BoilerFaultCodesJohnson Starley 21 Fault Code
The Johnson Starley 21 fault code means: your boiler’s internal computer is having trouble processing signals from its electronic sensors, which has caused the system to stop working for safety.. It appears on 5 Johnson Starley models in our database.
Technical description: ADC error.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
Find a Gas Safe engineer
Search the official register for a qualified engineer in your area.
Gas Safe Register →Avoid the next call-out bill
A cover plan would handle faults like 21: no call-out fees, no surprise bills.
Affiliate links. We may earn a commission at no extra cost to you.
Johnson Starley models that show the 21 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Johnson Starley 21 fault code information on this page is sourced and cross-checked against the official Johnson Starley service / installation manuals for each affected model.
- Johnson Starley C10d manual (PDF)
- Johnson Starley C16d manual (PDF)
- Johnson Starley C16u manual (PDF)
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 7 July 2026