Code 25 = Condensate drain blocked. Only appears on condensing models (Sensei, Sensei RX, RUC, RSC). The unit detected condensate level rising in the trap and shut down to prevent overflow into the burner area.
Where condensate goes (normal)
- Heat exchanger creates acidic condensate as a byproduct of combustion (1-2 gallons/day in normal use)
- Condensate drips into the trap at the bottom of the cabinet
- Trap routes condensate out through a ½" PVC drain line
- Drain line discharges to floor drain, sink trap, or condensate pump
- If running to iron drain pipe or septic, a neutralizer is required (raises pH 3-5 condensate to 7+)
Diagnostic sequence
1. Condensate pump (if installed)
If your install uses a condensate pump (no gravity drain available), the pump may have failed. Listen for it cycling. If silent during a recent draw, pump is dead or unplugged. Replace ($50-100).
2. Drain line freeze
If the drain line runs through an unconditioned space (crawlspace, exterior wall), winter can freeze it. Visible ice or frost on the line confirms. Insulate and run heat trace cable.
3. Drain line sediment
Condensate carries minerals; over years it deposits scale inside the drain line. Disconnect, inspect, and clear. Replace the line if heavily occluded.
4. Condensate trap blocked
The trap itself can accumulate sediment. Remove, inspect, clear. Document with a photo before disassembly.
5. Neutralizer media exhausted
If you have a neutralizer cartridge, the calcium media is consumed over 1-2 years. When fully consumed, the cartridge restricts flow. Replace media.
Reset procedure
- Identify and clear the blockage
- Verify free flow by pouring water into the trap inlet
- Power cycle the unit
- Monitor for normal operation
If you have Code 12 too
Code 25 + Code 12 together = the condensate blockage caused enough backup to disrupt flame detection. Same underlying fix.
Prevention
- Annual trap inspection
- Insulate drain lines through unconditioned spaces
- Replace neutralizer media every 12-18 months
- Verify drain line slope (¼" per foot minimum)
Bottom line
Code 25 is purely mechanical — find the blockage and clear it. Almost never a control-board or sensor issue. See our maintenance hub for the annual checklist.