My Daughter has a 1999 Subaru Legacy 2.5. The car always had put out an O2 sensor error but ran decent. The last week she had some battery issues and it required a few jump starts and I also discoered I needed to clean the battery terminals. Well since then he check engine ligh is off and the car runs terrible. The idle is at 1500 and if you try to normally accelerate the engine bogs and you have to back off the accelerator. If you lightly press the accelerator it will increase speed but once it reaches 35-40 mph the engine chugs and jerks. I replaced plugs, fuel filter,air filter, pcv valve.Thanks for listening

replace the o2 sensor.

ok, I can do that, but this will also be the third replacement of that. But I am willing to try it. Thanks

Check fuel pressure and injectors

o2 sensor replaced and still issues. The car Idles great. If I press the accelerator slightly the car will rev to 3000, if i press the accelerator as you normally would the engine bogs down and no revs at all. I took it on the road and as stated if a barely press the accelerator, I get to 40mph, if I press for normal acceleration the car goes nowhere, and chugs... I also noticed as I was driving if I feathered the pedal I could work the rpms up but the tranny would shift or slip. I will look into the fuel pressure and Injectors. Would the injectors throw a code if they were faulty?

No, but an 02 code would throw because as far as the car knows the 02 sensor isnt returning a good value. What was the o2 code? Rich or lean? Lean means no enough fuel. Rich means just the opposite,to much fuel not enough air. In which case you probably have a vacuum leak

Do you think this is an upstream O2 issue? The readings seem eratic.... I will do one more reading and post it.

What was the engine code on the car? With a scan tool

ok, ran scan tool... and now very odd. both upstream and down stream ran a constant voltage. The upstream was at .8 and the down ran a .4. Is something wrong with the ecu?