe. The fluctuations are do probably to the amount of water and rate of water going through the heater. If you have a shut off valve on the cold side of the heater, shut it back about half way. This will slow the water going through the heater and allow the heater to heat it up more efficiently. Then go to the shower, turn the hot on all the way (no cold) and allow it to heat up. Once it has heated up, test it by hand for a good temperature that you like. If it is too hot, turn the cold on just a bit until it comes to the temp you like. If it is too cold turn the hot down just a little (still no cold) and it will warm up. Ideally you should be able to turn the hot on full blast and then just turn on a little cold to get the right temp. Once you learn how these work then it makes it easier. You really cant run the water like you do with conventional hot water systems and expect to get the same results. Also, wanted to give you another little tip. Every 6 mos to a year, you should flush you heater with vinegar for about 30-45 minutes using a small cheap submerible pump like for a water feature in a garde. Just put in a bucket of vinegar and and run a hose to the "in" side of the heater and another from the "out' side of the heater and then back into the bucket. This will circulate the vinegar inside the heater and clean all the deposits that will DOA your heater. Funny how the manufacturer wont tell you this but these deposits will ruin the connectiosn on the heating elements. You can make your heater last for a long time if you do this.

Might be the flow sensor, these Bosch units have a lot of problems. Just to be sure, you do have electricity going to it and have not tripped a breaker? The specs. also say the inlet water should be 60 degrees at least. take care.

The crackling noises is just from air in the water and should be of no concern. If you are having a problem of heating up the water with the unit it is most likely do to water moving too fast through the lines. Ideally, there should be a shutoff valve installed on both sides of your heater (makes for easy removal if it needs to be serviced). But if you turn off the valve about 1/4 to 1/2 way on the one on the "in" side of the heater (where the cold water enters) then you will slow down the water going through the heater and this will give the water a better chance to heat up. This should alleviate the heating problems you are having. If you do have a shutoff valve on the cold (in) side of the heater, shut it off about 1/4 to 1/2 way and then go to your shower and turn the hot on all the way (no cold). Wait for the water to warm and test the water by hand to see if it is the temp you want. If it is still not warm enough, turn the hot on the shower DOWN just a little. This will actually make the water hotter. I know this doesnt seem right but it will. On the other hand, if the water coming out with the hot water fully turned on is too hot, then just turn on a little cold water at a time until the water is the temp you want. Remember, always give the water plenty of time to get from the heater to the shower before you decideof its the right temp. Ideally, you should be able to turn the hot water on all the way and get the hot temp that you want and if you want to cool it down, just turn the cold on just a little bit until it is where you want. I'm fairly sure the problems you are having are due to the water going too fast through the unit and once you adjust that and understand better how these tankless heaters work, you wont have any more trouble. They are difficult to get used to after using conventional water heaters and the manufacturers arent very good about giving you these hints so they will work properly. You will need a professional to uninstall and hopefully this will work and you want have to. But if you do, it requires shutting off the water to the unit. Shutting off the pwer to the unit. Unattaching the water lines to the unit and then unattaching the electric lines from the junction box or wherever they are attached. The heater then can be removed from the wall by unscrewing the screws from the hangers which hold it to the wall.

Call BOSCH 800-798-8161 tech support. If it is not too old even out of warranty they will help diagnose. You can get parts for free in warranty and buy parts at www.houseneeds.com out of warranty. If it gets hot fast and then stops, one of the "Thermal cutout switches" is cutting out. The water flow it probably blocked. (Just a guess) On many models they have a "Screen" and or a "Gromet" that you can clean out. If your water is really hard the whole unit may be clogged. I believe the "SAFE" thing to do is replace it but again I say CALL BOSCH TECH SUPPORT. I have also heard of people "Cleaning the whole units with vinigar, like cleaning a coffee maker. TURN OFF POWER! TURN OFF H20! REMOVE UNIT, then clean.

I dunno ... did you turn it on?

What kind of result do you expect from this kind of question???

If on a well, you can install a regulating valve in the line to help with this problem. You may have to play with the valve a little to find the right pressure to avoid those highs and lows. I installed a Bosch A125 and had this problem from the start. A little research un-earthed the regulating valve (I think around $25 at Lowe's or Home Depot) and we have had even hot water ever since.