My ranai tankless heater is not working

Do you have 3-phase water heater? Is this balanced or unbalanced 3-phase unit. Add comment and describe product in detail, including brand since i have some service resources for both types.
Or do you have ordinary residential split-phase 240Volt Bosch tankless computer that heats water, and has 2 or 3 separate elements, and a separate circuit breaker supplying power to each element? And testing across both hot leads coming from each breaker yields 238Volt?
Copy following link for Bosch tankless manuals, with brief troubleshoot section that does not provide detailed service procedures for repairing tankless computer:
http://waterheatertimer.org/How-to-wire-Tankless-electric-water-heater.html#manual
If this is new water heater, then call installer. If you installed yourself, then remove tankless and return to seller for refund.
Tankless computers require certain amount of water flow to activate computer sensors. Turn on Hot tap and listen to pipe to hear if water if flowing through hot pipe. Check for crossover:
http://waterheatertimer.org/Crossover.html
Clean water filter usually located just above where cold water connects to unit.
Delime tankless ... and use water softener. Tankless cannot be exposed to more than 11 grains hardness in water, or parts will fail within 2-3 years. Simply deliming tankless heat exchanger each year will not maintain full efficiency... although for typical family of 4, tankless comper water heater costs $100 more to operate than tank-type heater according to manufacturer's promo literature and independent studies.
Call local technician who specializes in fixing tankless computers. For plumbing problems call local plumber. These are two different things.
If tankless computer is older, then cut losses, and buy tank-type electric water heater at Lowes for $285 and make quick changeover and have hot water same day. Sell old tankless on ebay and cheat next guy, or sell at scrapyard for $3 per 100.
Gene
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If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

Replace single-handled faucet cartridge, and see if hot flows to that faucet again.
http://waterheatertimer.org/Crossover.html

Exposing tankless to hard water causes reduced efficiency within a few months, and can cause Total failure of parts within two years.
Do not expose tankless to more than 11 grains hard water.
Here is research pdf conducted by water softener industry comparing tankless andf tank-type:
http://waterheatertimer.org/pdf/Battelle_Final_Report.pdf

To possibly solve problem temporarily, install softening system, delime tankless unit using white vinegar. Delime yearly. It is fun for whole family. And who doesn't like a project schedule.
http://waterheatertimer.org/pdf/Rheem-De-lime-tankless.pdf

To solve problem long-long term, install tank-type gas water heater from local box store for $350.

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

http://waterheatertimer.org/Troubleshoot-Bosch-Tankless-water-heater.html#flow
1) Tankless require minimum flow rate before burner will activate.
2) Sounds like the water pressure drops quickly when faucet turns on. This implies combination of problems including sediment build up inside lines, crossover at single-handled faucet, or sediment filter clogged or water softener clogged.
3) Low flow faucets can reduce needed flow rate.
4) Other problems can contribute: sensor failure, loose wires, PC board failure, high temperature setting, cold incoming water temperature during winter months.

Can tankless electric water heater be repaired? Sure.
Is it worth the expense if problem is going to happen again? I can't answer, but can offer resources.

New tank-type electric water heater costs $207 and new tank-type gas costs $320 at local box store.
You can install new tank-type electric yourself in a couple hours. DIY repair on tankless is not easy since parts are not at local box store, and what part do you need, and tankless parts can easily cost more than new tank-type water heater.

Open following link for Bosch troubleshooting resources.
http://waterheatertimer.org/Troubleshoot-Bosch-Tankless-water-heater.html

GENERALLY, Tankless require .4 gpm (gallons per minute) water flow to activate heating cycle, and then require .25 gmp flow to keep heating cycle going.
For example .4 gallon per minute means faucet can fill 1 gallon container in 2-1/2 minutes
Some tankless units require more water flow, for example .75 gpm

Tankless water heaters have flow sensor that tells computer chip when enough water is flowing to activate heating cycle.
If the sensor is clogged, or if water filter inside tankless unit become clogged with dirt or sediment from hard water, then the unit does not signal enough water flow and will not activate.
Home filtration system and water softener can also slow the flow of water.
Low water pressure and crossover problem on single-handled faucet and crossover on recirculation system check valve and high thermostat setting can also cause heat cycle to not activate.

Tankless cannot be exposed to water with more than 11 grains hardness or parts inside tankless will coat with sediment and not work. Within 2 years the unit will not work, with diminished efficiency from the beginning.
http://waterheatertimer.org/Battelle_Final_Report.pdf

If water is hard, and you cannot soften the water, or if water is dirty and cannot be cleaned, then the best recommendation is to use tank-type heater instead of tankless.

All costs are energy. For average home: Tankless electric is NOT cheaper to operate than tank-type electric. Tankless gas saves $1-8 per month. However, cost of unit and installation, cost of potential repairs and cost of parts, difficulty of DIY repairs, required yearly-monthly maintenance, etc outweigh expected savings.
http://www.pmengineer.com/Articles/Cover_Story/8cf9e86f7c298010VgnVCM100000f932a8c0____

Upvote the help.
And take advantage of fixya expert assistance live.
For a price, expert works with you while you work on water heater or any do-it-yourself project.
Fixya is always less expensive than a service call.

Usually when a part has build-up that signals part replacement.
I am concerned with condition of other parts, since build-up occurs in more than one place.
Replacing one part is no guarantee that problem will be solved, and just as likely the new part will cause unit not to work.

