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Appliance Error Codes

E9 Error on AO-SMITH NNLP-199-H NG Tankless Water Heater — Fix Guide

Error E9 on ao-smith : Outlet water temperature limit exceeded. Follow the troubleshooting steps below to diagnose and fix this tankless-water-heater issue. ## What Error Code E9 Means on a ao-smith NNLP-199-H NG Error E9 on the NNLP-199-H NG tankless unit triggers the overtemperature safety when the outlet water temperature exceeds approximately 185°F (85°C). Low flow combined with high temperature setting is the typical cause. When E9 appears, the ao-smith tankless water heater pauses normal operation to protect internal components from potential damage. Not every E9 fault requires a professional engineer — many are resolved through careful inspection and routine maintenance. Work through each diagnostic step below in sequence, starting with the simplest check, to identify the root cause efficiently before considering part replacement or arranging a service call. ## Symptoms - E9 on display; unit shuts off

  • Outlet water was scalding hot before shutdown
  • Low-flow fixture was in use
  • Temperature setting may be at maximum ## Common Causes - Outlet water temperature limit exceeded ## Tools You May Need - Thermometer (to measure outlet temperature) ## Step-by-Step Repair Guide | Step | Action | Result | |------|--------|--------| | 1 | Reduce the temperature setpoint to 120°F (49°C) — the safe residential recommendation. | Done — continue to step 2 if the issue persists. | | 2 | Increase the flow rate by fully opening the hot water tap. | Done — continue to step 3 if the issue persists. | | 3 | If E9 trips at normal flow and reasonable temperature, the heat exchanger may be scaled, reducing flow area and concentrating heat. | Done — continue to step 4 if the issue persists. | | 4 | Schedule annual descaling. | Done — continue to step 5 if the issue persists. | | 5 | If E9 persists after descaling, the outlet thermistor or PCB may be faulty. | Done — if the fault remains, contact a qualified technician. | ## Safety Precautions Water at 130°F causes serious scalding burns in under 30 seconds. Keep the setpoint at 120°F for household safety. ## Estimated Repair Time and Difficulty Most tankless water heater error code E9 faults can be assessed and addressed in 30–90 minutes using the steps above. The diagnostic process requires only basic tools: a multimeter and standard screwdrivers cover the majority of checks described here. If a replacement part is required, allow 1–5 working days for delivery. A trained technician with access to the correct diagnostic equipment will typically resolve this type of fault within a single service visit. ## When to Call a Technician Contact a ao-smith authorised service technician if: the fault persists after following all the steps above; if PCB-level or refrigerant-circuit work is required; or if you are not comfortable working with electrical or mechanical components. When calling, have the full model number, the error code displayed, and a brief description of any sounds or symptoms you noticed before the fault appeared — this helps the technician prepare the correct parts and tools. ## Frequently Asked Questions Q: What temperature should I set my A.O. Smith tankless at? 120°F (49°C) is recommended by ASHRAE and most plumbing codes for residential use — hot enough for effective cleaning and dishwashing but below scalding temperature. ### Is it safe to keep using my ao-smith with error E9? If error E9 does not indicate an immediate safety hazard such as a gas leak, electrical short, or water flooding, you can continue limited use while monitoring the appliance. However, running the tankless-water-heater with an active error code increases the risk of further damage. Address the underlying issue as soon as possible. For high-severity errors, disconnect the appliance and contact a qualified technician. ### How much does it cost to fix error E9 on a ao-smith ? Repair costs for error E9 on the ao-smith tankless-water-heater depend on the root cause. DIY fixes such as cleaning filters, resetting the unit, or reconnecting loose wires cost nothing beyond basic tools. If a component replacement is needed, parts and labor from a certified technician are the main expenses. Contact a ao-smith-authorized service provider for an accurate estimate based on your specific situation. ### What does error code E9 mean on a ao-smith tankless-water-heater? Error code E9 on ao-smith tankless-water-heater models indicates Outlet water temperature limit exceeded. This error is detected by the appliance’s internal diagnostic system and displayed to alert the user that attention is needed. Refer to the troubleshooting steps above for guidance on resolving this issue. ### Is it safe to keep using my ao-smith with error E9? If error E9 does not indicate an immediate safety hazard such as a gas leak, electrical short, or water flooding, you can continue limited use while monitoring the appliance. However, running the tankless-water-heater with an active error code increases the risk of further damage. Address the underlying issue as soon as possible. For high-severity errors, disconnect the appliance and contact a qualified technician. ### How much does it cost to fix error E9 on a ao-smith ? Repair costs for error E9 on the ao-smith tankless-water-heater depend on the root cause. DIY fixes such as cleaning filters, resetting the unit, or reconnecting loose wires cost nothing beyond basic tools. If a component replacement is needed, parts and labor from a certified technician are the main expenses. Contact a ao-smith-authorized service provider for an accurate estimate based on your specific situation. ### What does error code E9 mean on a ao-smith tankless-water-heater? Error code E9 on ao-smith tankless-water-heater models indicates Outlet water temperature limit exceeded. This error is detected by the appliance’s internal diagnostic system and displayed to alert the user that attention is needed. Refer to the troubleshooting steps above for guidance on resolving this issue.