Lochinvar Water Heater Error Codes: WV8840 Blink Codes and Knight/Crest Control Board Faults Decoded

Common Symptoms

• Lochinvar gas tank model: green LED on the WV8840 gas valve blinking 1–7 times in a repeating pattern
• Status light blinking but no hot water — main burner fails to fire
• Pilot lights but goes out when the gas control knob is released
• LED showing 4 blinks after a period of no hot water — ECO/high-limit tripped
• LED showing 7 blinks — unit completely dead, gas valve safety lockout
• Lochinvar Knight/Crest: control board display showing Ign Fail, SHt, LoN, or an E-code
• Knight control board in lockout — unit locked out after repeated ignition failures
• Repeated fault codes returning within minutes of a power cycle

Most Likely Causes

1. 1

   WV8840 — 1 Blink: Standby (Normal) or Thermopile Voltage Low

   On the Honeywell WV8840 gas valve installed in Lochinvar residential gas tank water heaters, 1 blink per cycle is one of two conditions: (Normal) standby — the thermostat is satisfied and there is no call for heat; the unit is waiting. This is normal operation with no fault. (Fault) on some WV8840 firmware variants, 1 blink means the pilot is lit but thermopile voltage is insufficient to open the main gas valve. The WV8840 requires ≥325mV to open the main valve; a healthy Lochinvar thermopile produces 650–850mV; below 400mV is degraded; below 350mV will not reliably open the valve. To distinguish standby from fault: create hot water demand (open a hot tap). If the main burner fires, 1 blink is standby. If the main burner does not fire with confirmed demand, measure thermopile millivolts — below 350mV = replace thermopile (AO Smith 9005502 is compatible with Lochinvar models using the WV8840).

2. 2

   WV8840 — 2 Blinks: Thermopile Low Voltage Confirmed

   Two blinks on the Lochinvar WV8840 LED confirms thermopile voltage is below the main valve opening threshold. The pilot may be lit, but the thermopile output is insufficient for main burner operation. Causes: (1) Thermopile degraded from age (6–10 year typical life) — test with a millivolt meter, below 350mV = replace with AO Smith 9005502; (2) Pilot flame too small or misaligned — partially blocked pilot orifice produces a weak flame that cannot adequately heat the thermopile; clean with compressed air; (3) Thermopile wiring connector corroded or loose at the WV8840 — disconnect the TH/TP leads, clean pins with fine emery cloth, apply dielectric grease, firmly reseat; (4) Thermopile lead wire damaged near the burner. A 2-blink code with pilot confirmed lit and thermopile below 350mV is definitively a thermopile replacement.

3. 3

   WV8840 — 3 Blinks: Gas Valve Internal Fault

   Three blinks on the Lochinvar WV8840 LED signals an internal gas valve fault — the gas control electronics have detected a condition inside the valve that prevents safe operation. First step before condemning the valve: disconnect and firmly reseat all push-on wiring connectors at the WV8840 — thermopile leads (TH/TP), pilot lead, and igniter wiring. Corroded or loose terminals can generate a false 3-blink code. If 3 blinks clear after reseating connectors and the unit fires normally, the fault was a contact issue. If 3 blinks persist after confirmed clean, tight connections, the Honeywell WV8840 must be replaced. The WV8840 is a service part available from Lochinvar dealers and third-party suppliers — match the BTU rating from the existing valve.

4. 4

   WV8840 — 4 Blinks: ECO / High-Limit Trip

   Four blinks on the Lochinvar WV8840 LED indicates the Energy Cut-Off (ECO) high-temperature safety switch has tripped, shutting down all gas flow. The ECO trips when tank water temperature exceeds approximately 190–200°F — caused by a blocked flue vent, insufficient combustion air, or a gas control valve failing to close at setpoint (overfiring). Before resetting: inspect the flue vent from draft hood to exterior termination for any obstruction (bird nests, ice, debris). Verify the unit has adequate combustion air clearances. After clearing the root cause: restore gas supply, relight the pilot (60-second hold), turn to HOT, and open a hot tap to create demand. If 4 blinks return after one complete heating cycle, the unit requires service — do not reset repeatedly.

