HHHH or LLLL After Replacing a Furnace Probe: What to Check
Quick answer
If HHHH or LLLL remains after a probe replacement, the new probe may not be the whole problem. Check that the probe type, polarity, connector, lead routing, controller input, and installation match the exact furnace. Then document the result for ToAuto support.
Post-replacement checks
| Check | Why it matters |
|---|---|
| Correct part number | Similar probes may not match |
| Polarity | Reversed thermocouple leads can misread |
| Secure connection | Loose terminals can mimic sensor failure |
| Controller input type | Wrong configuration can create errors |
| Damage elsewhere | Heat-damaged wiring or controller issues remain possible |
What real users ask
Support questions often jump from “new probe” to “bad furnace.” A better article helps owners gather photos, part labels, error state, and timing before escalation.
Safety and setup
Only inspect after the furnace is cold and isolated. Do not bypass the sensor or operate with unresolved controller faults.
Frequently asked questions
Does HHHH always mean a broken probe?
No. It can point to the sensor circuit, connection, or controller interpretation.
Can I swap the two wires to test polarity?
Do not improvise unless the manual or support procedure instructs it.
What should I photograph?
Model label, controller display, probe label, connector, wiring path, and terminal area after safe isolation.
Bottom line
An error after probe replacement is a system diagnostic, not a reason to keep swapping parts. Verify match, installation, and evidence before operating again.
