What is a PID?

A Parameter ID (PID) is a request you send to a vehicle control unit. The unit replies with bytes. A formula turns those bytes into a physical value — engine speed, coolant temperature, or, on a hybrid, something the standard never defined.

Standard versus manufacturer

SAE J1979 (and ISO 15031-5) define a common set of emissions-related PIDs. The standard PID catalog lists every published Mode 01 / 09 PID with number, name, unit, and explanation. Engine RPM is the same request on almost every car. Hybrid battery SOC often is not. Manufacturers add their own requests (and sometimes UDS data identifiers). Those must not be merged with a standard PID just because the hex number looks similar.

The key this catalog uses

A protocol request is unique as service mode + PID bytes + ECU header (plus protocol family). Two platforms can expose “battery SOC” through different requests. The semantic signal stays the same; the protocol mapping does not.