Bosch AquaStar 125B Troubleshooting

Bosch AquaStar 125B Troubleshooting

This guide on Bosch AquaStar 125B Troubleshooting will explain common issues, including:

  • Pilot light won’t light
  • Pilot light comes on but flame goes out when button is released
  • Burner doesn’t ignite when the unit is turned on
  • Pilot light goes out during or immediately after hot water has been used
  • Water is not hot enough

Without further ado, let’s dive in and explore the solutions to say goodbye to those frustrations. 

Table of Contents

Bosch Aquastar 125B Troubleshooting [5 Common Problems & Solutions]

From this chapter, you will learn about the 5 common problems you will experience with your AquaStar 125B.

Pilot Light Won’t Light

Pilot light on the Bosch AquaStar 125B doesn’t come on if no gas is flowing to the water heater. 

Additionally, a jammed in-line AquaStar gas regulator can prevent the pilot light from turning on. 

Furthermore, a clogged pilot orifice, dirty air screen, or a filthy gas filter will cause the pilot light to not come on.  

And lastly, air in the gas line can also be the culprit behind this issue. 

How To Fix: 

To begin with, verify the gas is flowing to the water heater. In this case, ensure you open up the gas valve on the gas line. Besides, make sure you slide the gas valve button to Pilot position. 

Secondly, the regulator on this Bosch AquaStar Water Heater usually gets jammed up due to excessive pressure. Excessive gas pressure will lock up the regulator because of its by-default design. 

To unlock the regulator, you should reduce the gas pressure both on the gas tank (propane) and the water heater. In this case, you can call your gas supplier to adjust the gas pressure- which will be the best bait. 

If adjusting the gas pressure doesn’t unlock the regulator, replace the regulator itself. 

Thirdly, your water heater won’t heat the thermocouple sufficiently if the pilot burner gets clogged by dirt or debris. As a result, it will cause the pilot light to be weakened. 

For cleaning purposes, I recommend you clean the burner. Then, pull off the air screen filter, wash and blow it out. Nonetheless, clean the pilot orifice or replace it if needed. 

Finally, bleed air trapped in the gas line. Regarding this, turn the gas valve button to single flame position and keep depressing this button until all the air has escaped. 

During this process, continue to press the piezo ignition button until the pilot flame has ignited. 

Pilot Light Comes On But Flame Goes Out When Button Is Released

One of the following reasons is responsible for this issue, including: 

  • You may not hold the pilot push button in long enough or it doesn’t push in far enough.
  • Pilot flame improperly aimed or it’s too weak
  • Loose thermocouple connection at the electromagnet
  • Loose circuit connections at the ECO 
  • Defective ECO
  • Defective Thermocouple 

How To Fix: 

Once the pilot flame comes on, you must hold the push button for at least 15 seconds. It will help the pilot flame to heat the tip of the thermocouple properly. 

Secondly, verify the pilot flame is aimed properly to heat the tip of the thermocouple. Of course, the pilot flame should be sharp & blue.  

Afterward, inspect the thermocouple connection at the electromagnet. You will find the electromagnet on the right side of the gas valve behind the piezo push button assembly. 

At the electromagnet, check the thermocouple connection for tightness. If it comes loose, tighten up the thermocouple nut snugly- but not too tight. 

Next, loose connections at the ECO can result in millivolt current loss through the overheat safety device (ECO). I recommend you clean the terminals with very fine sandpaper and reconnect the circuit connections at the ECO. 

If cleaning the terminals attached to the ECO brings no luck for you, my next suspicion goes to the ECO itself. 

To determine whether the ECO is faulty or not, connect a jumper wire between the two wires (wiring connected with ECO) and relight the pilot. If the pilot light comes on, I bet the ECO is the culprit. 

In this case, replace the ECO to resolve this issue. 

On the other hand, if the pilot light still goes out, thermocouple is the next component I blame most. 

For testing the thermocouple electrically, detach the thermocouple lead at the ECO. Then, insert a multimeter probe into the thermocouple lead and attach the other lead at the metal gas valve. 

Now, light the pilot flame and hold the pilot button. The reading you will get at the meter should be 24 mVDC or more. It indicates the thermocouple is in good condition. 

Otherwise, you should replace the thermocouple with a new one. 

Burner Doesn’t Ignite When The Unit Is Turned On

It’s the operating nature of a machine or appliance. When you turn on the water heater, it will light the burner.  

But what happens when you turn on the unit but the burner won’t ignite? It generally occurs due to the following reasons: 

  • Pilot is not on or you fail to light the pilot correctly
  • Plumb the cold incoming water connection improperly
  • Water flow rate at the hot water tap is too low
  • Dirty cold inlet water filter
  • A crossover in household plumbing
  • Water valve parts get dirty or damaged

How To Fix: 

The first solution I suggest (if the burner won’t ignite) is to light the water heater according to the manufacturer instructions.  

If you don’t know how to light the pilot on Bosch AquaStart 125B, check the ‘How Do You Light A Bosch Aquastar Tankless Water Heater’ section in this article. 

