Code 99 = Exhaust restriction. The exhaust pressure sensor detected back-pressure suggesting the vent is restricted. The unit shuts down to prevent unsafe operation.
Diagnostic sequence
1. Visual inspection of vent termination
Walk outside and look at the vent termination. Common causes:
- Bird/insect nest — clean it out
- Ice buildup in cold-climate winter — most common Code 99 trigger
- Leaves/debris blocking the cap
- Snow accumulation burying the termination
2. Check vent length / configuration
If Code 99 throws on a new install: the vent may exceed Noritz's maximum equivalent length, or have too many elbows. Each elbow reduces effective length by 5 ft. See the model's installation manual for the maximum.
3. Concentric vent termination cap
Concentric vent caps (intake + exhaust in one cap) can frost over in cold weather. Some installs need a winter cap modification.
4. Internal exhaust restriction
Rare but possible — debris (insulation, animal nesting material) inside the vent pipe itself. Requires camera inspection.
Reset procedure
- Clear the visible restriction
- Power cycle the unit
- Monitor — if Code 99 returns, suspect internal vent restriction
Winter prevention
In cold climates, Code 99 is most common in mid-winter. Mitigations:
- Verify vent termination clearance from snow line (typically 12" above grade)
- Add a winter cap to concentric terminations
- Shovel snow drift away from terminations
- For chronic problems, relocate the termination to a less-drift-prone wall
Bottom line
Code 99 is almost always a physical blockage. Inspect the termination first — fixes 90% of cases. Persistent Code 99 after termination cleaning = internal vent inspection or installer call.