My grundfos pump auto-reset light is on...but water is not coming out of the faucets...however if I leave the faucet on and turn the pump off and on, it works well until I turn the faucet off again

As far as I know, tankless water heaters fire up when the water pressure DROPS 9opening a faucet). Having FLOW won't achieve that. I have never heard of having a recirc system on one of those water heaters, only with a tank. You would have to ask the maker of the water heater. I would say you have the wrong HEATER and SYSTEM for the application.

- changes in hydraulic conditions (clogged pipes, clogged filter, ...)
- pressure switch failure
- flow sensor failure
(in normal condition the pump starts when the consumption is higher than 1.2 l/min or the pressure is lower than 2 bar)

https://www.manualslib.com/manual/1569763/Grundfos-Mq-3-35.html?page=1#manual

Pump can be bad or clogged with debris loosened when pipes were moved.
Check installation, and check if crossover valve is installed correctly
Check if single-handled faucets are good
http://waterheatertimer.org/Water-heater-recirculation-system.html
http://waterheatertimer.org/Crossover.html

Gene

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

Sounds like 2 problems:
1) Crossover and 2) Setting timer
Recirculation system has check valve that stops backflow of water when pump is turned off.
When check valve is clogged with sediment, or has worn out, then cold water enters hot pipe or hot water enters cold pipe when faucet is turned on.
Take apart connection where grundfos connects to both hot and cold pipes, and clean with white vinegar or CLR.
http://waterheatertimer.org/Crossover.html
http://waterheatertimer.org/Water-heater-recirculation-system.html

Open following link for setting typical FM/1 analog timer module on Grundfos recirculation pump:
Your timer module may vary in appearance.
Slide switch selector might be located below timer dial.
http://waterheatertimer.org/images/IMG_9325-grundfos-timer-800.jpg

Sounds like you have sediment built up in the pipes, and maybe the furthest faucet is the gathering place.
Take off each shut-off valve at furthest faucet and connect a water hose that flows into nearby toilet bowl.
You can buy garden-hose-to-1/2-pipe adapter at hardware.
Open water so it flushes the line into toilet bowl.
Then clean out valves and reassemble shut-offs and grundfos connection.
Remove faucet aerator and flush line up to faucet.
Install new aerator.

It sounds like the installation loosened sediment inside pipe.
Remove sink aerator and see if you can flush open the faucet.

Also check if other faucets are receiving hot water from hot-and-cold taps that says if grundfos is pumping hot water from the water heater correctly.
http://waterheatertimer.org/Water-heater-recirculation-system.html

When recirculation pump is running, both hot and cold water lines have hot water.
If other faucets have hot water in both lines, then that tells you the blockage is localized in furthest faucet supply lines.
You may have to dissemble those supply lines and flush the sediment out.

Many times when pipes are worked on, the work loosens sediment that was built up in the pipe.
Sediment is probably why cold water line was replaced.
That's why faucet aerators get clogged up after pipes are worked on.

It sounds like your grundfos pump got clogged too.
Immediately unplug the pump so motor is not overheating.
Motors are meant to turn freely and can be damaged when blocked.

Take apart the pipes at grundfos pump and see if you can clean out the line.
If line cannot be cleaned or pump doesn't restart, then the grundfos has to be replaced.

Also look at adjoining pipes and see if there is a clog in nearby elbow, especially where galvanized and copper pipes join..

http://waterheatertimer.org/Water-heater-recirculation-system.html
troubleshoot resources
http://waterheatertimer.org/Crossover.html
Bad single-handled faucet cartridge?
Gene

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

This is a problem with your bladder in the pressure tank or the pressure switch. Ether one will produce this cycling on and off. Good luck.

Yes, you need to replace it.