Lennox furnace E204: Gas valve mis-wired

E204 means the control has detected the gas valve wired incorrectly. Lennox's action is to check gas valve operation and wiring; the code clears when the wiring is repaired.

Applies to: Lennox gas furnaces with a seven-segment diagnostic display on the control board, such as the ML193UHEK, per installation instructions 508565-01.

What it means

This is an installation and service fault rather than a wear fault. The control energises the gas valve and monitors the circuit, and E204 says what it sees does not match a correctly wired valve. That points at wiring rather than at the valve itself — a lead on the wrong terminal, a stage reversed on a two-stage valve, a connector on the wrong plug, or a valve substituted for one that is not the correct part. The practical significance is that this fault usually appears immediately after installation or after a repair, rather than developing over time. A furnace that has run correctly for years and then shows E204 is more likely to have had something disturbed than to have rewired itself.

Most common causes

Gas valve leads landed on the wrong terminals during installation or a repair.
The two stages of a two-stage valve reversed.
A replacement gas valve that is not the correct part for the furnace.
A damaged or partially seated connector at the valve.
Wiring disturbed during unrelated service work in the burner compartment.

How to diagnose it

Note whether the furnace has been installed, serviced or had parts replaced recently. That is nearly always the answer.
Do not open the burner compartment to look. The gas valve is not a homeowner-serviceable component.
If the furnace was recently installed and has never run correctly, mention that — it changes where a technician looks first.

How to fix it

Licensed technician work. It concerns the wiring of the component that admits gas to the burners.
Lennox's instruction is to check gas valve operation and wiring; the code clears when the fault is repaired.
If a valve was recently replaced, the part number should be checked against the furnace as well as the wiring.

Notes

Because this fault normally arrives with a person rather than with age, it is worth asking who last worked on the furnace before anything is replaced.

All codes for this manufacturer

Lennox furnace alert codes — every code, with what differs between models.

This code is not a homeowner repair. Faults in this group involve the gas valve, the burner compartment, or the heat exchanger — the parts whose failure can put combustion products, including carbon monoxide, into the air the house breathes. Do not reset it repeatedly to keep the heat on. Have it looked at by a licensed technician, and make sure there is a working carbon monoxide alarm on every level of the home, including the level the furnace is on.

Sources

Lennox ML193UHEK Installation Instructions 508565-01, May 2026 — Integrated Diagnostic Codes / Status of Equipment

This site is a reference, not a substitute for a licensed HVAC technician. Gas appliances can leak carbon monoxide and cause fire. Anything involving gas lines, the heat exchanger, or the flame sensing circuit should be handled by a licensed professional. Pages are written with AI assistance from the sources listed on each page and checked against them before publishing.