Skip to content
Appliance Error Codes

E11 Error on AO-SMITH NNLP-199-H NG Tankless Water Heater — Fix Guide

Error E11 on ao-smith : Cold inlet thermistor (NTC1) fault. Follow the troubleshooting steps below to diagnose and fix this tankless-water-heater issue. ## What Error Code E11 Means on a ao-smith NNLP-199-H NG Error E11 on the NNLP-199-H NG indicates the cold water inlet thermistor NTC1 is faulty. The unit uses this sensor to calculate the required heat output for the desired temperature rise. When E11 appears, the ao-smith tankless water heater pauses normal operation to protect internal components from potential damage. Not every E11 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 - E11 on display; unit will not fire

  • May appear intermittently after start
  • Sensor fault confirmed by resistance test ## Common Causes - Cold inlet thermistor (NTC1) fault ## Tools You May Need - Multimeter
  • Replacement NTC1 thermistor ## Step-by-Step Repair Guide | Step | Action | Result | |------|--------|--------| | 1 | Power off and turn off the cold water supply. | Done — continue to step 2 if the issue persists. | | 2 | Locate NTC1 on the cold inlet pipe inside the unit. | Done — continue to step 3 if the issue persists. | | 3 | Disconnect and measure resistance at current cold water temperature. | Done — continue to step 4 if the issue persists. | | 4 | Compare to the temperature-resistance chart in the service manual. | Done — continue to step 5 if the issue persists. | | 5 | Replace if out of specification; restore power and confirm E11 is cleared. | Done — if the fault remains, contact a qualified technician. | ## Safety Precautions Shut off the cold water supply before accessing water-side sensors. ## Estimated Repair Time and Difficulty Most tankless water heater error code E11 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: Why would the cold inlet thermistor fail? Physical damage during installation, freezing temperatures damaging the sensor housing, or general component aging are the most common causes. ### Is it safe to keep using my ao-smith with error E11? If error E11 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 E11 on a ao-smith ? Repair costs for error E11 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 E11 mean on a ao-smith tankless-water-heater? Error code E11 on ao-smith tankless-water-heater models indicates Cold inlet thermistor (NTC1) fault. 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 E11? If error E11 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 E11 on a ao-smith ? Repair costs for error E11 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 E11 mean on a ao-smith tankless-water-heater? Error code E11 on ao-smith tankless-water-heater models indicates Cold inlet thermistor (NTC1) fault. 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.