Weil-McLain Boiler E02 & E04 Error Codes — Ignition Failure & High Limit

Weil-McLain E02 and E04 Error Codes — Complete Guide

Weil-McLain gas boilers use the WM-CV (Ultra series) or WM-EC (Evergreen series) control boards to manage operation and display error codes. Two of the most common codes are E02 (ignition failure lockout) and E04 (high limit exceeded). These two codes often appear connected. A persistent E02 condition can lead to repeated firing attempts that eventually push the boiler past its high limit, causing E04.

This guide covers both codes, their diagnostic trees, and the control board differences between Ultra and Evergreen series boilers.

Weil-McLain Complete Code Reference

E02 — Ignition Failure Lockout

What it means: The boiler attempted to light the burner, but the control board did not detect a stable flame within the trial-for-ignition period. On Weil-McLain Ultra and Evergreen boilers, the trial period is approximately 7-15 seconds depending on the control configuration. The board commands the igniter to heat, opens the gas valve, and monitors the flame sensor for a microamp signal. If the signal does not register, the board closes the gas valve and locks out with E02.

How to reset: Press and hold the RESET button on the WM-CV or WM-EC control board for 1 second. On older models, the reset button is a red button on the Honeywell or Weil-McLain control module. If E02 returns immediately after reset, one of the ignition components has failed.

E02 Diagnostic Tree

Step 1: Check gas supply. Confirm the manual shutoff valve on the gas line to the boiler is fully open (handle parallel to the pipe). Test other gas appliances in the building to rule out a broader gas supply issue. Low gas pressure during high-demand periods (cold mornings) is common in under-sized supply lines.

Step 2: Inspect the spark igniter. On Ultra series boilers with spark ignition, the igniter electrode must have a 3 mm air gap to ground. Close the gap increases current draw. Wide the gap produces a weak spark. Remove the electrode assembly, clean carbon deposits with a wire brush, and verify the gap with feeler gauges. On Evergreen models with hot surface ignition, check the HSI for cracks and measure resistance. A cold HSI reads 40-80 ohms. Open circuit means the igniter has failed.

Step 3: Clean the flame sensor. The flame sensor rod sits in the burner flame path. Carbon and oxide buildup insulates it and blocks the microamp signal. Remove the sensor. Clean with fine steel wool or 400-grit emery cloth until the rod shines. Reinstall and reset the boiler. A clean flame sensor reads 2-6 microamps DC during burner operation. A reading below 1.5 microamps will not sustain the flame signal and triggers E02.

Step 4: Test the gas valve. With the boiler in a call for heat, measure 24 VAC at the gas valve solenoid terminals. If 24 VAC is present and the valve does not open, the valve solenoid has failed. If 24 VAC is absent, the control board is not commanding the valve to open. Check the board output circuit.

Step 5: Check the vent system. A blocked or restricted vent prevents proper combustion air flow and causes flame failure. Inspect the intake and exhaust PVC pipes for ice, debris, or animal nests. On direct-vent Weil-McLain boilers, both pipes must be clear. A blocked vent can also cause flame rollout, which downstream carbon monoxide detectors may catch.

E04 — High Limit Exceeded

What it means: The boiler water temperature exceeded the high limit setpoint (default 200°F on most WM-CV boards, adjustable). The limit control opened the burner circuit and the boiler stopped firing. E04 is a safety lockout that protects the heat exchanger from thermal stress and prevents boiling.

How it relates to E02: A boiler that cycles through repeated ignition failures (E02) dumps raw gas into the combustion chamber. When the gas finally ignites, the burner fires hotter than normal. This temperature spike can drive the supply water past the high limit. A technician encountering E04 should always check the boiler’s fault history for previous E02 events.

E04 Diagnostic Tree

Step 1: Confirm the circulator pump is running. The most common cause of E04 is no water circulation. Place a hand on the supply and return pipes. The supply should be noticeably warmer than the return. If the pipes feel equally hot or the return is cold, the pump is not moving water. Check for power at the pump. A stuck pump can often be freed by tapping the pump housing with a wrench handle.

Step 2: Check system pressure. A Weil-McLain boiler needs 12-15 PSI cold fill pressure. Below 10 PSI, the system may not push water through all zones, creating localized boiling at the heat exchanger. Add water through the boiler’s fill valve until pressure reaches 12 PSI. Bleed air from radiators or baseboard units to purge trapped air pockets.

Step 3: Verify zone valves. On zoned systems, check each zone valve for proper operation. A stuck-closed zone valve locks circulation to that loop. The boiler heat exchanger heats water that has nowhere to go, and the temperature climbs rapidly. Manually open each zone valve. If one does not open when the zone calls for heat, the zone valve motor or end switch has failed.

Step 4: Check the high limit setting. The WM-CV board’s high limit defaults to 200°F. Operating a boiler at a 180°F supply temperature with the high limit at 190°F leaves only 10°F of headroom. Normal overshoot during a call for heat can trigger E04. Raise the high limit to 200°F or increase the differential setting on the control board.

Step 5: Test the high limit sensor. The high limit thermistor reads supply water temperature. If the thermistor drifts (reads lower than actual temperature), the boiler overfires before the board sees the actual temperature. Measure the thermistor’s resistance. A Weil-McLain supply thermistor reads approximately 10 kΩ at 77°F (25°C). Compare resistance to temperature using the boiler manual’s resistance table.

Ultra vs Evergreen Control Board Differences

Weil-McLain’s two main condensing boiler lines use different control boards that display codes slightly differently.

The WM-EC board on Evergreen models is more helpful for live diagnosis because it displays flame current in microamps without requiring a separate meter. If you service both board types, familiarize yourself with the entry sequence. They use different button combinations.

When to Call a Pro

E02: If the flame sensor is clean, the igniter gap is correct, gas supply is confirmed, and E02 still returns, the issue may be in the burner orifices, blocked heat exchanger flue passages, or a failing control board. These diagnostics require combustion analysis and gas pressure measurement with a manometer.

E04: Repeated E04 events after checking the circulator, system pressure, and zone valves suggest a heat exchanger blockage or collapsed internal baffles restricting flow. Both require boiler disassembly. Additionally, an overfire condition (gas valve passing too much gas) requires a combustion analyzer to measure CO2 and O2 in the flue.

Both codes point to safety-critical systems. Gas pressure adjustment, combustion analysis, and heat exchanger service should only be done by a licensed HVAC technician.

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