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I have a 2002 Impreza outback with the 2.5L auto trans. It has 190,000 on the clock. I am the second owner. The previous owner had the car dealer services with a folder of receipts. At 134,000 miles the head gaskets, timing belt thermostat, water pump and IC304 coil was replaced. Car ran fine when i bought it at 170k the CEL was on when i bought it with the code (p0420) bad cat or o2. I had the cat os sensor, spark plugs and battery replaced 180k. The car ran great for about 1000 miles then it started to hesitate while driving. now at 190k The car isnt even drivable because it stalls at red lights and has no power. It starts fine when cold and runs ok for 5 min or so then it idles rough and stalls. This symptom is worse when the temp outside is cold. help please!
 

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Ok so i just replaced the plug wires, air filter and did an oil change. The car now idles fine but the loss of power is still a huge issue. It is so hard to pinpoint because it does it intermittently. Ill be driving down the road and all of a sudden Ill lose power! (by loss of power i mean drive trane wise not electrical) Ill push the peddle to the floor and the rps stay the same and i wont go any faster. it will do this for 5 seconds or so then the car will respond...... Is this a sensor, transmission or electrical problem... any ideas are helpful! I was going to change out the fuel filter but I really dont think that is the problem and I dont want this car to nickle and dime me to death. Should I try the up stream o2 sensor or would that be a waste of money too??? THANKS A LOT!!!!
 

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Ok so i just replaced the plug wires, air filter and did an oil change. The car now idles fine but the loss of power is still a huge issue. It is so hard to pinpoint because it does it intermittently. Ill be driving down the road and all of a sudden Ill lose power! (by loss of power i mean drive trane wise not electrical) Ill push the peddle to the floor and the rps stay the same and i wont go any faster. it will do this for 5 seconds or so then the car will respond...... Is this a sensor, transmission or electrical problem... any ideas are helpful! I was going to change out the fuel filter but I really dont think that is the problem and I dont want this car to nickle and dime me to death. Should I try the up stream o2 sensor or would that be a waste of money too??? THANKS A LOT!!!!
Check the grounds. A loose or corroded ground on the battery, engine block, body or intake manifold will cause this.

A battery that is low on amperage supply will also cause this. If the base amperage, not voltage, of the battery is below 400 amps, its a problem. It effects the alternator output, computer, transmission, ABS, everything.

If the battery is good amperage wise, you can check ground conductivity from the battery to the body, engine, trans case, intake and the alt casing. Also check the alternator output and this will only be reliable with a good battery.

I had a newer legacy 2.5 (09) in the shop yesterday with similar issues. Bad battery. Replaced, ran like a champ.

Not saying that's your problem. Always check the battery output first, then conductivity on the grounds, then alternator. After this you move on to the sensor output by pin point test or reading the data output on a scan tool or laptop data logger. Of course you will need to know how to "read" the information in the data, but if it comes to that, you can post it and we'll help.

There is a large number of input and output drivers that operate the vehicle. Try not to assume O2 sensors all the time. They rarely fail. My VDC has the original AF and rear O2 sensors at 179,000 miles. (I will most likely end up replacing them shortly with the extra exhaust heat its generating.)



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I think you run on a fixed factory a/f program with the O2 unplugged so, if it seems to run better - that points to some intermittent A/F problem. There are a lot of possibilities and getting freezeframe data or other info might help. At this age, the car could have so many issues but, I think your symptoms could be due to a bad MAF. I might not rule out the Engine Temp Sensor portion of the temp sensor. That might fit with the good cold-weather operation. Also, I think in another thread cardoc mentioned old vacuum lines that can alter with temperature.
 
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