I rebuilt the water valve of my Bosch aquastar 125B, and when I replaced it it is leaking from where it connects the system above it. Right where the set screws hold the valve . Any ideas?

I would look on the Bosch wed site for this information .

Thermocouple which on that model can be just the Thermocouple or a larger loom with the thermocouple in it depends. Call or look up Houseneeds.com they have parts.
If it was the magnet you wouldn't get any gas to manually light pilot.

Bosch Aquastar 125B Water Heater Repair Parts Pre January 2003

Bosch Aquastar 125B Repair Parts Selection Grid

This web site has Aquastar 125B parts. It appears your unit was manufactured 7/7/02, so that is what I pulled up for you. At least this will be a start for you to locate the part. I hope these links work.






Bosch Aquastar 125B Water Heater Repair Parts Post February 2003

Bosch 1600H NG??? I suppose. If so you need a digital manometer.
Take a static (off) reading then leave it hooked up and start flow. Watch pressure draw down and you should see the heater draw Water Column down below 5" as the pocket of NG in the pipe just before the unit draws down to minimal to keep heater lite. In that case problem in gas line size/gas line run length, Meter pressure.
If you have and maintain in operation 5" WC then is probably the water valve needs rebuilt along with a new diaphgram and also the water filter should be replaced too. This occurs esp after 2-3 year as Bosch specs rebuild at 3 yrs on the Maint Schedule. Also clean all the faucet aerators for good flow above 1.25 GPM Min at all faucets.
Under a single faucet flow (laundry sink) of 2.25 GPM the flame bed of the Unit should be 1"+ high and uniformly Blue.
I assume at this time you either have a Maint problem or you could be overflowing the heater's Winter capacity. This unit @ 70F rise only makes about 2.2 GPM. It's strictly a single use faucet heater at 117,000 BTU.
When you do the Maint look to see if there is corrosion between the GV pushrod and the water valve pushrod. Notice there is a huge mis- match in the 2 rods sizes.These units occasionally (more the 125B) corrode there and the GV will seize on and boil the heater dry and open a seam so you really want to maintain it to Bosch Specs and Schedule.
Note: This is a gas appliance & requires skill and knowledge to service. While Bosch may say you can DIY, I'd readily advise a Bosch approved Service Tech or Plumber do any rebuild. The Digital Manometer alone is a $100-150.00 item and it's the ONLY way to get accurate gas pressure readings.

1) If tankless is not showing error code, the problem might be single-handled faucet cartridge:
http://waterheatertimer.org/Crossover.html
http://waterheatertimer.org/Troubleshoot-Bosch-Tankless-water-heater.html

2) Catch up on maintenance, such as cleaning combustion parts, have gas pressure checked, etc.

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

Did you check to make sure the shut off handle is on the on position?

Possibly the flow switch is in need of replacing

Sounds like hard water. Service the water valve. Parts are available from your local Bosch Service Provider (Contact Bosch if you don't know who the service provider is).

Take the thermocouple out and match it up at any hard wear store. Maybe $10. Replace it .Make sure the flame is clean and hitting the rod in the center center. Ajust (Bend) accordingly.

Replace the thermocouple.

Thats the flame sensor.

If the gas valve is not sensing flame it will not stay lit.

T-couple's are about 9 bucks.