I am getting poor gas mileage and rough Idle on my 268000 mile infinity. Does the Rear Cat if clogged do this. The temp readings are low at the exhaust side and the O2-12 sensor readings vary. Or is this just a bad O2 sensor.
Poor mileage is a symptom of a clogged cat as is power loss at high rpm. Rough idle, not so much.
If you car is not showing any codes (like a P0420) and your oil consumption is not greater than 1500 miles per quart, then it probably is not the cat.
Is the check engine light on? What are the error codes? Is the car warming up properly, if not, that indicates a bad thermostat and that will cause poor mileage.
Having the same problems. No codes with 276000 miles. Cat not clogged. Fuel trims off. Very poor low idle. Warms up normally. High Idle on warm up.
You never answered Mr. Mustang 5 years ago so it is kind of hard to get concerned about you still having the problems.
Suggest to post the fuel trim numbers reported by your scanner. Beyond that, At 276k miles, many possibilities so probably a good idea to do a basic systems renewal
- Ignition system, replace spark plugs, dist cap/ignition rotor, spark plugs leads (if so equipped).
- Spark timing
- Valve timing
- Check entire vacuum system for leaks.
- Compression test.
- Intake manifold vacuum test
If you haven’t bothered to do anything, like, well you know had it checked by a mechanic in the last 5 years why would we even bother?
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Yeah, I normally would ask questions like, what fixed the issue 5 years ago, or did you just keep driving it like that for 5 years??
But since the OP posted close to the same thing after 5 years without any follow up much less having enough respect to answer Mustangman’s questions from 5 years ago before just basically asking the something again, then why would any of us even bother trying to help after 5 years just to wait another 5 years for the next question to be asked with NO follow up… Why even bother helping… 
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