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Phil, Mechanical Engineer.
Category: Plumbing
Satisfied Customers: 8437
Experience:  Retired mech. contractor, shop owner, 51 yrs experience.
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I have a Bosch tankless water heater. Model AQ 250 SX NG. It

Customer Question

I have a Bosch tankless water heater. Model AQ 250 SX NG. It is giving me an A2 Error Message. It fires up for 10 seconds and the quits. Any ideas?
Submitted: 1 year ago.
Category: Plumbing
Expert:  Phil replied 1 year ago.

Hello, its probably a flame sensing issue, resolved by cleaning the flame sensor rod off with a scotchbrite pot scrubber pad.

Look inside the cover of the unit for the wiring diagram it should have the code read out inside. If you want coaching on the trouble shooting

Please attach a well focused, close up photo of the wiring diagram glued inside the service panel, the picture needs to betaken from straight on, in very good light.

You can use the paper clip- icon at the right end of the tool bar at the top of this dialog box to attach it here. ,Include a picture of the thermostat wire connections to the printed circuit board in the same area.

If you have an error code showing, tell me what the code read out is on the wiring diagram or a sticker next to the diagram please.

If there is no paper clip-icon, look for the '+attach files' link at left of your send button.

I can mark it up with trouble shooting instructions.

You will need a multi meter ($30 at a hardware store) and some experience working with appliances, let me know a lot about your skill sets in those regards.

If I miss you tonight, I will pick up your response first thing in the morning.

Customer: replied 1 year ago.
attached is the diagram of the unit. What next? I have a multi meter. Thanks for your help.
Customer: replied 1 year ago.
A2 is the error message.
Expert:  Phil replied 1 year ago.

Thanks for the diagram

Let me know how those tests go, if the switch tests open, you can buy the part from your local dealer, if you have trouble with that let me know, we can go from there without a time limit.

If the switch tests closed and you do not find a bad wire connection, there is a problem with the printed circuit board.

We can do a heat stress test on it.

Use a hair drier to slowly warm the printed circuit card up to120F or a bit warmer, but not over 140F... if there is any change in the error codes or other water heater error code or behavior... the card has a cracked micro circuit that opens and closes as the card expands and contracts... that proves a bad printed circuit card.

No change however does not prove the card is good however... but a change proves the card is definitely bad.

Thanks for letting me work with you. When we are finished,if my answer has been polite, professional, and thorough, please locate the 'rate your expert button' and click one of the ratings .If you still need help let me know!. Expert is not paid for assisting you until you rate answer. Feel free to follow up for30 days at no additional cost.

Customer: replied 1 year ago.
I will try this in the morning. Thanks for your help. Toby
Expert:  Phil replied 1 year ago.

Hello again Toby. You can also use an electronic thermometer and focus it on the tube that the over heat switch is attached to and notice when it opens and shuts the burner off... if it shuts off below 160 to 180F then its bad, if it shuts off over 180F then there is a insufficient water flow due to water scale build up in the heat exchanger or inlet water filter.

Let me know what you can discover, we can go from there.

Customer: replied 1 year ago.
I have a laser temp gun. Would that work?
Expert:  Phil replied 1 year ago.

Hello again, the lazer gun might work, it depends on the surface and angle the laser beam strikes the tube, you could try it and see if it seems close enough.. hopefully the switch will be so far out of range that it trips just as the tube begins to get warm... then we know for sure that its bad.