If you have Tankless, then open following link for troubleshoot resources before you call service technician:
http://waterheatertimer.org/Troubleshoot-Bosch-Tankless-water-heater.html

Tankless cannot be exposed to water with more than 11 grains hardness or they will stop working.
Tankless must be delimed and maintained yearly.
Deliming will not restore efficiency to tankless water heater exposed to hard water and mineral build-up. Water softener and filter are required, and these things can reduce water flow and pressure that will cause low pressure and flow that will stop burner from activating.
Water temperatures above 135 can cause minerals to dissolve out of water and coat metal surfaces.
Read manual for troubleshooting and yearly maintenance requirements.

Tankless problems are like bowling on a ship.
Repair most tankless problems yourself, same day, any brand, for around $330.
http://waterheatertimer.org/How-to-install-gas-water-heater.html

Upvote the help.
And take advantage of fixya expert assistance live.
For a price, expert works with you while you work on water heater or any do-it-yourself project.
Fixya is always less expensive than a service call.

I could see how it might not work in parallel. Much water would go thru the conventional heater, so there would be low flow thru the tankless.
Now, the tankless is getting hot water from the conventional. Try shutting the conventional heater's power source off so the water gets cold, then see what happens with the tankless.
Gary

It will probably work if tankless is not older, and if water is very clean with less than 11 grains hardness, or you have frequently-maintained softener with also frequently-maintained particulate filter.
As a general rule, tankless require 1/2-2/3 gal per minute water flow to activate heater, and slightly less flow to keep heater going.
Your manual should have specific figures, because water-flow rate is definitely mentioned in troubleshoot section of every tankless manual, in every brand.
To download manual, open following link, and then select product that matches yours. I dont see a 600 series listed.
http://www.boschhotwater.com/HelpfulResources/BrochuresManuals/tabid/354/Default.aspx

Next step would be to check the water-flow rate on shower head.
The problem with tankless flow rates is they are rated for new product only.

Add a comment for more free help.
Also take advantage of fixya expert assistance live.
For a price, expert works with you via while you work on water heater or any do-it-yourself project.
Fixya is always less expensive than a service call.

Grundfos RCP-96 has Flow range: 0 - 27.5 U.S. GPM
According to Amazon spec sheet:
http://www.amazon.com/Grundfos-RCP-96-UP515-55SFC-Recirculator-Timer/dp/B00128BJAO

Most tankless require .4 GPM to start burner and .25 GPM to maintain burner
Rinnai does not show C85e-1 so you will have to consult your manual to see minimum water flow requirement.
http://www.rinnai.us/tankless-water-heater/
Sample manual from Rainnai mentions 'minimum flow rate' several times but doesn't give specific number. Yet promotion blurb on page 1 says "Ultra operates at a lower minimum flow rate."
http://www.rinnai.us/documentation/downloads/U290-665x0200_Condensing_Manual_EN_FR_1-2010.pdf
Another rinnai manual mentions flow rate several times in troubleshoot section yet fails to define what that rate is:
http://www.rinnai.us/documentation/downloads/U287-1833_VB_Series_R75LSi_R94LSi_R98LSi_Manual_EN_FR_EN_FR.pdf

It might seem that tankless promoters are CTA by failing to mention specific flow rate numbers.
I think tankless promotion is a definite overachiever, and vague flow rate is one tiny example.
It proves the necessity of reading whole manual and asking questions before buying a tankless.
There's another factor to consider:
All performance numbers are based on new products in ideal circumstance, and not based on a 4 year old product located in your home.

Your flow rate may vary, especially over time, and especially if water in your area is not crystal clear, and you are not diligent with correct water softener and filter.
Many factors can affect flow rates: water pressure, number of elbows, clogs in the line, sediment, size of pipe, aireators, water-saving shower heads, local water conservation policies, water well pressure, MUD district pressure issues, your home located near end of a water main, etc.

Read tankless efficiency vrs water softener
http://waterheatertimer.org/Battelle_Final_Report.pdf
Image of tankless parts vrs tank parts
http://waterheatertimer.org/images/Compare-water-heater-parts.jpg
Bradford White tankless vrs tank side by side
Lime scale does not flush out of tankless
Tankless must have water conditions with less than 11 grains hardness
"Savings falls far short of a payback," 'without comparing service costs'
http://www.pmengineer.com/Articles/Cover_Story/8cf9e86f7c298010VgnVCM100000f932a8c0____
Sediment clogs tankless pressure sensor without softener.

See image larger

-Tankless water heaters are susceptible to lime build-up if not de-limed each year.
-Hard water will clog pressure-sensor and cause failure within 2 years if filter and softener are not working.
-Water flow and water pressure sensors inside tankless are succeptible to hard water deposits.
-Low water pressure from clogged pipes to municipal water problems to water well pressure valve can affect tankless burner
-Low-flow shower heads may not engage burner.
-With tankless-gas water heaters, sensors inside combustion area detect when incoming air flow is not adequate ... inspect inflow
-Your manual probably has a troubleshoot section.
-The manufacturer will refer you to installer.
-If you self-installed unit, that will void warranty to my understanding.
-There are water heater forums 1 2 3 4
-There are plumbers who will each have a different opinion
-There are Fixya experts who get paid to tell you the above.
-And there are local plumbers who are authorized dealers of the brand of tankless who get paid to fix them

you probably had debris get into the filter or flow sensor when the water was turned back on. I'd recommend checking the filter first and if that doesn't work, call Noritz Tech support to help you with the flow sensor.

the filter is a black plug located on the cold water line (labeled cold on the bottom of the unit). It points torwards the left.