Rheem Tankless Sensor & Flow Codes (E1, E3, E4, CL, P1): Meaning & Fixes

Sensor codes are a heater's way of saying 'I can no longer trust my own measurements' — and a control that cannot trust its temperatures must either shut down or guess, and good controls refuse to guess. This guide covers the Rheem codes owners group under 'sensor errors': the electric tankless thermistor family E1, E3 and E4, the flow code P1 that keeps being filed with them, the scale warning hiding behind the mis-copied 'iL', and CL — which is not a sensor code, not a fault, and not bad news at all. Each gets its documented meaning, what actually fails, what an owner may check, and where the repair properly sits.

The shared system: thermistors, and why controls believe them or stop

Almost every temperature a modern Rheem heater knows comes from a thermistor — a small sensor whose electrical resistance changes predictably with temperature. Tankless gas models carry a small parliament of them: inlet, heat exchanger, outlet, and ambient/air thermistors, each with its own two-digit code when it fails (31, 32, 33, 34). Electric tankless models on the EcoSmart/RTEX platform compress the same idea into E1 (inlet thermistor), E2 (outlet thermistor) and E3 (both). The diagnostic principle is shared: Rheem's manuals test these sensors by resistance — around 7K-23K ohms in the tankless charts — and a sensor reading open, shorted, or wildly out of range is declared failed. Scale is the quiet villain in this family: a thermistor coated in mineral deposits reads the temperature of its own crust rather than the water, drifting further as the crust grows, which is why the manuals' flow charts so often read 'clean scale buildup on the thermistor, operate the unit, and replace it only if the code returns'. A control that catches a bad sensor stops, because every decision it makes — gas valve modulation, element power, overheat protection — is arithmetic on those readings. That is also why sensor codes should never be cleared and ignored: the code is not the malfunction, it is the control announcing it has gone blind in one eye.

Code E1: inlet thermistor fault (electric tankless)

On Rheem's electric tankless platform — the EcoSmart-built units Rheem sells under the RTEX line — E1 means the inlet thermistor has faulted: the sensor measuring incoming water temperature is reading outside its valid range or not reading at all. The inlet reading sounds humble, but the control uses it constantly: it calculates the temperature rise required, decides how much element power to stage, and sanity-checks the outlet sensor against it. Without a believable inlet value, that arithmetic has no anchor. The physical causes are the family usuals — a thermistor that has failed outright, a connector corroded or loosened by years of thermal cycling, or a harness fault — and because this platform's sensors live inside a cabinet carrying very high amperage, EcoSmart's troubleshooting guide treats thermistor faults as service items: there is no owner procedure beyond confirming the code and switching the unit's breakers off. What an owner can usefully do is observe and record: did E1 arrive with genuinely cold performance, or is the water heating normally under a lying code? Does it appear from a cold start, or only after long runs (heat-soaked wiring faults behave that way)? Those details steer a technician between sensor, harness, and control board. Do not run the unit indefinitely on E1 'because the water still gets hot' — the same sensor web backs the overheat protection summarised under E4, and a control working with one blind eye is not protecting you at full strength.

Code E3: both thermistors fault — the pattern that points at wiring

E3 is E1's bigger sibling: on the electric tankless platform it reports that both thermistors — inlet and outlet — are faulting together. Two independent sensors dying in the same hour is possible but improbable, and experienced diagnosis treats simultaneity as evidence: the shared elements are the suspects. Both sensors report through the same control board and, depending on the model, through adjacent harness runs and connectors; a corroded multi-pin connector, a harness pinched at a cabinet edge, moisture inside the unit, or a control board fault on the sensor-reading side will take both channels down together and present exactly as E3. That logic should shape the service visit: a technician who simply replaces both sensors without testing the harness and board has spent your money on the least likely explanation first. Rheem/EcoSmart's documented stance remains that these are internal, breaker-off repairs — the platform's troubleshooting guide lists E3 alongside E1 and E2 as thermistor faults for service. Owners contribute the timeline: E3 following a leak, a plumbing repair, or visible moisture is a different story from E3 arriving dry on a five-year-old unit, and saying so at booking can put the right parts on the truck. Until it is fixed, treat the heater as out of service: with both temperature inputs untrusted, the control has no safe basis to run the elements at all.

