Bosch Tankless Water Heater Error Codes (EA, E9, A7, C6, C7 Explained & Fixed)

If your Bosch tankless water heater is showing an error code like EA, E9, A7, C6, or C7, the unit has automatically shut down to prevent damage or safety risks.

These codes usually point to ignition failure, overheating, sensor faults, or ventilation issues. In many cases, the problem can be diagnosed within minutes – and sometimes fixed without replacing the unit.

This guide explains:

• when to call a professional
• what each Bosch error code means
• the most common causes
• how to fix it step by step

If your Bosch tankless water heater is blinking an error code, the most common causes are:

• Code EA: Ignition failure or dirty flame sensor
• Code E9: Overheating from massive calcium scale buildup
• Code A7: Broken hot water temperature sensor
• Code C6: Fan speed malfunction
• Code C7: Combustion fan failure

In many cases, you can identify the real problem in minutes by reading the digital screen.

Why Your Bosch Tankless Shows Error Codes

Your water heater is a very powerful machine. It burns raw explosive gas to heat your water fast. It needs a way to protect your house. A blinking code is a smart safety shutoff. The machine feels something going wrong inside the metal box. It prevents massive fires or dangerous toxic gas leaks.

The computer brain stops the machine on purpose to keep your family totally safe. Old water heaters did not have these smart computers. They would just keep running until they melted or caught fire. Your new Bosch unit refuses to take that terrible risk. It shuts down and flashes a light to call for help. You can read the Energy.gov Tankless Heaters page to see how these smart machines save energy safely.

Bosch Tankless Error Codes

Error Code	Category	Meaning	Severity	DIY Fix	Main Cause
EA	Ignition	Ignition failure (no flame detected)	Medium	⚠️ Sometimes	Dirty flame sensor / gas issue
EC	Ignition	Flame instability	Medium	❌ No	Burner or gas pressure issue
E0	Gas/Air	Combustion problem	High	❌ No	Gas supply / air mix issue
E1	Gas/Air	Ignition or combustion fault	High	❌ No	Low gas pressure
E2	Temperature	Temp sensor malfunction	Medium	❌ No	Faulty sensor
E3	Electronics	Internal system error	High	❌ No	PCB failure
E4	Water Flow	Low water flow detected	Medium	⚠️ Sometimes	Clogged filter
E5	Water Flow	Flow sensor issue	Medium	❌ No	Faulty flow sensor
E6	Temperature	Heat sensor communication error	High	❌ No	Wiring issue
E7	Electronics	Control board fault	High	❌ No	PCB damage
E9	Overheating	Heat exchanger overheating	High	❌ No	Scale buildup
E12	Overheating	Outlet temp too high	High	❌ No	Blocked flow / scale
A7	Sensor	Outlet temp sensor fault	Medium	❌ No	Broken sensor
A9	Electronics	Communication error	High	❌ No	PCB or wiring
C1	Airflow	Air supply problem	High	❌ No	Blocked intake
C2	Airflow	Air pressure abnormal	High	❌ No	Vent issue
C4	Airflow	Fan pressure issue	High	❌ No	Fan malfunction
C6	Fan	Fan speed fault	High	❌ No	Motor failure
C7	Fan	Combustion fan failure	High	❌ No	Dead fan motor

Bosch Tankless Error Code EA

Why It Happens

The machine is trying to light the gas but cannot see the fire. Gas flows into the burner chamber. A spark tries to light the gas. A tiny metal flame sensor watches for the bright fire. The tiny metal flame sensor gets covered in dark black carbon soot.

The computer brain thinks the fire never started. It shuts the raw gas off instantly to prevent a massive boom. Read our No Hot Water In House guide to see other reasons your water stays cold.

Signs

You will hear a loud clicking sound but the fire never starts. The machine tries to spark three different times. The clicks sound like a gas grill trying to light. Then the unit goes completely silent. The water stays completely freezing cold. You might smell a very faint hint of gas near the vent pipe.

Fix

We have to open the metal box and clean the tiny sensor. We use a special soft pad to rub the black soot off the metal rod. Cleaning a dirty flame sensor costs between $100 and $200 depending on the carbon buildup. We make the metal rod shiny and clean again.

If you keep trying to force a dirty system to run, the gas valve will break. The electrical parts can fry from too many restarts.

Bosch Tankless Error Code E9

What This Means

The heavy copper heat exchanger inside is getting way too hot. Hard water leaves thick white calcium minerals inside the tiny metal tubes. The water cannot flow fast enough through the clogged pipes. The thick white minerals trap the intense heat inside the metal.

The water boils and shuts the safety limit off. The machine thinks it is melting down. It cuts the fire off immediately. It flashes E9 to warn you about the extreme heat danger.

Symptoms

The machine shuts off randomly while you take a shower. The water goes from boiling hot to freezing cold instantly. You jump out of the cold water in total shock. The machine might make a loud rumbling noise like a boiling kettle. The metal pipes on the bottom feel incredibly hot to the touch.

