1988 Porsche 924 failed my emissions what can I do?

As combustion efficiency decreases, the oxygen content in the exhaust rises and carbon dioxide CO2 falls. An engine that is running at a nearly ideal air/fuel ratio of 14.5:1 will show about 14.5 percent carbon dioxide and 2.5 percent oxygen in the exhaust. CO2 readings of less than about 13 percent and oxygen readings greater than about 4 or 5 percent indicate poor combustion efficiency. This translates to an over-rich or over-lean air/fuel ratio, poor compression, or an ignition problem