I need parts for a Junker gas water heater in Spain. I can't find a parts outlet anywhere on line.

make sure the water supply linee is on,and water outlet line is open.,and unit is plugged in.
If all are...u got a bad unit.

You need make model number do search online in America we have appliance repair site which may ship to SPAIN with repair videos

Did service man clean combustion parts and check for loose wires, proper venting etc?
In other words, is the tankless up-to-date on yearly required maintenance?
Did you check outlet for power?
Is the unit properly grounded?
Is the unit vented properly, and set for correct altitude?
Is the thermostat set too high?
What does the manual say about maintenance?
It certainly says to clean water filter frequently, and clean combustion parts yearly, and bleed air from gas line, and delime yearly.
Is your water heater exposed to more than 11 grains hardness in water that will cause failure due to clogged parts?

Does service man have a service manual for your specific water heater?
Does anyone have a service manual for your water heater? Probably not, which means service man will be guessing at one thing and then guessing at the next thing.

Problem could be PC board failure.
Similar to failed motherboard on computer.

Let's be frank:
Do you want the cheapest solution?
Or do you want to chase around trying to find why a complex computer chip connected to electric outlet, electric sensors, gas valve, water valve, water line, gas line, and gas burner with vent to outside has decided to stop working?

Cheap solution, Buy tank-type gas water heater for $350 at local box store. Just get a simple one off the shelf. Don't worry about warranty, because it means nothing.
Select Whirlpool, AO Smith, American, State, Reliance, Craftmaster, Kenmore because these are made by same company and can be repaired and maintained by homeowner, and service manuals are available, and any plumber knows how to fix em..
Avoid Rheem, Ruud, Richmond, GE, for all same reasons except in reverse.
Read the manual, perform the required maintenance.
Keep the air intake clean.
Install water heater yourself same day, turn up the temperature wait, 1/2 hour for tank-full of hot water, and take hot shower.
http://waterheatertimer.org/How-to-install-gas-water-heater.html

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

Before calling service technician, open following links for common error codes, Rheem chat, and possible manual.
http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html
http://waterheatertimer.org/pdf/ErrorcodeTroubleshooting.pdf
http://waterheatertimer.org/pdf/Tankless-maintenance.pdf

Error codes are start point for troubleshoot.
C2 11 is 'no ignition' or 'gas shut off vale is not open'
Gas shut off valve handle must sit parallel with gas line.
This kind of error code can be air in gas line, improper grounding at electrical outlet, bad igniter, shorted solenoid, loose connections, etc.
Most tankless manuals recommend yearly service, and that type of work usually covers cause for most common error codes: for example checking gas pressure, gas line, air in gas line, clean and check burner and combustion parts and igniter, inspect that vent condensate is properly draining and vent has good draft, clean water filter, clean air intake, check for loose wires, delime heat exchanger, test outlet with circuit analyzer etc.

Are you asking about tankless gas or tankless electric?

You posted under Tankless gas water heater: gas type tankless plugs into standard 120Volt household outlet to power circuit board. You need larger gas line than used for tank-type gas water heater.

If you are asking about volt and amp draw of Tankless electric, then that depends on specific model number.
For example, if each tankless element draws 3500Watts, and unit has 3 elements, you will need 3 additional 20 amp 240Volt circuit breakers. Volts x amps = watts. 3500 watts divided by 240Volts = 14.58 amps. At full blast, this unit would draw 43.75 amps and will cost more to operate yearly than tank-type electric water heater. If unit has 4500 watts elements, then it will draw 56 amps at full blast, and use a good percentage of household power.

Tankless water heaters have no hot water in power outage. This is true for gas and electric models.

Additional resources:
http://waterheatertimer.org/images/Compare-water-heater-parts-650.jpg
http://waterheatertimer.org/pro-n-con.html
http://waterheatertimer.org/pdf/Bradford-White-on-demand-tankless-service-manual.pdf

You're probably in for a service call and/or possibly water heater replacement if unit is more that 5-6 years old.
Your water heater plugs into outlet. Check outlet with cheap circuit analyzer to make sure the unit is receiving correct polarity without line interference.
If unit is in the attic, then add more venting to soffit and roof so pilot is not being snuffed out by hot air. If unit is in basement, then open some doors and a window and see if unit needs more air.
Most gas water heaters require yearly cleaning and service.
Clean the combustion parts and tubes.
Clean the air intake.
Have gas company check gas pressure.
Open following link for possible illustrations:
http://waterheatertimer.org/How-to-install-gas-water-heater.html#troubleshoot

The problem can be bad gas control valve, air in gas line, low gas pressure, gas shut off handle not fully parallel with gas line, dirty combustion parts, lack of combustion air, improper venting. Or combination of all things. It's an electronic gizmo, anything can be bad or shorted, and repair may not last long.

Without a trained service technician to ohm-test the parts using correct service manual, then we are guessing at the cause.

Power vent water heater implies that you have side wall venting which must be replaced with another side-wall vented unit -or- it implies rapid reheating of water.

If water heater is more than 6 years old, you might be money ahead by installing a new one from local box store. Avoid the power vent and electronics. Local box store will have passive roof-venting type water heater BUT box store does not sell side-wall power vent. If your power vent goes up through the roof, then box-store variety gas water heater will work. Total cost $350 do-it-yourself for 40 gallon gas.
Or get 40 gallon 240Volt electric for $220 and connect to circuit breaker box. Requires no venting, and is easy to repair for years to come.
Or small 120Volt electric that plugs into outlet to tide you over until gas repairs can be made.
Look for scratch-n-dent models and always offer 10% off.
http://waterheatertimer.org/How-to-install-gas-water-heater.html
Shower uses 5-9 gallons hot water.

Hello Mike,
Check gas pressure, burner parts need to be cleaned, check electrical outlet, hot surface igniter is bad

White Rodgers intellivent error code:
http://waterheatertimer.org/How-to-adjust-water-heater-temperature.html#Intellivent
http://waterheatertimer.org/pdf/Intelli-vent-gas-valve2.pdf

Error code says: system in lockout, but you know that.
1) Low gas pressure, no gas to water heater: call gas company to check pressure
2) Hot surface igniter not positioned correctly: inspect and clean burner parts.
http://waterheatertimer.org/images/Reliance-hot-surface-ignite.jpg
3) Low voltage to water heater/ outlet has wrong polarity: Use circuit analyzer on electrical outlet to make sure polarity is correct

Typical power vent manual:
http://waterheatertimer.org/pdf/Reliance-40-50-power-vent.pdf
Cleaning water heater:
http://waterheatertimer.org/How-to-install-gas-water-heater.html#clean
http://waterheatertimer.org/pdf/How-to-clean-combustion-chamber.pdf

i have a junker water heater. what side does the cold water pipe go the the geyser

They do make a 3/4 inch flex. You may need to go to a plumbing wholesale outlet to find one.

onlines yuor best hope