5. 5

   WV8840 — 7 Blinks: Thermal Switch Open or Internal Gas Valve Failure

   Seven blinks from the Lochinvar WV8840 is the most serious fault code — a safety lockout. It indicates either: (a) the thermal switch in the combustion chamber has opened due to a severe overtemperature event — sometimes resetable if the root cause is corrected; or (b) an internal failure of the WV8840 gas control electronics — requires valve replacement. Perform a power cycle: turn the gas control to OFF for 5 full minutes, then back to PILOT. If 7 blinks return within 30 seconds with no pilot attempt, the WV8840 has failed internally. Gas valve replacement requires a licensed plumber in most jurisdictions. Do not attempt to bypass or force the valve to operate during a 7-blink lockout.

6. 6

   Lochinvar Knight/Crest — Ign Fail: Ignition Failure

   The Ign Fail code on Lochinvar Knight condensing water heaters and Crest commercial models indicates the control board attempted ignition but did not detect a stable flame within the trial-for-ignition period (typically 4–7 seconds). The board will attempt 3 ignition trials before entering lockout. Causes: (1) Failed spark igniter — no visible spark at the burner; test continuity of the igniter cable; (2) Oxidized or fouled flame sensor rod — the most common cause of nuisance Ign Fail lockouts; the ceramic flame sensor rod develops an oxide layer over time that prevents the board from detecting the flame; clean with very fine steel wool or emery cloth; (3) Gas supply interruption — verify gas supply is on and adequate; (4) Low gas pressure at the manifold — manifold pressure should be 3.5 inches WC (natural gas); test with a manometer. A power cycle (disconnect 120V for 30 seconds) clears the lockout and allows a fresh ignition attempt.

7. 7

   Lochinvar Knight/Crest — SHt: High-Temperature Shutdown

   The SHt (or equivalent high-temp/overheat) fault code on Lochinvar Knight and Crest control boards indicates the unit's high-limit aquastat or outlet temperature sensor has detected an overtemperature condition and shut down operation. Causes: (1) Blocked or restricted flue/venting — condensing Knight models vent through PVC; check for blockage at the exterior termination; (2) Fouled heat exchanger — heavy scale or lime buildup on the heat exchanger surfaces reduces heat transfer efficiency and causes localized overtemperature; descaling service required; (3) Low flow rate through the unit — insufficient water flow causes the heat exchanger to overheat; check for closed valves or scale in the inlet filter screen. After correcting the root cause, a power cycle will reset the SHt lockout on most Knight/Crest models.

8. 8

   Lochinvar Knight/Crest — LoN and E-Code Faults

   LoN on Lochinvar Knight control boards typically indicates a low-water, low-flow, or temperature sensor fault — the inlet or outlet thermistor is reading outside its expected range, or water flow through the unit is below the minimum required for safe operation. Check the cold water supply valve is fully open, inspect the inlet filter screen for scale, and verify water pressure at the unit. E1–E9 (or similar numeric) codes on Lochinvar Knight/Crest models are model-specific — the exact meaning varies by board generation and model series. Common E-code assignments: E1/E2 = inlet or outlet sensor fault (open or shorted thermistor); E3 = flue/exhaust sensor fault; E5 = ignition or flame sensing circuit fault; E7 = gas valve feedback fault. Always cross-reference the specific code against the Lochinvar installation and service manual for your model number for the authoritative definition and repair procedure.

