Grand Am trouble codes

Posted similar posting in alt.autos.pontiac. My '91 Grand Am (2.5L 103k miles )has error code 45 - rich exhaust, low gas milage (17mpg) and overheats quickly.
Expected the Coolant Temp Sensor, but this seems to work fine after testing. I blame to much gas in the cylinders for all my problems. Is this right? And what are possible causes for this??
Thanks
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You may have a head gasket leak.
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