Tankless problems almost always trace back to a handful of causes: mineral scale, gas supply, ignition components, sensors, or venting. Here's what the common symptoms usually mean.
Water goes cold mid-showerThe "cold water sandwich" or a unit cutting out under load — often a flow sensor issue, scale buildup restricting the heat exchanger, or an undersized gas supply that can't sustain the burn.
Temperature swings hot and coldFluctuating output usually points to scale in the heat exchanger or a failing flow/temperature sensor. Descaling resolves a large share of these calls.
Unit won't igniteNo hot water and clicking with no flame typically means an ignition component, gas valve, or gas supply problem — one of the most common tankless repairs we perform.
Reduced hot water flowWhen hot-side pressure drops but cold is fine, the inlet filter or heat exchanger is usually restricted by scale or debris. Cleaning or descaling typically restores full flow.
Error code on the displayModern tankless units self-diagnose. The code narrows the cause fast — see the common codes below, and tell us the code when you call so we arrive with the right parts.
Unit running loud or smelling offUnusual combustion noise or odors can indicate venting or combustion-air problems. Shut the unit off and call — combustion issues shouldn't wait.
Common Error Codes We See in Santa Cruz
| Code | Brand | What It Commonly Indicates |
|---|
| 11 | Rinnai | Ignition failure — no flame on startup. Often gas supply or ignition components. |
| 12 | Rinnai | Flame failure — burner lit, then lost flame. Gas supply, flame rod, or venting. |
| 10 | Rinnai | Air supply or exhaust blockage — venting obstruction or fan issue. |
| LC | Rinnai | Scale buildup in the heat exchanger — the unit is asking for a descale. Very common in Santa Cruz. |
| E003 | Navien | Ignition failure — similar causes to Rinnai code 11. |
| E012 | Navien | Flame loss during operation — gas supply or flame sensing. |
| E110 | Navien | Exhaust/intake blockage — venting obstruction or fan problem. |
| E407 | Navien | Temperature sensor fault on the hot water outlet. |
Codes are a starting point, not a final diagnosis — the same code can have several causes, and we confirm on site before recommending anything. Have a different brand or code? Call (831) 818-6331 and read us what's on the display.