Quick DIY Checks

1. 1Step 1 — Identify your Lochinvar model type and fault code system: examine the front of the Lochinvar water heater. If you see a knob with PILOT/ON/HOT/VERY HOT positions and a small circular LED window on a valve body — your unit uses the Honeywell WV8840 gas valve and displays blink codes (1–7 blinks per repeating cycle). If you see a control panel with a digital LED or LCD display showing alphanumeric codes — your unit is a Knight condensing or Crest commercial model using a proprietary control board. Record the exact displayed code or blink count before proceeding. For Knight/Crest models, also note the model number from the data plate — the Lochinvar installation manual for your specific model contains the authoritative fault code list.
2. 2Step 2 — WV8840 models: measure thermopile millivolts for 1-blink or 2-blink codes: turn the gas control knob to PILOT. Press and hold while clicking the igniter — hold for 60 full seconds after the pilot lights (the thermopile must warm to operating temperature). Slowly release the knob — the pilot should stay lit. If the pilot extinguishes on release, the thermopile output is insufficient. With the pilot lit and warmed for 2 full minutes: set a digital multimeter to DC millivolts (mVDC). Disconnect the two thermopile lead wires from the TH/TP terminals on the WV8840. Measure across the two leads. Targets: ≥350mV minimum to hold the valve open; 600mV ideal; 650–850mV = full healthy output. Below 350mV = replace the thermopile with AO Smith 9005502. On older models with a thermocouple (no separate TH/TP leads): test at the TC terminal on the valve — ≥17mV minimum. Below 17mV = replace with AO Smith 9003972.
3. 3Step 3 — WV8840 models: investigate and reset a 4-blink ECO trip: a 4-blink code indicates the ECO high-limit has tripped inside the WV8840. Turn the gas control to OFF. Inspect the flue vent pipe from the draft hood at the top of the unit all the way to the exterior vent termination — look for any obstruction (bird nests, ice, collapsed cap, debris). Confirm the unit has at least 12 inches of clearance and adequate combustion air. After clearing any blockage: restore gas supply, relight the pilot (full 60-second hold), turn the knob to HOT, and open a hot tap to create a call for heat. If 4 blinks do not return after a complete heating cycle, the obstruction was the root cause. If 4 blinks return quickly, call a licensed gas technician — the unit may be overfiring.

Repair vs Replace

Recommended Tools & Parts

• AO Smith 9005502 Thermopile (Lochinvar WV8840 Compatible)

  Replacement thermopile for Lochinvar gas tank water heaters with the Honeywell WV8840 gas valve. Resolves 1-blink and 2-blink low-voltage fault codes. Generates 650–850mV when healthy. Push-on spade terminals. Replace when output drops below 350mV.

  $25–$45

  Buy on Amazon →

• AO Smith 9003972 Thermocouple (Lochinvar Compatible)

  Replacement thermocouple for older Lochinvar gas water heaters with standing pilot thermocouple-type gas valve. 36-inch lead. Replace when thermocouple output drops below 17mV at the TH terminal.

  $12–$25

  Buy on Amazon →

• Honeywell WV8840 Gas Control Valve

  Replacement gas control valve for Lochinvar gas tank water heaters showing 3-blink or 7-blink fault codes. Match BTU rating from the existing valve or Lochinvar data label. Gas valve replacement requires a licensed plumber in most jurisdictions.

  $80–$150

  Buy on Amazon →

• Digital Multimeter (DC Millivolt Mode)

  Essential for testing thermopile millivolt output at the WV8840 gas valve (TH/TP terminals), thermocouple output, and Knight/Crest thermistor resistance. Look for a meter with a dedicated mVDC range for accurate thermopile testing.

  $15–$35

  Buy on Amazon →

• Fine Steel Wool 0000 Grade (Flame Sensor Cleaning)

  Ultra-fine steel wool (0000 grade) for cleaning the oxide layer from the Lochinvar Knight/Crest ceramic flame sensor rod. Cleans without scratching the rod surface. Resolves most recurring Ign Fail lockouts without part replacement.

  $5–$10

  Buy on Amazon →

Links are Amazon affiliate links (tag: fixitfastai-20). Prices are estimates.

Related Repair Guides

This section combines editorial `relatedSlugs` from the article source with metadata-based matches so each guide links deeper into the repair library.