EcoSmart Tankless Error Codes: E1 to E5, S103 and d85

By Ray Bennett · Updated 2 October 2026 · 7 min read · Checked against the maker’s manual

EcoSmart Tankless Error Codes: E1 to E5, S103 and d85

Short answer

On an older EcoSmart ECO 18 to 36, E1, E2 and E3 are the water temperature sensors, and holding the round knob gives you hot water at 125 degrees until the part comes. E4 is water coming in too hot, E5 is more flow than it can heat, and S103 is low voltage. The newer ECOS models show A70, A74, b00, b10, c05 and d85 instead.

An EcoSmart electric tankless heater shows its codes on the same small display that shows the temperature. Which list applies depends on the model. The older ECO 18, 24, 27 and 36 have a round knob and use E codes, and the newer ECOS 12, 18 and 27 have plus and minus buttons and use letter and number messages.

Everything on this page comes from EcoSmart’s own troubleshooting guide for the ECO 18 to 36 kW models and its use and care manual for the ECOS models. The newer manual carries Rheem’s name, which owns the EcoSmart brand.

Before you start

What to carry: a torch and a pen to note the code. Reading it takes five minutes. The checks inside the cover also need a multimeter that reads volts and ohms.

Gas or electric? Electric. These heaters run on 240 volt circuits, from one to four double pole breakers depending on the size.

Which model is it? The model is printed on the label, ECO 27 or ECOS 18 for example. A round knob means the older list, buttons mean the newer one.

Codes show with water running. The display lights when water flows through the heater, so open a hot tap before you read it.

Breaker off before the cover comes off. EcoSmart’s first rule. Never clean, inspect or repair the heater without first switching off all its power at the breaker box. EcoSmart’s check: turn the knob once the breaker is off. If the display still lights, you switched off the wrong breaker.

Air burns out the elements. After any plumbing work that lets air into the pipes, EcoSmart says to keep the power off and run hot water until it flows without air, then switch the breakers back on. Skipping this can damage the heating elements for good and void the warranty.

Never switch it on frozen. EcoSmart says not to turn the breaker on if the water inside the heater may be frozen.

The older E codes, ECO 18 to 36

EcoSmart's six codes on the ECO 18 to 36. E1 to E3 are the water temperature sensors, and the round knob gives you a stopgap for those three.

Check 1Free, two minutes

Read the code

E1, E2 and E3: the temperature sensors. EcoSmart calls them thermistors. E1 is the inlet one on the yellow wire, E2 the outlet one on the red wire, and E3 means both. EcoSmart says the part may need to be replaced.

E4: water coming in too hot. EcoSmart says the heater is reading incoming water at 150°F or more. Its thermostats are rated 140°F, and the fix is to bring the incoming water under 140°F.

E5: too much flow. More water is passing through than the heater can bring up to temperature.

S103: low voltage. EcoSmart says these heaters need at least 208 volts to work correctly.

Hot water today with CE1, CE2 or CE3

Check 2Free, five minutes

Use EcoSmart's stopgap

Turn it on. With hot water running and the display showing E1, E2 or E3, hold the round knob down for three to five seconds and let go. The display should read CE1, CE2 or CE3.

Wait for the heat. Keep the water running for two to three minutes and check it is hot.

Know what it does. EcoSmart says that in CE mode the heat defaults to 125 degrees, the way a tank heater would run, and you cannot change it. Mix in cold water at the tap if you need it cooler.

It is a short term fix. EcoSmart is clear that you still need a new thermistor. If the heater is under warranty, the part is covered.

E5 and the flow rate

The ECO diagnostics screens: hold the button 10 seconds, then turn the knob. The flow screen hides its decimal point, so F049 is 4.9 gallons a minute.

Check 3Free, ten minutes

Read the flow, then slow it

What things draw. EcoSmart’s figures: sinks 1 to 2.5 gallons a minute, showers 1.5 to 3, bathtubs 3.5 to 8 and washing machines 1.5 to 2.5.

See the flow. From the temperature screen, hold the button for 10 seconds to open diagnostics. Turning the round knob counterclockwise steps through three screens: the water coming in, l088 for 88°F, the water going out, O105 for 105°F, and the flow.

