Check the time window first
The Congestion Charge operates Monday-Friday 07:00-18:00 and Saturday-Sunday 12:00-18:00 (with bank holiday exemptions). If your PCN was issued outside the charging hours — or on Christmas Day / between Christmas and New Year — it is void.
Pre-registered exemptions
Blue Badge holders, NHS staff on eligible duties, some emergency vehicles, and certain residents in eligible postcodes are exempt or get a discount — but the vehicle must be pre-registered with TfL. If you are exempt but your vehicle was not on TfL's system at the time of the contravention, register first, then appeal.
Payment race conditions
You can pay in advance, on the day, or up to midnight on the third charging day after you travelled — the later payment costs more, but it is still on time. If you paid within that window and TfL still issued a PCN, that is a system-error defence. Attach your payment confirmation and the PCN should be cancelled.