Lennox SLP98UHV Error Code E 204: Gas Valve Mis-Wired
What Does Code E 204 Mean?
The E 204 fault on the Lennox SLP98UHV means the SureLight control has detected the gas valve wired in a way that does not match the expected configuration. The board watches the gas valve circuit, and when the connections are reversed, crossed, or landed on the wrong terminals, it flags a mis-wire and refuses to run gas heat. On this communicating modulating furnace the gas valve is driven precisely by the control, so correct wiring is essential for the board to command the right firing rate.
This is a safety-related fault because an incorrectly wired gas valve can behave unpredictably — it could fail to open when heat is called for, or in the worst case energize when it should be closed. To keep that from happening, the control shuts down gas heating as a protective measure. E 204 most commonly shows up right after a gas valve replacement, a control-board swap, or other service work where the valve wiring was disturbed.
E 204 is one of two gas-valve control faults on this board. It is distinct from E 205 (gas valve relay contact shorted), which is an internal board relay that has welded closed rather than a wiring error. Both prevent normal, controlled gas delivery, and neither is a homeowner repair.
What You'll Notice
- No heat — the furnace will not fire the burners even though the thermostat is calling
- The blower or inducer may run briefly, but the burners never light
- The fault appeared right after a gas valve, control board, or wiring repair
- The furnace locks out at the start of a heat call and the display shows E 204
Common Causes
| Cause | Likelihood | DIY? |
|---|---|---|
| Gas valve wiring connected incorrectly during installation or service | Most common | ✗ Call a pro → |
How This Is Diagnosed
A technician compares the actual gas valve wiring against the unit wiring diagram, verifying that each lead lands on the correct control terminal and that no connections are reversed or crossed. They confirm the valve type and connector match the SLP98UHV's requirements, since a substituted or aftermarket valve can wire differently.
Once the wiring is corrected, the technician cycles power to clear the fault and test-fires the furnace, confirming the board commands the valve open and closed cleanly across the modulating firing range.
When to Call a Professional
This code involves components that are not homeowner-serviceable, so have a licensed HVAC technician diagnose and repair it. Keep in mind:
- The furnace will not produce heat and displays E 204
- The code appeared after recent service work or a gas valve replacement
- You smell gas near the furnace at any time
- The fault returns after power is cycled
Frequently Asked Questions
Can I fix the gas valve wiring on my Lennox SLP98UHV myself?
No. Gas valve wiring controls the flow of gas to the burners, and an error here is a safety issue. On this sealed communicating furnace it should be corrected by a qualified HVAC technician who can verify the connections against the wiring diagram and test-fire the unit.
Why did E 204 appear right after a repair?
This code most often shows up when the gas valve or control board was recently replaced and a lead was landed on the wrong terminal or reversed. The board immediately detects the mismatch and blocks gas heat, which is why it can appear as soon as service work is finished.
✓ Verified against manufacturer service manual — March 2026