Check Engine Light illumination and performance issues can be caused by oxygen sensor faults.
Problems for specific Porsche 911 years:
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Newest reported 1990 Porsche 911 problems
The accumulation of moisture at major connectors and control units can cause various ABS/PDAS faults due to corrosion.
A normal operating clock may cause the airbag warning lamp to illuminate or a chronic dead battery.
The front hood and rear deck lid support struts may fail; replacement is required.
Engine issues related to valve guide wear are common. Oil consumption, smoking at first startup, and valve train noise are all symptomatic of loose guides and worn valve stems. The valve train is complex and should be thoroughly inspected if a problem arises.
An accumulation of small air leaks at various locations around the intake manifold/intercooler area can cause performance problems.
Various repairs and adjustments to the drive mechanism (including cables) are commonly needed on the Cabriolet tops.
If engine misfiring, rough running, or fuel consumption issues are not resolved quickly, the catalytic converter can overheat and fail.
Climate control modules can be problematic. Symptoms vary but include failure or persistent dead battery after not using the vehicle for a while.