How much does it cost for a diagnostic test for a 1997 BMW roadster

Check on Bimmerfest,com its a bmw forum, I have gotten alot of great answers and ideas for my 1997 z-3 roadster great site.

1st when the airbag light is on the system is disabled, so if u have an accident they will not work, as to the common cause read below.
When the airbag light is on it means a fault has occurred in the system and the airbag diagnostic monitor has disabled the system, the most common problem that turns the light on is a break in the connection to the steering wheel mounted airbag, this is due to a rotating ribbon contact that breaks under the steering wheel that connects the airbag to the wiring harness and the diagnostic monitor, the part is called th clock spring contact, a coiled up ribbon electrical contact that turns with the wheel

DTC P1423 - Secondary Air Injection Incorrect Flow detected

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Negative, like every other modern vehicle. It's only some old classics from before the late 1960's which use positive grounding.

a 1997 roadster does not have a drive belt it has a chain and i have never heard of anyone needing to replace it at any mileage

No, that is a specilty part that dealers world wide do not carry in the parts department, you have to wait it out. I never see those in junk yards unlesss they have crashed so sorry for the bad news

it probably does. you will have to contact the dealer and see if they have any advice on what you can do to fix it.

Isn't there a rear sensor by the catalytic converter also?

Bushings in seat are shot. See this link: http://www.youtube.com/watch?v=DM4KSnA58EI

Could be the immobiliser.  Do you have the original keys?  If your key does not have a remote unlock, it's likely to be a "spare" key, which doesn't have an immobiliser chip in it.  If you take your VIN to your dealer, they can get you a new key - at a cost.  I've been quoted over $400 to have a key replaced.
Hope this helps.