air injection faulty - bank 1

I have a '95 E320 (104 engine) and got an "air injection faulty - bank 1" error today (I read the flash codes and it blinked four times). Could this
be due to a loose gas cap (it was slightly loose) or is this the infamous air injection pump that runs for 90 seconds or so when the car is cold (I already replaced this beast about three years ago)?
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