Most tankless codes linked to scale fall into three groups: service reminders that come on by schedule, warnings that the control board has detected scale, and safety shutdowns when the heat exchanger runs too hot. Reminders and scale warnings call for a flush. Overheating codes often do too, but they’re worth a technician’s look. Flow codes can be scale or something else. The table below sorts each brand’s codes by group.
Scale, flow and service codes by brand
| Brand | Code | What the manufacturer says it means | Group |
|---|---|---|---|
| Rinnai | LC (RE Series) | there is scale build-up in the heat exchanger and it needs to be flushed to prevent damage | scale warning |
| Rinnai | SS (SENSEI) | it is time for service and the heat exchanger should be flushed | service reminder |
| Rinnai | 14 | the heat exchanger has overheated | overheating |
| Navien | E016 | the heat exchanger is overheating | overheating; flush listed as a fix |
| Navien | E030 | exhaust overheating | overheating; inlet filter and flush listed |
| Navien | E760 | a flushing/service alarm (time for routine service) | service reminder |
| Noritz | C1# | a service reminder for periodic maintenance, such as flushing scale from the heat exchanger | service reminder |
| Noritz | 90 | a possible blocked air supply or exhaust vent, abnormal combustion, or a clogged condensate drain | combustion; not scale |
| Rheem | 1L | lime build-up has been detected in the heat exchanger | scale warning |
| Rheem | 88 | a reminder to flush the heat exchanger and/or replace the water treatment filter | service reminder |
| Rheem | P1 | there is not enough water flow to operate the unit | flow |
| Takagi | 651 | a flow adjustment valve fault | flow; scale is one cause |
| Bosch | E9 | the temperature limiter has sensed over 220 F, which may indicate scale build-up in the heat exchanger | overheating |
| Bosch | E1 | the hot water sensor is reading above 185 F, possibly due to scale build-up | overheating |
| Bosch | A7 | a problem related to the outlet hot water temperature sensor | sensor; listed with scale symptoms |
Codes and their meanings vary by model. The same number can mean different things across brands, and sometimes across a single brand’s product lines. Check the manual for your model, or the brand page, before acting on a code.
How the groups differ
Service reminders: SS, E760, C1#, 88
Most of these run on a timer or a usage count, not a sensor. They come on when the interval a technician picked has passed. Noritz’s warranty text describes C1# as a reminder indicating scale build-up, so treat that one as a scale signal too. Navien’s reminder can be set to no alarm, 6, 12 or 24 months. Rheem’s can be set to 500, 750 or 1,000 hours of operation. The heater still works, but the manufacturer is telling you service is due. The fix is a flush and descale, and then the technician resets the reminder.
Scale warnings: LC and 1L
These mean the heater has sensed scale. Rinnai notes that LC is the only code that lets the unit keep running: the display switches between LC and the set temperature. That makes it easy to put off. Rheem’s 1L says the unit must be drained and flushed. Don’t wait for the heater to stop.
Overheating shutdowns: 14, E016, E030, E9, E1
Scale insulates the inside of the heat exchanger. Heat can’t pass into the water fast enough, so metal and water temperatures climb until a safety limit trips. Rinnai’s manual tells technicians to check for hot spots that indicate blockage due to scale build-up under code 14. Bosch says E1 can come from scale or pre-heated feed water. These codes can also have other causes, so a technician should confirm before flushing alone is the answer.
Flow codes: P1 and 651
A tankless heater needs a minimum flow of water before it will fire. Rheem’s P1 says flow is too low. Takagi lists a locked motor drive due to scale buildup among the causes of 651. Scale, a clogged inlet filter, a partly closed valve or a fixture problem can all cause these.
Seeing a code? Call to book a descale
What about Rinnai LC0 to LC9?
People search for Rinnai codes LC0 through LC9 and “LC#” a lot. We didn’t find those codes defined in any official Rinnai document. What Rinnai does document is LC on RE Series heaters, meaning scale build-up that needs a flush, and SS on SENSEI heaters, meaning service is due. If your display shows LC followed by a number, treat it as a scale warning and have a technician check it against the manual for your exact model. We won’t guess at what each number means.
A.O. Smith tankless heaters
A.O. Smith doesn’t have a page of its own on this site, so here’s what our sources say.
- Hardness limit. The A.O. Smith-hosted manual for 110U, 310U and 510U models says hardness must not exceed 7 gpg for single-family homes.
- Warranty. The warranty in that manual says hard water above 7 gpg (120 ppm) is excluded. Most Wasatch Front cities that publish a figure are above that.
- X3 descaling. A.O. Smith says its X3 scale prevention technology eliminates routine descaling (X3 models).
- X3 warranty. A.O. Smith says X3-equipped units carry a limited warranty with no exclusions for scale-related failures (X3 models).
The A.O. Smith-hosted manual sets the same single-family hardness limit as Takagi’s manual for its 110, 310 and 510 models. Even so, we haven’t verified an A.O. Smith code list, so we don’t print one. If an A.O. Smith heater shows a code, have the technician read it against your model’s manual. The Takagi page and scale prevention page cover X3 in more detail.
Seeing a code? Call to book a descale
Codes that aren’t about scale
Many codes point away from scale. Ignition and flame codes (Rinnai 11 and 12, Navien E003 and E012), vent blockage (Rinnai 10) and circuit board faults (Navien E515) need a repair diagnosis. A flush won’t clear them. The repair page groups those by failure, with HomeGuide’s cost ranges.
What to do when a code appears
These checks are safe for any homeowner:
- Write the code down exactly. Note whether it’s steady, flashing or alternating with the temperature.
- Note what was happening. Was one tap running, or the shower and washer together? Did the water go cold, or keep flowing?
- Look it up. Use the table above, your brand page or the manual for your model.
- Check the obvious. Is the heater plugged in and the breaker on? Do other gas appliances work?
- If it’s a leak code, Rinnai’s manual says to turn off the water supply and contact a licensed professional.
- Don’t open the cover or keep resetting. Repeated resets can hide a safety problem.
If you smell gas, leave the house first and call your gas utility from outside.
If the code is a reminder or scale warning, a flush is the usual answer. For anything else, start with repair. If the heater has stopped making hot water, the no hot water page sorts the symptoms. Either way, call to book a descale or a repair visit with a technician near you, from American Fork to Herriman.