Code E4: high temperature — the overheat that shuts you down to protect you

E4, on the same electric tankless platform, means high temperature: the outlet water has exceeded the safe ceiling and the control has shut the elements down. Where E1-E3 are about sensing, E4 is about an event — and it deserves to be read as the protection working, not failing. The physics of an electric tankless make overheats easy to arrange accidentally: element power is sized for a design flow, so if flow collapses while power stays high — a nearly closed outlet valve, a fixture throttled to a trickle, a kinked or scaled passage strangling movement through the exchanger — the small volume of water inside absorbs the full output and its temperature spikes in seconds. Scale contributes twice: it insulates the exchanger so heat sheds poorly, and it narrows passages so flow falls. A failing outlet thermistor can also cry wolf, reporting heat that is not there — which is why E4 and the sensor codes are neighbours in this guide: the technician's first job is to establish whether the water truly overheated or the messenger lied. Owner response is disciplined and short: stop drawing hot water, let things cool, check the obvious external restrictions (inlet filter condition, valves fully open, no newly fitted fixture throttling flow to a thread), and try once at healthy flow. An E4 that recurs at normal flow is a hard stop — do not keep resetting an overheat shutdown. The whole point of the code is that something in the heat-versus-flow balance is broken; resetting re-runs the experiment with your household as the test rig.

CL: the display that means 'reset complete', not a sensor fault

CL appears in this guide because owners keep meeting it while hunting sensor problems — and because it is the most misread display in the Rheem tankless world. CL is not a fault code and not a sensor report. It is the confirmation message in Rheem's documented reset procedure for tankless hard lockouts: with the remote turned off and the unit plugged in, the front cover comes off, DIP switch settings are manipulated on the control board, the MIN and MAX buttons are held — and the remote flashes 'CL' and then shows it solid, which the manual states indicates the heater has been reset, specifically that the fault history has been cleared. A companion procedure ends with the remote flashing 'UL' for the lockout reset itself. So a CL sighting means someone — you, or a technician before you — has just run the clearing procedure, or you are watching its final step. Nothing needs fixing, nothing needs clearing, and no sensor is involved. The genuine lesson inside CL is about its context: the faults that require this procedure (the hard lockouts, including repeated poor-combustion 13s and vent-blockage 99s) are the serious ones, and the procedure exists to make clearing them a deliberate act rather than a casual button press. If your heater shows CL unprompted and you did not run any procedure, note it and mention it at the next service — a remote or board that displays reset confirmations on its own is telling its own small story.

Code P1: not enough water flow — the honorary sensor code

P1 is filed with sensor codes on list after list, and it is really a flow verdict: Rheem's chart defines it in five words — not enough water flow for operation. The tankless control measures incoming flow, and below a minimum threshold it will not fire the burner, because heating a trickle with a burner sized for a flood produces exactly the overheating the rest of this guide worries about. So P1 is the control protecting the exchanger, announced at the flow sensor rather than a thermistor — close enough in spirit to explain the filing error. The causes are refreshingly physical and mostly owner-checkable: a clogged inlet water filter (these units carry a small screen in the cold inlet connection that loads up with sediment, especially after municipal work or on well water), inlet valves not fully open after plumbing work, low house water pressure, or a single very-low-flow fixture asking for heat below the unit's activation rate. The fixes follow the causes: remove and rinse the inlet filter screen (the manuals treat this as routine owner maintenance), confirm every valve in the cold line is fully open, try a fuller flow at the tap to see the unit fire happily, and if flow is weak at every fixture in the house, the conversation moves to the home's pressure and supply, not the heater. P1 only becomes a service call when flow is demonstrably strong and the code persists — then the flow sensor itself or its wiring joins the suspect list, and that is internal work.

The code behind 'iL': 1L, the lime buildup warning

Every sensor guide for Rheem eventually meets 'iL', and it does not exist. The real code is 1L — digit one, letter L — Rheem's lime buildup warning for tankless heat exchangers, and the misreading is understandable on a seven-segment display but consequential, because searching 'iL' finds nothing authoritative and searching '1L' finds the manual. Rheem's tankless troubleshooting manual gives 1L its own section as a warning (not a shutdown): scale has built up on the exchanger surfaces enough for the control to notice, typically through the heat exchanger thermistor working harder and drifting — the sensor family again. Scale is the tax hard water levies on tankless heating: mineral crust insulates the exchanger, forces higher surface temperatures, stresses every temperature sensor reading through it, and unremoved, matures into the overheat codes (14, 15) and thermistor codes (31-34) elsewhere in this system. The remedy is the famous flush: isolator valves, a pump, hoses, and a descaling solution circulated through the exchanger — a job confident owners do with a kit and technicians do as routine service, and which Rheem's related maintenance code 88 pairs with replacing the water treatment filter where fitted. Treat 1L as the cheapest warning in this entire guide: it is the heater asking for maintenance while maintenance is still all it needs.

