(Just my opinion on what possible problems there may be):
If engine temp sensor is giving a false reading then that may cause high fuel consumption (also if the temp gauge doesn't show that the engine reaches running temp then the thermostat might always be open and that too can cause higher fuel consumption). If the lambda sond isn't working properly, then that too. If the spark plugs are worn or dist. cap and rotor then that as well. A dirty air filter can cause similar effects. If alls well and the engine runs fine yet the emission reading isn't good then it may be the cat. converter.
There the things i can think of, hope you're able to find out what the problem is.
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