Code 991 = Exhaust restriction. The exhaust pressure sensor detected back-pressure suggesting vent restriction.
Diagnostic sequence
1. Visual inspection
Walk outside and inspect the vent termination:
- Bird/insect nest
- Ice buildup (cold-climate winter — most common 991 trigger)
- Leaves/debris
- Snow accumulation
2. Vent length / configuration
If on a new install, vent may exceed maximum equivalent length. See install guide.
3. Concentric vent cap frost
Cold weather can frost over the cap. Add winter cap modification if chronic.
Reset
- Clear visible restriction
- Power cycle
- Monitor
Winter prevention
- Termination clearance from snow line
- Winter cap on concentric terminations
- Shovel snow drift away
Bottom line
Almost always a physical blockage. Inspect termination first.