Pontiac 1999 Firebird - Manual (Page 268)

yellow blue
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4. Close the cover without overlap (the clamp will not
latch with overlap) and latch the clamps.
Automatic Transmission Fluid
When to Check and Change
A good time to check your automatic transmission fluid
level is when the engine oil is changed.
Change both the fluid and filter every 15,000 miles
(25 000 km) if the vehicle is mainly driven under one or
more of these conditions:
D In heavy city traffic where the outside temperature
regularly reaches 90
_F (32_C) or higher.
D In hilly or mountainous terrain.
D Uses such as high performance operation.
D When doing frequent trailer towing.
D Uses such as found in taxi, police or delivery service.
If you do not use your vehicle under any of these
conditions, change the fluid and filter every
50,000 miles (83 000 km).
See “Scheduled Maintenance Services” in the Index.