E5 Error on AO-SMITH NNLP-150-H NG Tankless Water Heater — Fix Guide

• Source: Official ao-smith service manual
• Reviewed by: Geo Technical Editorial
• Published: March 1, 2026
• Updated: March 1, 2026

Safety notice: Start with user-safe checks only. Do not open sealed refrigerant systems, inspect gas-train components, or work on energized high-voltage parts unless you are qualified to service this appliance.

Error E5 on ao-smith : Insufficient water flow or flow sensor fault. Follow the troubleshooting steps below to diagnose and fix this tankless-water-heater issue. ## What Error Code E5 Means on a ao-smith NNLP-150-H NG Error E5 on the NNLP-150-H NG means water flow through the unit has dropped below the minimum activation threshold (typically 0.5 GPM), or the flow sensor turbine is fouled with scale. When E5 appears, the ao-smith tankless water heater pauses normal operation to protect internal components from potential damage. Not every E5 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 - E5 displayed; cold water delivered

• Unit does not fire when demand is low
• Works at full tap opening but fails at half flow
• Cold water inlet filter may be blocked ## Common Causes - Insufficient water flow or flow sensor fault ## Tools You May Need - Screwdrivers
• Small brush (for filter cleaning)
• Descaling kit ## Step-by-Step Repair Guide | Step | Action | Result | |------|--------|--------| | 1 | Open the cold water inlet valve fully at the unit. | Done — continue to step 2 if the issue persists. | | 2 | Turn off water supply and remove the cold water inlet filter screen; rinse under tap water. | Done — continue to step 3 if the issue persists. | | 3 | Test with a single fully open tap to confirm the unit fires at maximum flow. | Done — continue to step 4 if the issue persists. | | 4 | If the unit fires at high flow but not low flow, the minimum flow fixtures need aerators removed or the setpoint needs adjustment. | Done — continue to step 5 if the issue persists. | | 5 | For persistent E5 at normal flow rates, flush the unit’s heat exchanger and flow sensor with a descaling solution. | Done — if the fault remains, contact a qualified technician. | ## Safety Precautions Shut off gas and cold water before removing the inlet filter. Hot water stored in the heat exchanger can cause burns. ## Estimated Repair Time and Difficulty Most tankless water heater error code E5 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 won’t my tankless unit work at low flow? Tankless heaters need minimum flow to operate safely — too little water would be heated to dangerous temperatures. If minimum flow is an issue, consider a recirculation system. Q: How often should I descale my A.O. Smith tankless? In hard water areas (above 7 gpg), descale annually. In soft water areas, every 2 years is typically sufficient. ### Is it safe to keep using my ao-smith with error E5? If error E5 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 E5 on a ao-smith ? Repair costs for error E5 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 E5 mean on a ao-smith tankless-water-heater? Error code E5 on ao-smith tankless-water-heater models indicates Insufficient water flow or flow sensor 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 E5? If error E5 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 E5 on a ao-smith ? Repair costs for error E5 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 E5 mean on a ao-smith tankless-water-heater? Error code E5 on ao-smith tankless-water-heater models indicates Insufficient water flow or flow sensor 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.