Fix

We must pump strong chemicals through your machine. We use a bucket and a small pump to run safe acid through the copper tubes. A chemical acid flush descaling costs between $200 and $350 depending on the calcium thickness. The acid eats the white minerals and clears the blockage completely. Read our Tankless Water Heater Cost guide to see typical maintenance pricing.

If you leave the thick scale inside, the extreme heat can destroy the metal. The copper tubes will split open and flood your basement.

Bosch Tankless Error Code A7

Causes

The hot water outlet temperature sensor is totally broken or disconnected. The machine has tiny wire probes that touch the water pipes. These probes read the water heat level. The computer brain cannot feel how hot the water is anymore.

The thin wires might melt from a bad power surge. A tiny mouse might chew through the delicate yellow wires. The computer gets completely blind. It refuses to guess the temperature. It shuts down to prevent burning your skin with boiling water.

What to Check

The water temperature will bounce up and down constantly. It will never feel steady and comfortable. You will see the A7 code flashing right after you turn the sink faucet on. You might pull the front metal cover off and see a loose wire hanging down. Do not touch any bare wires by yourself.

Fix

We have to replace the tiny wire sensor completely. We test the broken wires with a digital meter. We unplug the bad piece. We plug a brand new sensor into the green computer board. We clip the new sensor tightly to the copper water pipe. The computer brain can see the heat levels perfectly again.

Bosch Tankless Error Codes C6 And C7

Why It Causes Shutoffs

These codes mean the massive combustion fan is broken. The machine creates deadly exhaust smoke when it burns gas. The fan must push the toxic exhaust smoke out of your house. It blows the smoke through a thick white plastic pipe.

The fan motor might burn out from old age. A bird might build a nest inside the white plastic pipe outside. If the fan stops, the machine shuts the raw gas off instantly to save your life. Toxic carbon monoxide gas will kill your family if the fan fails. This is a brilliant safety feature.

Fix

You must check the outside vent pipe for hidden bird nests. Look for leaves or sticks blocking the plastic hole. If the pipe is clear, the heavy fan motor is dead. A dead fan requires a complete heavy metal motor replacement. We unbolt the broken fan and install a fresh new blower assembly.

Step-by-Step Fix for Bosch Tankless Codes

Step 1 – Turn Off The Power

Unplug the machine from the wall completely to clear the temporary memory. Wait a full five minutes. The green computer board needs time to drain its hidden power.

Step 2 – Check The Water Valves

Make sure the blue and red water handles are turned completely open. The handles must point in the same direction as the metal pipes. Closed handles will trigger overheat warnings fast.

Step 3 – Inspect The Exhaust Vent

Look outside the house to make sure nothing is blocking the white plastic vent pipe. Clear away any tall green bushes or thick snow piles blocking the air flow.

Step 4 – Reset The Digital Screen

Plug the thick black power cord back into the wall outlet. Press the glowing power button on the front panel. Watch the digital screen boot up.

Step 5 – Test The Hot Water

Turn on your kitchen sink faucet to the hottest setting. Listen for the machine to click and roar to life. Check the digital screen to see if the red error code comes back.

Bosch Tankless Repair Cost (2026)

Repair Type	Cost
Flame sensor cleaning	$100–$200
Descaling (E9 fix)	$200–$350
Sensor replacement	$150–$250
Fan replacement	$400–$700
Full unit replacement	$2,000–$4,000

When You Should Call a Technician

You can fix simple power glitches by unplugging the cord. But some problems require a licensed professional. You are dealing with raw explosive gas and deadly exhaust fumes. This is not a fun weekend project. You must call an expert when:

• You smell rotten eggs or raw gas near the machine.
• The machine makes a loud banging noise when it starts.
• You see water leaking from the bottom of the metal cabinet.
• The error codes come back immediately after a hard reset.

FAQ

What does Bosch error code EA mean?

Code EA means the machine cannot light the gas burner. The tiny metal flame sensor is covered in thick black soot and needs a professional cleaning.

How do I fix Bosch error code E9?

Code E9 means the internal copper pipes are dangerously overheating. You must hire a plumber to perform a chemical acid flush to remove the thick white hard water scale.

Can I reset my Bosch tankless water heater?

Yes. You can clear temporary computer glitches by unplugging the main power cord for five full minutes. If the code comes right back, you have a broken mechanical part.

Why does my hot water turn cold randomly?

Your heat exchanger is likely clogged with hard water minerals. The machine shuts the fire off to prevent melting the internal copper pipes.

How much does a tankless acid flush cost?

A professional descaling service usually costs between $200 and $350. This necessary maintenance adds many years to the life of your expensive water heater.

Conclusion

Taking a freezing cold shower in the morning ruins your entire day. Bosch hvac blinking codes like EA and E9 warn you about dangerous gas ignition failures and severe overheating. You must act quickly to clean the dirty flame sensors and flush the hard water scale out before the heavy heat exchanger cracks open. You should never try to open the sealed gas combustion chamber by yourself. Call our friendly local experts for safe and fast Plumbing Services. Use our Contact Us page or call us directly to schedule a fast repair. We will fix the blinking codes and get your hot water flowing perfectly again right now.

🚨 Emergency Bosch Tankless Repair

• Same-day service available
• Licensed technicians
• Fast diagnosis & repair