"All sensor ground returns are isolated from chassis ground. Do not continuity test between T121/13 and chassis ground – use ECU ground T60/4 only."
Most Bosch ECUs have one to three rectangular or trapezoidal connectors (often labeled A, B, C). The pins are numbered. Manufacturers number pins differently. Some start top-left to bottom-right; others follow ISO standards. Always check the “connector view” legend. Bosch Ecu Pinout Datasheet
Using the wrong pinout datasheet can destroy your ECU or harness. Avoid these errors: "All sensor ground returns are isolated from chassis ground