Seisco Tankless Water Heater Troubleshooting
The Short Answer: A Seisco (RA / Signature / Flagship) that will not heat is almost always missing a leg of 208-240 V, air in the chamber, a tripped high-limit (code 122), a leak/moisture detect (126), or a sensor the board no longer trusts. Green blink is normal. Repeating red digits are not. Lock every double-pole off before you open the cover. Do not ohm a live element.
Licensed local technicians
Seisco tankless down or throwing codes?
Connect with a licensed 24/7 technician in your zip code right now.
Search Intent & The Real Problem
Showers are cold. The Seisco on the wall is beeping or the LED is flashing red in a three-digit sequence. This is an electric tankless with one or more heating chambers, a control board, and thermistors that the board uses as a flow signal. It is not a 40-gallon dual-element tank. Signature models can have two to four dedicated breakers. One off and the board may still glow while the chambers stay lukewarm. Related outage behavior: beeping after a power outage.
What other guides get wrong
They treat it like a 4500 W tank: one 30-amp, two screw-in elements, a red ECO. A 14 kW RA is ~58 A at 240 V; a 28 kW is ~117 A. They skip the other breakers. They tell you to replace the high-limit because of 122 - the limit is the messenger; a failed sensor or a dry chamber cooked it. They mix generic tankless 11/12 gas codes onto an electric board.
Step-by-Step Troubleshooting
Step 1: Identify the platform and kill every circuit
| Family | What you are looking at |
|---|---|
| RA / CA Signature (2-4 chamber) | Multiple double-poles, LED on the board, fittings on top |
| Flagship 1-chamber | Two-part flash codes; still 240 V |
| Point-of-use SH / small kW | Still prove dead; do not treat as a 120 V plug-in |
- What to look for: Model and serial on the rating plate. Every breaker labeled for the heater ON or tripped. Inlet/outlet not reversed (cold is on the right on many Signature units). Mounted vertical, fittings up.
- Required tools: Flashlight, phone photo of the plate. Panel from a dry location.
A 4500 W / 240 V tank element is ~18.8 A and ~12-13 Ω. Seisco chamber elements are sized off that unit’s kW. Compare ohms only with all breakers locked off and wires lifted.
Step 2: Read the LED - green vs a red three-digit code
Boot: a few red flashes and beeps, then a repeating green blink is normal. A fault is a three-part red sequence (two-part on some Flagship), then green, then it repeats. There may be more than one code. The blue button on the board will beep the code for a tech on the phone.
| Signature code (confirm the service sticker) | Meaning | Homeowner action |
|---|---|---|
| Green blink | Normal | None if water is hot |
| 111-115 | Thermistors TH-IN / TH-1 through TH-4 | Power off; licensed sensor test |
| 117 / 118 | Shorted / open temp sensor | Power off; do not keep cycling |
| 121 | Disable jumper missing on some boards | Licensed board setup |
| 122 | High-limit open | All power off; do not keep resetting the button |
| 123 | Level detect / air in chamber | Fill and purge; never leave the detect jumpered |
| 124 | High-temperature shutdown | Flow, sensors, sizing |
| 126 | Moisture / leak detect | All power off. Find the leak. |
| 132 / 133 | High / low voltage | Utility or wiring; code often clears with voltage |
| 134-137 | Element 1-4 | Ohm dead; sensors often fake an element code |
| 142 | Data / A/D error | One 30-second power-down of all breakers |
- What to look for: The full sequence twice. Photograph it.
- Required tools: Eyes on the LED with the cover off only after 0 V is proven, or listen to the beep through a closed jacket if you can hear it.
Step 3: Water first - fill, purge, flow, no dry fire
- What to look for: Isolation valves open, air purged, a solid stream at a hot faucet before power. New install: purge, shut the fixture, then energize. The unit may refuse to fire if you power it with a tap already wide open. Reversed hot/cold. Inlet screen packed. House static over 80 psi is a regulator/expansion job, not a Seisco “boost.”
- Required tools: Faucet, valves. No live cover.
123 with sputtering water is air. 122 after a dry start is a cooked chamber - same physics as turned on without water. Never energize empty.
Step 4: 122, 126, and the safeties - stop clicking
- What to look for: 122: high-limit at the top of a chamber. All power off. A single press of a manual reset on older Signature limits is a procedure after you find why it opened - failed sensor, scale, no flow, a stuck relay. Replacing the limit and walking away is how you get 122 again. 126: moisture on the detect pad or a real leak at an element O-ring, fitting, or sensor. Rust powder around elements is a leak, not dust. Leave it off.
- Required tools: Paper towel, dry cabinet. Breakers locked.
Do not ground the level-detect spades as a “fix.” That test exists in the service guide for technicians. Leaving it jumpered kills the dry-fire protection.
Step 5: Sensors and elements - meter, power proven dead
- What to look for: Thermistors cooled with water running, power off: readings within about 10% of each other. High-limit closed (near 0 Ω) after a valid reset. Elements: ohms across the red/black screws per the kW on that chamber; any screw-to-chassis continuity is a ground. An “element” code with a good ohm reading is often a thermistor - the board is guessing.
- Required tools: Digital meter, all breakers locked, wires lifted. Clamp-amp checks are licensed live work.
Matching procedure (Signature) after a sensor swap is in the Seisco service sheet. Skip it and the board lies. Lukewarm with all elements drawing nameplate amps is undersized flow, not a mystery part - restrict the fixture or resize.
A breaker that will not reset is a bolted short. Do not hold it on.
When DIY Ends: Calling a Licensed Pro
You may confirm every breaker, purge air with power off, photograph a code, and dry a cabinet after a 126 only if the leak is a fitting you can isolate. Stop at live lugs, element replacement, board replacement, a repeating 122, and any leak into the chamber. Multiple 240-volt circuits, a wet board, and a grounded chamber element are licensed electrician or plumber work. Seisco still supports these; a licensed tech with the service guide beats a second used board from the internet.
Frequently Asked Questions
Q: Why is my Seisco tankless not producing hot water? A: A breaker off, air in the chamber, reversed piping, a 122 high-limit, a 126 leak detect, or a sensor the board does not trust. Green blink with lukewarm water is often flow or missing a second circuit.
Q: What does a flashing red light on a Seisco mean? A: A three-digit diagnostic. Count the red flashes in groups. 122 is high-limit, 123 is low water/air, 126 is moisture. The sticker and Seisco sheet win over a generic tankless chart.
Q: Can I replace a Seisco heating element myself? A: Only with every breaker locked off, 0 V proven, the chamber drained, and a new O-ring. If you cannot prove dead, or you have 126 moisture on the board, hire a licensed tech. Never work it live.