Slow it. EcoSmart recommends a flow restrictor, sold at hardware stores, or closing the incoming or outgoing water valve a little. Do it until the display shows the set temperature again: why a shower goes cold halfway through.

The newer ECOS messages

Message	What EcoSmart says	What the heater does
A70	Water leaving more than 20°F above the set point	Stops heating until it falls below the set point
A74	Water in or out below 35°F	Stops heating until both are above 35°F
b00	Inlet thermistor disconnected	Keeps heating, assuming 65°F water coming in
b10	Both thermistors disconnected	Keeps heating on an assumed 65°F, the flow and its last self calibration
c05	Outlet thermistor disconnected	Keeps heating the same way as b10
d85	Flow too high or water in too cold, demand over 100%	Keeps running, but will not reach the set temperature

Check 4Free, five minutes

Read the ECOS screens

Not faults at power up. The display first shows Sr and a software number, then P012, P018 or P027, which is the heater’s size in kW. Then comes SAFE and a count such as -1.5G.

The SAFE count. EcoSmart calls it Safestart: the gallons that still have to flow through before the heater counts as ready. Run hot water until it reaches 0.0 and the temperature screen appears.

d85 in use. EcoSmart says it shows when the heater is beyond its capacity and at least 5°F under the set point. Use less hot water at once, or ask about EcoSmart’s flow regulator, which fits on the outlet.

A74. Water near freezing. EcoSmart says the heater must not be installed where it can freeze, and not switched on if the water inside may be frozen: what to do about frozen pipes.

No code, but no hot water

Check 5

Work the checks with no code

The display stays dark. Check every breaker for the heater: why a breaker keeps tripping. The ECOS needs about 0.5 gallons a minute before it starts, so open the tap further.

Dark on a heater that used to work. On the ECO, EcoSmart’s guide cleans the flow meter. Breakers and water off, open a hot tap, take out its four screws and rinse the small propeller. It must spin freely when it goes back. Then run hot water for three to five minutes to clear the air before the power goes back on.

Warm but not hot. EcoSmart lists too much flow, voltage under 240, an anti scald valve at the tap mixing in cold, and heat lost along long pipes. On the ECO, EcoSmart says power under 220 volts cuts the heat by 25%.

Hard water. When a heater that used to heat well no longer does, EcoSmart’s guide checks the elements and then the scale. For owners of the older models it gives a vinegar soak every 6 to 12 months, or a pump flush once or twice a year. For the ECOS, it says flushing is for a qualified technician: how a tankless heater is flushed.

When it needs an electrician

Check 6A service call and a part

Know when to stop

S103, or low heat on a new install. EcoSmart’s test measures 208 to 240 volts across each pair of terminals with one breaker on at a time. Crossed wires at the breakers make the heater run wrong even though it turns on. That is a job for an electrician.

An element. EcoSmart’s guide says a good element reads 6 to 10 ohms with the power off, and below 5 or above 10 it needs replacing. It names air pockets, scale and freezing as the usual causes.

Scale damage. EcoSmart says damage from scale is not covered by the warranty, which is why the flush matters: the other reasons a house has no hot water.

Questions people ask

What does E5 mean on an EcoSmart?

Too much water is flowing for the heater to bring it up to temperature. Slow the flow with a restrictor or by closing a valve a little until the display shows the set temperature.

How do I get hot water with an E1 code?

With water running, hold the round knob for three to five seconds. The display shows CE1 and the heater runs at a fixed 125 degrees until the thermistor is replaced.

What does P018 mean on an EcoSmart?

It is not a fault. On the ECOS models it is the heater’s rated power, 18 kW, shown at power up. P012 and P027 are the other two sizes.

What do SAFE and -1.5G mean?

Safestart. The number counts down the gallons that must flow through before the heater starts heating. Run hot water until it reaches 0.0.

What does d85 mean?

The heater is beyond its capacity, because the flow is too high or the water coming in is too cold. It keeps running but will not reach the set temperature.

What does S103 mean?

Low voltage. EcoSmart says these heaters need at least 208 volts, and the next step is measuring the supply at the terminals.

Sources: EcoSmart Troubleshooting Guide for models ECO 18 to 36 kW (2018) and the EcoSmart use and care manual for the ECOS 12, 18 and 27 tankless electric water heaters