All Boiler Fault Codes

Every boiler fault code in our database — 1,408 codes across all UK manufacturers. Click any code to see which brands and models display it, with plain English explanations and fixes.

Most common fault codes

These appear on the largest number of boiler models across the UK market.

10 151 models 20 149 models 12 129 models F1 111 models 15 108 models 28 102 models 13 93 models 24 93 models 06 93 models 05 86 models F7 85 models 02 83 models F.10 82 models FA 80 models 40 80 models 30 79 models 04 78 models 14 76 models F.13 75 models 01 75 models F.22 74 models F2 73 models 29 73 models F.11 73 models 25 72 models F.28 72 models F.29 72 models F.62 72 models F.63 72 models F.67 72 models F.24 70 models F.64 70 models F.65 70 models 16 69 models F.61 69 models F.23 69 models F.27 69 models F.32 69 models S.53 67 models S.54 67 models F4 67 models 110 67 models 09 67 models 31 67 models L1 67 models F0 66 models 11 66 models F.01 66 models 03 66 models 18 65 models

0-9 (453 codes)

.0 <0,6 bar 00 0 / A 0 / H 0 bar 0.0 0.5bar 0A 0E2 0E8 0E37 01 001 0001 1 01 1 02 1 03 1 04 1 05 1 06 1 07 1 08 1 09 1 10 1 12 1 14 1 16 1 18 1 green flash 1 P1 1 P2 1 second ON 1 second OFF 1. 1.0 1.1 1.2 1.C 1A 1B 1C 1F 1P4 02 2 01 2 02 2 03 2 04 2 05 2 07 2 08 2 flashes 2. 2.0 2.2 2.3 2.4 2.5 2.7bar 2.9 2.31 2.C 2.F 2A 2H 03 3 3 0 3 3 01 3 02 3 03 3 04 3 05 3 06 3 07 3 bar 3 flashes 3 flashes red 3 green flashes 3 red flashes 3.1 3/0/6 3A 04 004 4 / A 4 07 4 17 4 flashes 4 red flashes 4.5 4.7 4.8 4.9 4.10 4.11 4.12 4.13 4.14 4.15 4.16 4.F 4# 4A 4C 4P 4Y 05 5 5 01 5 02 5 28 5 flashes 5 flashes red 5 P 2 5.0 5.A 5A 06 6 6 10 6 12 6 flashes 6.5 6.A 6a 6C 07 7 7 01 7 02 7 03 7 04 7 05 7 06 7 07 7 22 7-1 7A 7F 7L 08 8 8-9 8.1 8.3 8.4 8.6 8.79 08E 8E 09 9 9 flashes 9.1.37 9.b 9.F 9A 9C 9F 9L 9U 10 10, 11, 12, 13, 14 10.2 10E 11 12 13 14 15 16 016 17 18 19 20 20, 21, 22, 23, 24 21 21:0 22 23 23:0 24 25 26 27 28 29 29.30 30 30 40 50 60 70 80 31 32 33 34 35 36 36.0 37 38 39 40 41 42 43 44 45 46 47 48 49 50 50/70 51 52 53 54 55 56 57 58 58:0 59 60 61 62 63 65 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 114 115 116 117 118 119 121 122 125 126 127 128 129 130 132 133 134 135 140 146 151 154 158 160 162 164 164-026 169-083 170 171 180 183 200 201 202 203 204 205 210 214 215 216 217 218 220 226 229 / EA 231 232 234 / EA 235 241 241 / D1 242 249 250 253 254 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 280 281 282 283 284 285 286 287 288 289 290 290 / F0 300 301 302 303 304 305 306 307 308 309 310 311 312 315 317 321 322 323 324 325 326 327 328 329 332 332 E5 333 341 342 343 344 345 346 347 348 349 350 351 352 353 354 357 359 365 366 369 370 372 373 374 375 375 699 376 376 E4 377 378 380 382 384 385 386 387 388 395 396 400 402 403 410 424 425 426 429 432 471 501 506 517 522 594 612 809 810 2917 布 & 皿 neons flashing 閃 neon illuminated

A (85 codes)

A A 01 A 02 A 03 A 10 A 36 A 37 A 39 A 40 A 41 A 77 A short pulse every 4 seconds A-4 A.12 A.18 A.19 A.20 A0 A1 A01 A02 A2 A03 A3 A04 A4 A5 A05 A06 A6 A7 A07 A8 A08 A9 A09 A10 A11 A12 A15 A16 A17 A18 A19 A20 A24 A30 A33 A40 A41 A42 A44 A61 A62 A63 A64 A70 A80 A97 A98 A99 AF Air Flow Monitor AL 56 AL 82 AL 99 AL10 AL20 AL21 AL26 AL28 AL34 AL40 AL41 AL52 AL55 AL60 AL71 AL73 AL74 AL79 Alert AM001 Amber LED illuminated Anti-cycle

B (37 codes)

