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I had 3 error codes. Three little piggies now there are none. No Check Engine light. Nothing. Gone. Only difference, it's cold out now and engine is smooth again. Maybe something fouling an injector for a while. Put in Lucas fuel treatment, dry gas, and do a dance to prevent this happening again?

The first to go was P0303 which was 3rd cylinder misfire. Sounds about right. Car was rough for a bit, but after warming up it was smooth. Easily a possible misfire. 124,000 miles. 2.0 liters. Elantra 1999, sedan. GS or GLS trim? Black, in case that matters. Interesting, shop manual, too many misfires can fry the converter, the catalytic converter. Uh oh.

The other 2 errors, 0422 and 0722, one is about output speed sensor from the transmission, not from the wheels but from the tranny. The other other is about a bad catalytic converter.

Hmmm, may downstream oxygen sensor is marginal? But I should get an error message for just the sensor unless it's a tiny bit marginal. Yes?

Bad signal ground just screwing thing up in general. How to get? Take voltage reading from a signal ground or ground strap wire? If can find a ground wire and take it from battery then if low, it's a bad ground? Any help here?

Can't find the second downstream oxygen sensor. Probably above the pipe leading from the catalytic converter to the flex pipe or the muffler? Has to be somewhere. Hard to see without a car lift. The up stream sensor is easy. Replaced that. Can see it behind the engine just looking down with the hood popped open. Well, after someone pointed it out, it is easy to find :)
 

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--> 0303, misfire detect, cylinder #3, look at plug wire ends for burn/arc through, look at plugs if at a 60,000 mile multiple, and especially the intake gasket between intake and cylinder head.. with engine running, squirt something (carb, WD-40, or just water from water bottle in stream) at the seam along head where intake meets, if you have leak, you'll hear the vacuum suck the liquid through the crack... water willl hiss real loud, flammables will make engine do goofy things.

--> 0422, catalyst efficiency... misfire will kill a cat... if it not dead already

--> 0722, output speed sensor fault
 
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