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Catalitic converters

how do you know when your catalitic converter needs to be replaced?

My MPG has dropped fron 42 down to 36 but I don’t think the car has lost power.I have changed the spark pluds and air filter but that has not helped.I have an 04 hyundai accent 4 cylinder standard trans with 222000 miles. I am not kidding

Is the Check Engine light on?


The engine is running rich. If there is a check engine light, the DTC (Diagostic Trouble Code) will give us a clue as to what may be causing the richness. Auto parts stores do a free code scan. Just ask.

Don’t worry about the cat unless you have some indication it’s not working correctly. Reduced mileage is more likely caused by something else. I’d look very closely at the O2 sensor(s).

I suggest having the car’s computer scanned for codes even if the check engine light is not on.

JMHO, but whenever the spark plugs come out a compression test should be run. This will verify whether or not the engine has any abnormal wear in the top end and which will in turn lower engine performance.

Even a partially clogged cconverter can lead to loss of fuel mileage and the best method of checking for a converter problem is by using a vacuum gauge.
It’s possible to have a partial clog, no CEL, and no apparent lack of performance.

(About 6 months ago I ran across this on one of my cars; a SAAB. The car was running great, etc. but since I’m a tinkerer I put the vac. gauge on it one weekend and it showed a possible slight clog. The converter is easy to remove and I replaced it with a known good one out of my parts stash. Later I knocked some of the substrate out of the converter and a rough estimate showed that it had about a 30% clog in it.)

You could also lose mileage from: tire pressure, wheel alignment, over thick oil (watch out for quick lube places using the wrong viscosity), ignition timing and advance (if it has a distributor), sticky coolant thermostat, or a temp sensor that off but not enough to trip the engine light.