how does one "service air bag"? pops up on display every now and then.

 

That's something for the dealer to look at..... "service air bag" means there's a fault in the system.

 

X 2 Have a dealer check it out A.S.A.P

 

air bag defect - is unlikely.
Most of all - a temporary loss of contact within the electrical network.
This may be connectors of seat belts. They suffer most because are "underfoot", in addition - the danger of getting wet. Another - connector control module air bag may pose a problem.
The control module airbag is located between the front seats. Under a plastic panel (with cupholders). This is the only connector that can be easily tested. Just dismantle the plastic cover, a visual inspection (dust, moisture), disconnect the connector and plug the connector again.
If the problem persists - a visit to a professional car electrician. The remaining connectors and parts are difficult, not recommended for home repairs in the garage.

 

Take it to the dealer. Mine did a "goodwill" gesture and repaired the problem fairly quickly at no charge. There must be a TSB on it and the fix does work, because my HHR was out of the 3/36 warranty.

 

What was the repair that fixed it?