Vehicle compliance — check first, then dispute
Before disputing the PCN, run your registration through the TfL vehicle checker to confirm whether your vehicle is compliant. If TfL's own checker says you ARE compliant but the PCN says otherwise, that is a category-1 appeal ground: TfL's database is wrong.
ANPR mismatch — a misread plate
ULEZ enforcement is ANPR (camera) only. Cameras misread plates regularly — "0" as "O", "I" as "1", a clean plate misread as a dirty one. If your V5C registration document shows the correct plate and TfL's photo shows a misread, the PCN is defective on the evidence.
Signage defects at boundary entries
TfL must sign the ULEZ boundary clearly at every entrance. If you entered at a poorly-signed point — temporary works obscuring the sign, a junction without warning — that is a statutory ground under the Road User Charging (London) Regulations 2001.