Most Bosch problems display an alphanumeric error code on the controller — start there. This page covers symptom-based diagnostics for cases without a code or where the code alone doesn't explain the user-visible problem.
No hot water at all
Sequence: (1) confirm power (display lit); (2) verify gas supply (other gas appliances working); (3) check controller for error code — most likely Code A1 (ignition failure); (4) confirm water flow exceeds minimum activation (0.4-0.5 GPM on most Greentherm models). For Tronic Heat electric: check breaker first — undersized service trips repeatedly.
Lukewarm water
Three causes: (1) temperature setpoint too low; (2) cold-sandwich effect on Greentherm without recirculation; (3) scaled heat exchanger reducing efficiency — see flushing guide.
Cold-sandwich on Greentherm
Greentherm units don't have a buffer tank (unlike Navien NPE-A2). Cold-sandwich on brief inter-draw pauses is real. Fixes: (1) install external recirculation pump; (2) accept the brief cold-sandwich (most homeowners don't notice after a few weeks); (3) at next replacement consider Navien NPE-A2 if cold-sandwich is the specific complaint.
Greenstar combi — heat or DHW not both
If heat works but DHW doesn't (or vice versa) on a Greenstar combi:
- 3-way diverter valve failed — internal valve switches between heating loop and DHW. Dealer replacement
- DHW flow sensor failed — burner not firing on hot-tap demand. See flow sensor
- Heating-loop circulator pump failed — heat-only symptom; pump replacement
Greenstar combi — short cycling
Burner firing repeatedly during a heating call usually means hydronic system design issues:
- Heating system oversized for unit's modulation range — needs primary/secondary loop or buffer tank
- Insufficient hydronic loop volume — add buffer
- Outdoor reset not configured or sensor failed
- Thermostat anticipation incorrect
Tronic POU mini-tank issues
- Tank leaking from base: tank shell failure — replace the unit (tanks aren't repairable)
- T&P valve drips: normal during heat-up; if persistent, check thermal expansion (need expansion tank in closed plumbing systems)
- Water not hot: check 120V outlet, breaker, then thermostat. Reset button on most models
- Rotten egg smell: anode rod reacting with sulfate bacteria — switch to aluminum-zinc anode (see sulfur smell)
Tronic Heat 3000 electric tankless issues
- Breaker trips repeatedly: undersized electrical service. US9 needs 150A; US12 needs 200A. Common install failure mode
- Water lukewarm during simultaneous fixture use: demand exceeds unit's GPM capacity. Stagger fixture usage or upgrade unit size
- Unit cycling on/off: flow below activation threshold — check inlet filter and aerator restrictions
Water heater leaking
Common Bosch leak points: pressure relief valve (thermal expansion — see expansion tank); condensate trap on condensing models; inlet/outlet isolation valves; heat exchanger (rare, terminal).
Bosch Home Comfort app connection
If Home Comfort app won't connect: 2.4 GHz Wi-Fi requirement (not 5 GHz), router DHCP issues, account verification. See dedicated app hub.
Bottom line
Error codes solve 80% of cases. For Greenstar combi, hydronic-side issues (short cycling, heating/DHW asymmetry) are install-design problems more often than unit faults — review the install with a hydronics-experienced installer.