Just found out tonight that the OBD-0 PR4 has 2 versions. The 90 PR4 has an external PA sensor and the 91 has an internal PA sensor.

I found out because I have a 91 Integra harness in a 89 Civic with a PR4 and was getting a code 13. Went to rewire my friends 90 integra to OBD-1 for his 1.7 GSR and he had a PA sensor. checked my harness and no connector for it. checked my ECU internals and no PA sensor. checked his ECU and no PA sensor. checked another 91 ECU that is waiting for a chip and it had a PA sensor.

Now my car is ready for smog ref.
Just had to rant and provide a little info