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Fix a tankless water heater that flashes an error code

We'll confirm the error code, check flow, gas, venting, and power, then clear or fix the fault—or tell you when to call a pro.

Troubleshooting · Home maintenance
15–45 min
What you'll need
  • Owner manual or access to manufacturer website
  • Screwdriver (for filter access if applicable)
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Steps

Goal: Confirm the error code, check flow, gas, venting, and power, then clear or fix the fault.

  • Note the exact error code from the display. Check the owner manual for your model.
  • If you smell gas, evacuate immediately. Call 911 or your gas utility from outside.
  • Good: No gas smell—proceed to Check flow.
  • Bad: Gas smell—evacuate and call 911.

Check flow

Goal: Confirm water flow. Many tankless faults are caused by no flow.

  • Confirm the cold water supply valve to the unit is open. Confirm the hot water outlet valve is open.
  • Turn on a hot water faucet. You should see flow. If a filter screen is at the inlet, check it for debris.
  • Good: Flow is adequate. Try a hard reset. If the code clears, fix complete.
  • Bad: No flow—open valves or clean the filter.

Check gas and vent

Goal: For gas units, confirm gas supply and vent are OK. Do not work on gas or vent yourself.

  • Confirm the gas valve to the unit is on. Check that other gas appliances work.
  • Check the vent termination outside—no debris, snow, or nests. Check the vent pipe is not damaged.
  • If the code indicates ignition or vent issues, call an HVAC pro. Do not run the unit with a blocked vent.
  • Good: Gas on and vent clear—try hard reset. If code persists, call a pro.
  • Bad: Gas or vent issue—call a professional.

When to get help

Call a plumber or HVAC professional if:

  • You smell gas (evacuate and call 911 first).
  • The code indicates gas or vent issues.
  • Ignition fails after checking flow and power.
  • The unit needs descaling.
  • You are not comfortable with gas or electrical work.

Verification

  • No error code on the display when you run hot water.
  • Hot water reaches fixtures at the expected temperature.
  • No gas smell.

Escalation ladder

Work from the device outward. Stop when the problem is fixed.

  1. Note error code Read the code and check the manual for meaning.
  2. Check flow Confirm water valves are open and filter is clean.
  3. Check gas and vent Gas supply on; vent clear. Call a pro for gas or vent work.
  4. Hard reset Power cycle the unit to clear transient faults.
  5. Call a pro Code persists, gas/vent/ignition issue—call a plumber or HVAC pro.

What to capture if you need help

Before calling support or posting for help, have these ready. It speeds everything up.

  • Exact error code
  • Unit brand and model
  • Whether flow, gas, and vent were checked
  • Steps already tried

Do you smell gas?

If you smell gas, evacuate and call 911 or your gas utility from outside.

Yes: evacuate immediately. Do not turn on lights or appliances. Call 911 or your gas utility from outside. No: proceed to check the error code and causes.

Yes No

You can change your answer later.

Evacuate and call 911

Evacuate. Call 911 or your gas utility from outside. Do not call a technician for an active gas leak.

Done

Is the error code related to flow, ignition, or vent?

Check the manual for your model. Flow codes: check valves and filter. Ignition/vent codes: often need a pro.

Note the code. Check the manual. Flow: check cold and hot valves, inlet filter. Ignition/vent: check gas supply and vent; call a pro for repairs.

Flow Ignition or vent

You can change your answer later.

Is water flowing when you open a hot faucet?

Open a hot water faucet. Confirm the cold supply and hot outlet valves to the unit are open. Check the inlet filter for debris. Yes: try hard reset. No: open valves or clean filter.

Yes No

You can change your answer later.

Open valves and clean filter

Open the cold supply and hot outlet valves. Clean the inlet filter if present. Restore flow and test. If the code clears, fix complete.

Done

Did a hard reset clear the code?

Turn off power at the breaker for 30 seconds. Turn it back on. Run hot water. Yes: fix complete. No: call a pro.

Yes No

You can change your answer later.

Fix complete

The code has cleared. Monitor for a few days.

Done

Call a professional

Call a plumber or HVAC pro for ignition, vent, or gas issues. Do not work on gas lines or venting yourself.

Done

Reviewed by Blackbox Atlas

Frequently asked questions

Why would a tankless water heater flash an error code?
Error codes indicate a fault—common causes include no water flow, no gas, vent blockage, ignition failure, or overheating. Each brand has different codes; check the manual for the specific meaning.
Can I fix a tankless error code myself?
Yes, for flow issues (open valves, clean filter) and some ignition resets. Gas, vent, and ignition repairs often require a professional. Never work on gas lines yourself.
When should I call a professional for a tankless error code?
Call a pro if you smell gas (evacuate and call 911 first), the code indicates gas or vent issues, ignition fails after checking flow and power, or you are not comfortable with gas or electrical work.

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