Temperature swings with no code yet: the pre-failure zone

Many sensor faults announce themselves behaviourally weeks before any code posts, and owners who recognise the pre-failure zone can schedule a repair instead of suffering a shutdown. The signs are consistency failures: water that overshoots after long draws then runs cool, showers that need the mixer nudged further each month to feel the same, a tankless unit whose display claims the set temperature while the tap disagrees, recovery that slows for no plumbing reason. A drifting thermistor causes exactly this — still inside its valid electrical range, so no code, but reading a few degrees of fiction that the control faithfully acts on. Scale on the sensor is the usual mechanism, which is why the zone correlates with hard water and skipped flushes. You can test the story with a kitchen thermometer: draw hot water at a steady moderate flow for two minutes, measure at the tap nearest the heater, and compare with the set point. A small fixed gap suggests sensor drift; wild swings suggest the sensor or its connection failing intermittently; a perfect match sends the investigation elsewhere — to mixing valves and demand. Write the numbers down with the date. A technician arriving to 'it feels wrong sometimes' can do little; one arriving to a dated log of set point versus measured temperature, trending worse over six weeks, can go straight to the sensor with a meter.

Diagnosis order for sensor-family complaints

When a heater presents with a sensor code — or with the vaguer complaint of erratic temperatures that usually precedes one — resist the urge to order the named sensor immediately. Work Rheem's own logic. First, verify the symptom against reality: run hot water and judge it — is delivery actually wrong, or is a healthy heater reporting a lying sensor? A cheap thermometer under the tap settles arguments. Second, clear the environmental suspects: inlet filter screen clean, valves open, flow healthy, and on tankless models the exchanger's flush history known — scale explains more sensor weirdness than failed sensors do. Third, power-cycle once, cleanly (controller off, supply off, five minutes, restore): connectors and controls both occasionally recover from a transient, and a code that never returns was an event, not a failure. Fourth, read the pattern of simultaneity: two sensors down together (E3, or paired 31/33 codes) indicts shared wiring or the board; one sensor down alone indicts the sensor. Fifth, and only then, internal testing — resistance against the manual's ranges (7K-23K ohms in the tankless charts), harness continuity, board substitution — which is technician work on every Rheem platform, performed with the power off and the manual open. The thread through all five steps: sensors are witnesses, and good diagnosis cross-examines the witness before executing it.

Key takeaways

• E1 = inlet thermistor, E2 = outlet, E3 = both, E4 = high temperature, on Rheem's electric tankless (RTEX/EcoSmart) platform — do not import these meanings to gas models.
• Two sensors failing at once (E3) usually means shared wiring or the control board, not two dead sensors — say so when you book the repair.
• CL on the remote means the reset/fault-history-clear procedure completed. It is a confirmation, not a fault, and needs no action.
• There is no 'iL' code: the display shows 1L, the lime-scale warning. A descale flush is the fix, and it is the cheapest code on this page.
• P1 is a flow verdict, not a sensor failure — clean the inlet filter screen and open the valves before suspecting any part.

Resetting the Rheem water heater

Each Rheem platform resets the way its manual documents, and only that way. Tankless models: close the hot faucet, turn the controller off, switch off the electrical supply, wait 5 minutes, restore power, turn the controller on, and open a hot tap; IKONIC displays also clear alarms via ENTER and 'Clear Alarms?' — Yes; hard lockouts (10, 13, 99) have a DIP-switch procedure that ends with the remote flashing CL (fault history cleared) or UL (reset confirmed). Tank gas controls have no reset button: the status LED clears when the underlying fault clears, and Rheem's chart routes 4, 5, 7 and 8 flashes straight to a qualified technician. Hybrids and smart electric tanks: read and note the alarm first (Next / Down arrows on the hybrid display), fix the cause, then clear; a leak alarm such as A107 can be overridden for 24 hours only by holding Clear Alarm for 5 seconds — time bought to arrange the repair, not to skip it. One reset after fixing the cause is the documented pattern on every platform; repeated resets against a live fault are not a strategy.

Frequently asked questions

Related guides

Facts on this page were checked against manufacturer documentation: files.myrheem.com · files.myrheem.com · www.manualshelf.com · www.manualshelf.com · www.manualshelf.com · mans.io · manuals.plus · manuals.plus · manuals.plus