While taking a shower the water will go cold and the display on the tankless water heater display will flash P1 and then the temperature setting.

I do not know how old this unit is or what model but many water heaters are not sophisticated enough to measure the temperature, but heat based on water flow and temperature rise. To start, tankless water heaters turn off every time you turn the hot water off. Lets use an example. You use the facilities, wash your hands then turned off the water and take a shower. You will bring the already heated water (from washing your hands) from the pipe up to the shower. Then the cold water that comes into the pipe from the reactivation of the water heater comes up the hot water pipe with warm then hot water just behind.

If this is not your problem, keep reading. Every unit I have been called to see were installed wrong. Check your installation manual to see if the problem with the system started with the contractor.

First, check the temperature of the water from the heater. If it is too hot and you are mixing too much cold water in, the unit will turn off because there is not enough water flow to keep it running.

Second, turn off the valve at the water heater and turn on the hot water at a faucet. If the water does not stop running, there is a "plumbing cross over" usually caused by a failing pressure balance valve in a shower.

Third, if the unit has not been serviced, go to bosch-climate.us to find a tankless water heating professional in your area. Totally Tankless LLC

P1 error code is not enough water flow to operate the unit: increase water flow to unit.
http://waterheatertimer.org/pdf/Rheem-Error-code-P1.pdf
http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html

Error codes are considered start-point for troubleshooting.
Page 18 of maintenance manual shows how to clean water filter monthly.
http://waterheatertimer.org/pdf/Tankless-maintenance.pdf

Other causes can include faulty sensor, clogged in-line filter, clogged water softener, sediment in pipe, sediment at elbow, bad check valve, water leak, etc.

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Before calling service technician, open following link for Rheem troubleshoot resources including Rheem chat:
http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html

Error codes are considered start point for troubleshooting, and can sometimes mislead.
Error P1 is 'not enough water to operate the unit:'
http://waterheatertimer.org/pdf/Rheem-Error-code-P1.pdf

This problem can be caused by plumbing problems such as crossover, or clogged water lines, or dirty water filter, or clogged water softener, or temperature of unit set too high. Or combination of these problems.
Remember if temperature is set very high, then more cold water is used at the faucet. This means less hot water is drawn through the tankless water heater, so the flow rate may drop below activation threshold.
http://waterheatertimer.org/Crossover.html
http://waterheatertimer.org/Low-hot-water-pressure.html

Ok i intall tankless water heaters and sell them and repair them . The trouble your are hving is more than likely ,the mixing valve on your shower has a ajustment on it to prevent you from burning your self . that is the number one reason in showers when water never get to temp . What it is doing is addng cold water to your shower for you bummer . Take handle off and adjust water temp to your hottest setting . Some times you have to remove the low flow washer in shower head to get the right flow . also every time you start up your hot water it shouldbe set all the way ot other wise withe the battery 2 D models the unit will not sence flow and shut off the unit ,with some units it is a fine balancing act to get them adjusted right. If your buying a tankless water heater always buy a blower type unit that has a computer that runs it . You will be happy . If the Battery operated unit is for a cottage or a smallcabin then your just going to have to fix the pumbing to make it work right many of the safety features of the newer showers are not made to work well with tankless water heaters .

just clean shower head or faucet with CLR to increase water flow. I have same problem

P1 is reportedly defective check valve that can cause water flow to decrease, and tankless not heat water smoothly. But can also be other causes, or combination of causes. Call service technician, contact Rheem, or replace check valve and see if problem solves. Then add a comment with solution so others can benefit from your experience.

Open following link for Rheem/Paloma/Richmond/Ruud tankless troubleshoot resources including error codes, self help, manuals, parts links, tips, and contact information.
http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html

Also 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.

1) I cannot find Rheem error code P1
http://www.palomatankless.com/owners/homeowner+trouble.lasso
http://www.palomastore.com/paloma_error_codes.html

2) BOSCH does not show P1 either:
http://www.boschhotwater.com/HelpfulResources/TechnicalSupport/BoschAquaStarModels/2400ESTechSupport/Troubleshooting/tabid/1295/Default.aspx

3) Best I could find is that folks with P1 error code report tankless fires for short period, and then turns off.
This implies air, fuel or vent problem, and can be internal, external or combination.

4) P1 appears frequently on control board descriptors, which might imply the error is control board malfunction.

5) Call Rheem 1-800-432-8373, and then add a comment to this thread and let folks know what you discover.

6) I will also call Rheem when time permits, and post what I discover.

Add a comment for more free help.
Also 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.

your cold water supply check pressure

Water inlet temperature to the heater is the same as preset on the controller.
I had this happen to me becouse I have a solar tank with temp of
135 farenheit.
Once I bypassed the sollar tank..no problem

It might be a question of inadequate flow. All tankless water heaters have a minimum flow rate that must be met, typically .5-.9 gpm, before the heater will operate. In the summer time the incoming ground water temperature is higher, causing you to mix more cold water and less hot water to get a lukewarm temperature. This can reduce your flow rate near or below the minimum flow rate.
Here's a test for you, try running running a sink while taking your next shower. If the hot water remains mostly constant throughout, then your problem is with flow at the showerhead. The flow at a shower head can be reduced by buildup of scale and sediment. Also all showerheads have a flow restrictor that limits flow to 2.5gpm (some even less). Removing the restrictor will increase the showerhead flow rate as well.
Also look for any error code displayed on the digital read out of the heater. Some heaters will flash on their burner indicate light as well.