OBD2 Mode 1 PID 12

Question

What are the meanings of the returned values for OBD2 Mode 1 PID 12?

One source shows me

A0     Upstream of catalytic converter
A1     Downstream of catalytic converter
A2     From the outside atmosphere or off
A3-A7  Always zero

Another source (wiki's PID) shows me

1   Upstream
2   Downstream of catalytic converter
4   From the outside atmosphere or off
8   Pump commanded on for diagnostics

I would like to know which one is correct and what it means when it returns a value corresponding to:

1. Upstream of catalytic converter
2. Downstream of catalytic converter
3. From the outside atmosphere or off
4. Zero?
5. Pump commanded on for diagnostics

Answer 1

This information is from SAE J1979DA, June 2014, edition, hence should be considered authoritative:

PID 12 is Commanded Secondary Air Status. Expected is one return byte with the following semantics:

• bit 0: 1 = upstream of first catalytic converter
• bit 1: 1 = downstream of first catalytic converter inlet
• bit 2: 1 = atmosphere / off
• bit 3: 1 = pump commanded on for diagnostics
• bit 4 -7: SAE-reserved, should be 0