B b 18 b 19 b 24 b 25 b 26 b 28 b 29 b 30 b 38 b 65 B1 B02 b3 b4 b5 B05 b7 B07 b7 257 B08 b18 Bat bb BC BCC Fault BE BLANK DISPLAY BLANK DISPLAY SCREEN Blue indicator light off Boiler On Boiler Overheat Thermostat Boiler Type Card Fault Burner fails to ignite Burner lockout Burner On Burner On Neon

C (30 codes)

C c / 2 C0 C1 C1 05 C1 10 c1 17 c1 18 c1 54 C2 C3 C4 C5 C6 C7 C12 C13 C32 C123 CA Casting Thermistor Fault CC CE CF CO COdE Communication fault Condensate Drain Blockage Connection Cylinder Thermistor Fault

D (88 codes)

D d.00 D.0 d.01 D.1 d.02 d.04 d.05 d.06 d.07 d.08 d.09 d.10 d.11 D.13 D.14 D.15 d.16 D.17 D.18 d.19 D.20 d.21 D.22 d.23 D.24 d.25 D.27 D.28 d.31 d.33 d.34 d.35 d.36 d.39 d.40 d.41 d.43 D.44 d.45 D.47 d.50 D.51 d.58 d.60 d.61 d.62 d.64 d.65 d.66 d.67 d.68 d.69 d.70 d.71 d.73 d.75 D.77 d.80 d.81 d.82 d.83 D.84 d.85 d.88 D.90 D.91 D.93 d.94 d.95 D.96 D1 d2 D3 D4 D5 D6 d7 d8 d9 dd DHW DHW PUMP indicator FLASHING DHW thermistor fault DHWsensor dP Dry-Fire dU

E (227 codes)

E 00 E 01 E 02 E 03 E 04 E 05 E 06 E 07 E 08 E 09 E 10 E 11 E 12 E 13 E 14 E 15 E 16 E 17 E 18 E 19 E 20 E 21 E 22 E 23 E 25 E 26 E 27 E 28 E 29 E 30 E 31 E 32 E 33 E 34 E 35 E 36 E 37 E 38 E 44 E 60 E 61 E 65 E 99 E.04.03 E.04.23 e/\/ E00 E0 E00-684 E00-688 E00.04 E01 E1 E01.12 E2 E02 E02-313 E02.15 E03 E3 E04 E4 E04.07 E04.23 E5 E05 E06 E6 E07 E7 E08 E8 E9 E09 E10 E11 E12 E012 E13 E14 E15 E16 E17 E18 E19 E20 E21 E22 E23 E24 E25 E26 E28 E29 E030 E30 E31 E031 E32 E33 E34 E35 E36 E37 E38 E39 E40 E41 E42 E43 E44 E45 E46 E48 E50 E52 E54 E060 E62 E64 E65 E66 E74 E76 E77 E78 E79 E80 E83 E83-013 E83-082 E83-086 E83-101 E83-122 E83-126 E83-127 E83-142 E83-145 E085 E89 E90 E92 E94 E95 E96 E97 E98 E99 E110 E117 E118 E119 E125 E127 E128 E130 E131 E133 E160 E193 EA EC ED EE EEPROM LOCKOUT EF EF 349 EL Er 01 Er 02 Er 03 Er 04 Er 05 Er 06 Er 07 Er 08 Er 09 Er 18 Er 19 Er 25 Er 25 + RESET Er 69 Er 99 Er.1 Er.2 Er.3 Er.4 Er.5 Er.6 Er.7 Er.8 Er.9 Er.10 Er.11 Er.12 Er.13 Er.14 Er.15 Er.16 Er.17 Er02 Er09 Er14 Err Err / 110 Err.5 ERROR Error 01 Error 02 Error 03 Error 04 Error 05 Error 12 Error 19 Error 20 Error 28 Error 125 Error 130 ERROR CODE 1 ERROR CODE 2 ERROR CODE 3 ERROR CODE 4 ERROR CODE 5 ERROR CODE 6 Excessive temperature on primary circuit Explosive ignition Ext’ Interlock Off

F (204 codes)

F F 0U F 04 F 14 F 15 F 16 F.00 F.01 F.02 F.03 F.5 F.10 F.11 F.12 F.13 F.17 F.20 F.22 F.23 F.24 F.25 F.26 F.27 F.28 F.29 F.32 F.33 F.42 F.46 F.49 F.52 F.53 F.54 F.56 F.57 F.61 F.62 F.63 F.64 F.65 F.67 F.68 F.70 F.71 F.72 F.73 F.74 F.75 F.76 F.77 F.78 F.83 F.84 F.85 F.86 F.87 F.88 F.89 F.91 F.92 F.93 F.97 F.359 F.381 F.401 F.456 F0 F00 F000 F0-237 F0C F1 F01 F1A F2 F02 F2 or Fn or Ln F3 F03 F4 F04 F4 OR L4 F5 F05 F5 OR L5 F6 F006 F7 F07 F8 F08 F9 F09 F9 or L9 or F8 or L8 F10 F11 F12 F13 f014 F14 F15 F16 F17 F18 F19 F20 F21 F22 F23 F24 F25 F26 F27 F027 F28 F29 F30 F31 F32 F34 F35 F36 F37 F39 F40 F41 F42 F43 F47 F49 F50 F51 F52 F53 F61 F62 F63 F64 F65 F66 F67 F68 F70 F71 F72 F73 F74 F76 F77 F81 F83 F84 F85 F86 FA False Flame Fault False Flame Lockout FAN Fan does not run Fan Fault Fan Hunt FAN LOCKOUT Fan Purge Fan runs Fast flash Fast flash, Lockout reset: Off Fault Fault on DHW sensor Faulty air pressure switch Faulty flame detection Faulty gas valve Faulty primary circuit FC FD FE FH FL Flame Flame detection error Flame Failure Flame Failure Neon flame lock-out Flame Loss Flame On Before Gas On Flame on before gas valve on FLASH FLASH ON Flash Off Flash FLASH OFF ON FLASH ON FLASH FLASH ON ON Flashes red Flashing light Flashing pressure flashing red LED Flow return reversed Flow Switch Flow Thermistor Flow Thermistor Fault Flue overheat FLUE SENSOR Fn FU FUSE fuse blown

