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My check engine light came on, the traction control light is also on and the cruise light flashes. I plugged it in at the local parts store and got the following codes and information:

2 Codes P0420 P0303 MIL ON MONITRS 0 Inc 8 ReadyMisfire Fuel Comp Catlyst Evap O2 Snsr O2 Htr EGR

I replaced both the O2 sensors. The engine has about 7,000 miles ago the car has 175,000 miles. The spark plugs were replaced at the same time as the engine but not the wires.

The car runs rough when it is in gear at a stop and when going up mountain passes. I am a young female on a budget looking for some guidance. I am fairly capable of replacing parts on my own. Any advice is greatly appreciated!
 

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2019 Forester CVT
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What year is the car? P0420 is catalyst below efficiency. P0303 is misfire on cylinder 3.
The misfire could be contributing to the catalyst problem so it should be addressed first.
Pull the # 3 plug and look for carbon or wet with gas. The old plug wires are also suspect.
Clean the plug and re-install. Check the connections on both ends of the wire. Next try swapping the #3 injector
with one of the others and see if the problem follows the injector. If this does not solve the problem,
diagnosis rather than parts replacing will be faster and cheaper. There is a sticky in this forum for the P0420 code
and the NASIOC forum has extensive Misfire and catalyst trouble shooting info.
Good Luck
 

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older subarus are prone to spark plug wire failure and to coil pack failure, it is common, start with the spark plug wires , move them to see if the code follows ,................... remember to DELETE the codes prior to starting after the move ......., then if not ok, replace wires, and delete again , , coils can be tested for spark using simple cheap tool available most any car parts stores it simply plugs in between the coil end of the wire and the wire and when running should show visible spark, ................you can also test for spark using a screwdriver and see if it grounds to the body when the screwdriver is close to the engine, this is more difficult than using the tool, but it does the same thing, but also requires 2 people to do, one holding the screwdriver and or wire to ground, [ rubber cover used to prptect you ] and one to start car harder to do but nealry free to do............ and if deemed ok, then you move on to the plug and or injector, injectors do fail and your millage is old enough for that then conpression test to see if bad valve, , it is usually however the wire or coil pack
 
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