You turn on the hot water, wait, and nothing happens. Then you notice the small display on the tankless unit is flashing an error code. No hot water, no obvious explanation, and a code that means nothing to you. If that is where you are, the good news is that many tankless ignition problems have simple causes, and the rest are worth understanding before you decide what to do next.
How Tankless Ignition Differs From a Pilot Light
If you searched for “water heater pilot light won’t stay lit,” you may be thinking of a traditional tank. Older tank water heaters keep a small standing pilot flame burning, backed by a thermocouple that shuts off the gas if the flame goes out. When that pilot keeps going out, it usually points to a dirty pilot tube, a worn thermocouple, or a draft problem.
Most tankless water heaters do not have a standing pilot at all. They light on demand, using a sequence of parts working together:
- Flow sensor. When you open a hot tap, the sensor detects water moving through the unit and tells the control board to start.
- Fan or blower. It clears the combustion chamber and pushes exhaust through the vent.
- Igniter. An electronic igniter or spark creates the flame when gas flows.
- Flame sensor. It confirms the flame is lit. If it does not detect a flame, the unit shuts down and shows an error code.
Because so many parts must agree before the burner stays lit, an ignition fault can come from the gas supply, the vent, the sensors, or the water flow, not only the igniter. See our tankless water heater installation page for how these systems are set up.
Common Causes of Ignition Faults
Gas supply issues. A partly closed gas valve, a gas line that is too small, low gas pressure, or an interruption in service can keep the unit from lighting. Other gas appliances, such as a range or furnace, behaving oddly are a clue. Learn more on our gas water heater page.
Venting blockages. The unit needs clear intake and exhaust. A blocked vent, a damaged vent section, or debris at the termination can trip a safety and prevent ignition.
Sensor faults. A dirty or worn flame sensor, a failing flow sensor, or a faulty igniter can prevent the unit from lighting or staying lit.
Low water flow. Tankless units need a minimum flow to fire. A clogged inlet filter, a partly closed isolation valve, or scale buildup from San Diego’s hard water can reduce flow below that threshold.
Control board or other component failure. Less common, but possible on older units.
What You Can Safely Check Yourself
A few checks are safe for a homeowner and often solve the problem:
- Make sure the unit has power and that nothing has tripped a breaker.
- Check that the gas valve to the unit is open, and see whether other gas appliances work.
- Check that the hot and cold water isolation valves are fully open.
- Look at the vent termination outside for visible debris or an obstruction, without opening or taking apart anything.
- Reset the unit by turning off power for a short time and turning it back on, as your manual describes.
- Look up the error code in your manual to see what the manufacturer says it means.
What Needs a Licensed Technician
Do not open the unit, adjust gas pressure, remove or clean gas components, or modify the vent. Gas and combustion work carries real risks, including gas leaks and carbon monoxide, and it must be done by a licensed professional.
If you smell gas, leave the home, avoid switching anything on or off, and call your gas utility or 911 from outside. For urgent hot water or gas-related emergencies, see our emergency plumbing service.
Cost of a Typical Ignition Repair
Costs depend on the cause. As a general range, many ignition repairs, such as cleaning or replacing a flame sensor or igniter, or clearing a vent or filter, run roughly $150 to $800. A flow sensor or fan replacement can cost more, and a control board or heat exchanger problem can cost significantly more, which is when replacement is worth weighing. A diagnostic visit finds the actual cause so you get a firm quote before any repair.
Why San Diego Homeowners Choose Repipe Home Hero
Repipe Home Hero technicians are trained specifically on tankless systems, not just standard tank units, so they understand how the flow sensor, igniter, fan, and flame sensor work together and can diagnose the cause rather than guess at parts.
Owner Stefan Mirkov holds a California C-36 plumbing license (#1075463), and every job is handled by Repipe Home Hero’s own licensed crew, with no subcontracted labor. Repipe Home Hero has completed more than 950 projects across San Diego County and holds over 250 five-star Google reviews.
Frequently Asked Questions
Does a recurring ignition fault mean the unit needs replacement? Not necessarily. A repeating fault usually points to an underlying cause, such as gas supply, venting, a failing sensor, or scale restricting flow. If the unit is relatively young and the cause is a replaceable part, a repair often restores it. Replacement becomes worth considering when the unit is near the end of its 20-year range, the heat exchanger or control board has failed, or repair costs are approaching the price of a new unit.
Why does my tankless water heater show an error code? The unit is telling you which safety check failed, such as no ignition, no flame detected, a vent problem, or low flow. Your manual lists the codes for your model, and a technician can tell you what is behind it.
Can hard water cause ignition problems? Indirectly, yes. Scale buildup can reduce water flow through the unit and the inlet filter, which can keep the flow sensor from triggering. Annual descaling helps prevent this in San Diego’s hard water.
Is it safe to keep resetting the unit? An occasional reset is fine, but repeated faults are a warning. Have it diagnosed rather than resetting it again and again.
Schedule a Same-Week Diagnostic Visit
If your tankless water heater will not ignite or keeps flashing an error code, Repipe Home Hero offers a same-week diagnostic visit for San Diego County homeowners. We find the cause, explain your options, and give you a clear quote before any work starts.
Call (619) 386-0375 or request a diagnostic visit online today.
Learn more about our tankless water heater installation, our gas water heater services, or our emergency plumbing service.
