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Loss of power on hills

My 2004 Subaru Outback has had the head gasket replaced, the O2 sensor replaced, spark plugs replaced. Still I cannot go up big hills anywhere near the speed limit.

I think it may be the catalytic converter is bad from driving it with a leaky head gasket. Any ideas?

Could be the converter, in fast that would be near the top of my list. I did not see anything about air filter or other regular maintenance.

You could have a myriad of possibilities, bad air filter, bad fuel filter, bad plugs, bad vacuum advance, bad cat, bad sensor etc. My idea get a qualified mechanic to fix it.

The top two things on the list are fuel pressure and clogged exhaust. I’m not even a mechanic and I could tell you about those within about 20-30 minutes in my driveway - its very simple stuff to check.

However - I’d also ask whether anyone checked the compression after doing the head.

Is the check engine light on?

I just had the O2 sensor changed again, as the check engine light had been on and sending an O2 snesor code. Turns out the first guy apparently broke the electrical connector installing it.The car ran better, and hill climbed better, though still runs a little rough.

It’s quite possible that the problem could be a clogged converter but another possibility could be a weak engine.

Seeing as how the spark plugs were out this would have been an opportune time to run a compression test. This would be especially true if the reason for the head gasket replacement involved overheating.

You also use the word gasket in the singular. When performing this job on a Subaru one always replaces both gaskets.