Secondly, verify the cold water inlet connection is on the right side of the unit when you are facing the water heater. 

Thirdly, ensure you get ½ gallon per minute flow rate when turning the flow control knob all the way clockwise. Otherwise, you can’t activate the water heater to heat the water. 

On the other hand, If you turn the flow control knob fully counterclockwise, the flow rate should be 1.1 GPM to activate the burner. 

Fourthly, the burner won’t also come on despite having a turned on water heater if the cold water inlet filter is dirty. 

To let the burner ignite again, clean that filter. In this case, just locate the inlet filter at the inlet side of the water valve and take it out of its place. Then, clean it with cold water. 

Fifthly, you can’t ignite the burner though you turn on the water heater if there is a crossover in household plumbing. It typically occurs due to a bad washer at a single lever faucet, incorrect plumbing, or a mixing valve in the line. 

When there is a crossover in plumbing, it will create a back pressure in the system. As a result, the water is entering the unit from both sides and the burner won’t come on. 

Now, the question that may spin around your head is- how can I confirm if there is a crossover in the plumbing or not? 

It’s just simple. 

Turn off cold water supply to your AquaStart 125B and open hot water taps. No water should flow. If any water is flowing, I bet a crossover is present in the plumbing. 

Fortunately, it’s a problem with the plumbing, not with your water heater. So, call your plumber to fix the crossover issue. 

And last not the least, the water valve and its components should be totally free of dirt & debris. Check the ventuni and verify there are no dirt particles. 

Of course, don’t forget to service the water valve assembly components immediately if you observe any sign of moisture at the joint of the water valve and the gas valve. 

Pilot Light Goes Out During Or Immediately After Hot Water Has Been Used

The following causes are responsible for this issue: 

  • Too low gas pressure
  • Burner won’t shut off immediately when hot water is turned off
  • ECO gets tripped due to overheating
  • Flue gas safety device gets tripped

How To Fix: 

To begin with, check the gas pressure and ensure it’s within specifications. If you power up your AquaStar 125B with LP gas, the gas pressure should be between 9” WC to 10.5 WC while operating the unit. 

On the other hand, the gas pressure should be between 4.2” WC to 5.7” WC when running the AquaStar.  

It’s better to call a certified technician to measure & adjust the gas pressure if you don’t know how to check the gas pressure. 

In addition, the pilot light goes out or the burner won’t come on when the hot water is turned off. When you go through this issue, the unit will overheat and cut off the ECO. In terms of resolving this problem, just call a professional to rebuild the water valve. 

Furthermore, shut off the water heater when the ECO button gets tripped. Then, reignite the water heater 10 minutes later. If the problem still persists, contact a service guy. 

Finally, reigniting the issue will also work for solving the tripped flue gas safety device. 

Water Is Not Hot Enough

The follow culprits are to blame if you get insufficient hot water out of your Bosch AquaStar 125B: 

  • Too low temperature selection
  • BTU input is too low due to insufficient gas pressure
  • You may not open the valve fully

How To Fix: 

First off, check the temperature setpoint and ensure you don’t set it to the lowest temperature setting. 

If needed, increase the temperature by turning the temperature adjustment knob clockwise. Alternatively, set the gas control knob to the winter mode setting to increase the burner output. 

Secondly, make sure the gas pressure on the unit is within specifications. 

And lastly, ensure you turn the gas valve fully on to let the gas flow freely through the water heater. 

How Do You Light A Bosch AquaStar Water Heater?

The lighting instructions are labeled on the front cover of the heater. Or, follow the lighting procedure below to light your Bosch AquaStar Water Heater like a pro: 

1. Read the safety guide on the plate first.

2. Turn off the gas valve by sliding it to the far left under the Off mark.

3. Wait for five minutes to clear out any gas. If you smell gas, don’t turn on the unit and call the gas supplier for further assistance. If not, go to the next step.

4. Locate the pilot burner behind the peephole in the front center of the jacket directly below the instruction plate.

5. Set the gas valve button to the right, under the flame mark. 

6. Depress the gas valve button fully and light the pilot by pushing the ‘Pilot Igniter’ button. You may need to repeat this step if needed.

7. Take a look at the pilot flame through the peephole. You should hold down the gas valve button for at least 15 seconds with pilot burning. When you release the gas valve button, the pilot should continue to burn. 

  • If the gas valve button doesn’t pop up when you release it, call a professional right off the bat.
  • Repeat the steps from 1 to 7 if the pilot light doesn’t stay lit.
  • If the pilot light won’t stay lit after several tries, slide the gas valve button to the Off position and call a certified technician. 

8. Release the gas valve button once the pilot light remains lit. 

Read More About: Bosch Tankless Water Heater Reset Button


I can feel your frustration when your Bosch AquaStar 125B doesn’t function normally. Fortunately, I broke down everything you need to know to get hot water out of your water heater. 

Just follow the troubleshooting steps and fix those water heater issues. Keep in mind; you must turn off both the gas supply and electric supply to the unit when troubleshooting the unit. 

Don’t forget to call a professional if you fail to resolve your issue by following this guideline. 

Happy Plumbing!

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