G (11 codes)

G G20 Gas valve(s) GREEN GREEN blinking Green led flashing GREEN LED Flashing 8-times a second GREEN LED ON every 0.5-secs GREEN LED ON every 6-secs GREEN LED Solid Green Light

H (9 codes)

H E H n H-- H03.05 H5 H9 H9/L9 H21-052 HE

I (17 codes)

I i1 i2 i3 i4 i6 i7 i14 i22 Ignition failure Ignition fault Ignition indicator IGNITION LOCKOUT II - INDICATOR LIGHT FLASHING Internal fault IPX5D

J (3 codes)

J J46 JP6

K (2 codes)

K Keys fault

L (46 codes)

L 01 L A L E L- L1 L01 L2 L4 L4 or F4 L5 L5 or F5 L6 L7 L8 L9 L47 LA/HA Lack of burner ignition LC LC5 LCE LD2 LD3 LE LED 1 LED 2 LED 3 LED 4 LED 5 Lh Lights: Flash, On, Flash Lights: Flash, On, On Lights: Off, Off, Off Lights: On, Flash, Flash Lights: On, On, Flash Ln lock-out lock-out light LOCKOUT lockout light Low mains voltage Low system pressure low water pressure low-pressure LP Lt

M (3 codes)

m MAIN BURNER DOES NOT LIGHT main burner operating

N (19 codes)

N/A neon (flashing) Neon 1 Neon 1 lit Neon 1 not lit Neon 2 Neon 2 lit Neon 2 not lit Neon 3 Neon 3 lit Neon 3 not lit Neon 4 Neon 4 lit Neon 4 not lit Neon I3 Neon3 No DHW temperature control No display No Water Flow

O (17 codes)

OF OFF On / Off ON FLASH FLASH ON FLASH ON On Off Flash On Off On ON ON FLASH On On On Orange LED flashing Orange Light Outside Sensor Fault Overheat Overheat condition OVERHEAT LOCKOUT Overheat Neon Overheat thermostat

P (44 codes)

P 0.9 P.00 P.01 P.1 P.02 P.03 P.04 P.05 P.06 P.07 p/-.- P00 P1 P01 P2 P02 P3 P03 P4 P04 P05 P06 P7 P07 P08 P09 P10 P11 P12 P13 P19 P20 P21 P27 PCB Fault Pictogram of a spark Pilot will not light Power On Power On Neon Pr00 Pressure of 3 or greater Pump Fault or Low Pressure Pump Overrun Pump runs

R (24 codes)

r2 RED Red - blinking Red flashing light Red LED RED LED BLINKS Red LED Flashing Red LED illuminated RED LED LIGHT Red LED On RED LED ON every 0.5-secs RED LED Solid Red Light Red light (Egg timer symbol) RED NEON Red reset button illuminated Reset LED is lit Reset Light / Overheat Reset neon Return Thermistor Return Thermistor Fault Return Thmr Fault Reverse polarity Running lamp flashing

S (67 codes)

S.0 S.00 S.01 S.02 S.03 S.04 S.05 S.06 S.07 S.08 S.10 S.11 S.13 S.14 S.15 S.16 S.17 S.20 S.21 S.22 S.23 S.24 S.25 S.26 S.27 S.28 S.30 S.31 S.32 S.33 S.34 S.36 S.37 S.39 S.40 S.41 S.42 S.46 S.53 S.54 S.58 S.59 S.76 S.88 S.91 S.92 S.95 S.96 S.97 S.98 S.99 S.108 S.109 S.O Safety LOCK-OUT Safety Thermostat Safety thermostat activated SE Sensor Fault SENSOR STUCK LOCKOUT SF SH Short Cycling SL Slow flash Spare PCB Not Set STOP

T (11 codes)

t - T1/T2 t2 t6 T8 t80 Tap symbol light ON TEC THERMISTOR Too many resets Too Many Restarts

U (4 codes)

UF uP UU/FF UU/tt

V (1 codes)

VF

W (4 codes)

Water pressure water pressure switch Water temperature low Water temperature too hot

Y (2 codes)

YELLOW LED ON every 0.5-secs YELLOW LED Solid

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 31 August 2026