Bosch SSB255 - Errors Not Shown on Display; Sensor Resistance Table

6720818454 (2016/02) US SSB
44 | Troubleshooting
6.2 Errors not shown on display
Symptoms Possible solutions
Combustion noise too loud; rumbling noises
Check gas type.
Check inlet gas pressure; adjust as needed. Check ue gas system; clean or repair as needed.
Check gas/air ratio in the combustion air and ue gas; replace gas valve as needed.
Flow noises
Set pump speed correctly to match maximum output.
Set pump mode.
Purge system.
Heating-up takes too long
Set pump speed correctly to match maximum output.
Set pump mode.
Flue gas readings incorrect; CO levels too high
Check gas type.
Check inlet gas pressure; adjust as needed.
Check ue gas system; clean or repair as needed.
Check gas/air ratio in ue gas; replace gas valve as needed.
Hard ignition, poor ignition
Check gas type.
Check inlet gas pressure; adjust as needed.
Check power supply.
Check electrodes for visual damage; replace as needed.
Check ue gas system; clean or repair as needed.
Check gas/air ratio; replace gas valve as needed.
Check gas valve; replace as needed.
Check burner; replace if required.
Tab. 20
6.3 Sensor Resistance table
Temperature °F (°C)
Testing tolerance ±10%
Resistance [Ω]
32 (0) 27396
41 (5) 22140
50 (10) 17999
59 (15) 14716
68 (20) 12099
77 (25) 10000
86 (30) 8308
95 (35) 6936
104 (40) 5819
113 (45) 4904
122 (50) 4151
131 (55) 3529
140 (60) 3012
149 (65) 2582
158 (70) 2221
167 (75) 1918
176 (80) 1663
185 (85) 1446
194 (90) 1262
203 (95) 1105
212 (100) 970
221 (105) 855
230 (110) 755
239 (115) 669
248 (120) 594
257 (125) 529
Tab. 21 Resistance table for: Supply temperature sensor, Return
sensor, Flue sensor, DHW tank temperature sensor, LLH
sensor, Outdoor temperature sensor