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So my CEL came on recently, and had it read at autozone, then again at the dealership, and both say its a code that indicates my cats are going bad. my vehicle is a 2004 obw 5spd, with EJ259 motor, and from what I know my model has far more expensive cats than earlier 2000's outbacks. 120k seems early to me for the cats to go, having friends with 200k on origional cats. the cost of new cats seems to be too astroniomical to me based on the book value of the car...any thoughts? thanks!
 

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O2 sensors cause this reading cat's rarely need replacing though people get hosed by parts sales people all the time when really all they need is a new O2 sensor
 

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Since you car has 120K miles you are past the emissions warranty of 8Year/80,000miles. I suggest a salvage yard or muffler shop. Our prior vehicle was a 2000 Celica GT with 105K miles when the cat went bad. Dealer wanted $1200 to replace the cat. Had a local muffler shop cut and weld a new cat for $100.
 

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Cats rarely fail.

What is the code?

A P0420 code is most often a failing oxygen sensor, exhaust leak, or some such. Not the cat.
 

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I put this up on another thread. Maybe if its up in enough places, people will know what they need to check for. CATs last as long as the engine performs as it was designed. They fail when performance is down for a long period.

A short list of things that will cause a 420 code:

1. Crappy, cheap gasoline. Especially this time of year when the oil companies are switching to "winter blend".
2. A stuck thermostat. If the engine is not getting to proper temperature, the ECM continues to enrich the fuel to try and warm it up. More fuel means the CAT must work harder to get the numbers the ECM wants from the rear O2 sensor, but the CAT can only do so much before extra HC's get through. Also, when your engine doesn't maintain proper temperature, it reduces the combustion temperature which lowers the rate of burn and efficiency of the engine itself.
3. Ignition system. Old, worn plugs, wires, weak coil, dirty injectors (bad spray pattern or streamed gas shot due to varnish build up)
4. Vacuum leak.
5. Clogged fuel filter
6. Exhaust leak.
7. Failed O2 sensor. O2 sensor feeds info back to the ECM as to how the CAT is functioning, or the amount of HC's mixed with the oxygen on exit.
8. RARE Faulty AF sensor. If it is sending incorrect readings to the ECM, and the ECM hasn't picked up that the AF sensor is slacking, it only knows what the O2 sensor is sending, the CAT is not working as designed or there is a problem upstream of the CAT.

Now, 1-6 you can check easily in your driveway. You know where you buy your gas. If its a Murphy station, stop going there. Its "shelf" gas. The temp gauge should stay at the middle all the time after warm up, and you can also use a digital thermometer to check the temperature at the base of the water pump where the thermostat is to check the block temperature above the stat. Steady reading above 195 is great. The thermostat opens at 195. Ignition, pull a plug, check the gap, color, tip wear and go from there. Vacuum leaks tend to make a whistle noise. Fuel filter needs to be replaced every 15k unless you use Murphy gas, then every 10k or less. And exhaust leaks, well you know what that sounds like, right.

P0420 means the engine performance has gotten to the point that the CAT can't keep up and its performance or "efficiency" is below programmed levels within the ECM ROM.
 
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