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2004 Subaru Legacy

My mechanic can’t find the problem. Engine light comes on. Along with the flashing cruise control. His diagnostic doesn’t show him the exact problem as the voltage changes. He has checked everything related, changed out the bank 2 sensor 1 oxygen sensor, but can’t find the problem. I don’t know the diagnostic code. But he says that it doesn’t give him an exact code. Can only the dealer do this? Car runs fine.

A reader will always give you a code. He may be over his head or is just replacing parts.
Go to autozone or pepboys and have them read the code for you for free.
Don’t buy anything from them, nor let them touch your car but report back with those code here.

Remco is correct, and I suggest that you follow his advice.

Additionally, the flashing cruise control light is merely the fall-out of the engine problem that turns on the CEL. When the engine is not running at its optimum, the cruise control, the traction control, and the vehicle stability control systems are all disabled–hence the flashing light(s).

Get the codes, report them here, and perhaps we can help you with a solution, but I believe that the solution will also include a new mechanic.

The code is PO420. But my mechanic was talking about how the voltage fluctuations make diagnostics difficult. Perhaps the Bank 2 Sensor 1 Oxygen Sensor that he put in is no good? I really don’t want to take it to the dealer, but do I have a choice? Thanks.

It could be the o2 sensor but I wouldn’t start replacing it yet.
That condition is determined by monitoring both o2 sensors, the one in front of the catalytic converter and the one behind it. Once the car gets hot, they both read a certain voltage at a certain engine speed and its ratio (one over the other) needs to be a number plus or minus some tolerance.
If it isn’t, the PO420 or related code is thrown. It could be that one of the O2 sensors is lazy in that is doesn’t respond quickly enough or that it doesn’t respond at all.

Those two O2 voltage can be monitored, usually with the same reader he used to pull your code (unless he has a cheap reader - they can only reset and read codes).

Very often it is due to an exhaust leak. That’s where I’d start.

This is a bit hazy to me but is your mechanic saying that the O2 sensor voltage is supposed to be a constant?

read ok4450 - it seems to make sense and describe the condition. seems like his reader is ok, but as stated depending on the engine speed the ratios change (?) so the exhaust leak would be in the muffler? or the pipes? should I go back to the same guy. he has always been great, but not many mechanics around here work on Subarus much. thanks

The definition for code P0420 is Catalyst System Efficiency Below Threshold (Bank 1) and is one of the more difficult engine operation problems to solve in my opinion since it can be caused by more than one thing. The front or rear O2 sensors for the CAT could be causing the problem but as others have already stated there could be other things causing the trouble. An air leak after the intake MAF sensor or a leak in the exhaust system before the CAT could be causing the trouble along with a bad CAT. If a tech doesn’t know how to go about solving this kind of problem it could result in a repair bill that would make the dealer service shop repair price look cheap, especially if the CAT is replaced needlessly. If the CAT is really causing the trouble I recommend you replace it with an OEM one and not a less expensive after market unit. If the CAT is causing the trouble by being clogged up sometimes it can be saved by adding an additive to the fuel that helps clean it out and work more efficiently again. That would be the cheap way out of this problem if it worked.

The front O2 sensor output signal should always be changing as it sends data to the ECU to make corrections to the fuel mixture. The rear sensor should have a steady output if things are working as they should be. Your mechanic should already be aware of these things. If he isn’t, I recommend you go to the dealer for help with this issue. Subarus are a little different but really are basically just another engine when it comes to the operation. The tech would use the same trouble shooting procedures as he would on a Chevy or a Ford vehicle.

Thanks for your help. What does it cost to replace the CAT?

OEM, 919 clams, labor not included. They bolt off and on. Maybe an hour worth of work for a mechanic.
http://subarupartsforyou.com/cp_partdetail.php?partid=2943

They are expensive. It clearly isn’t a part you want to replace, just to see if it fixes your problem.

How many miles on this legacy?

Yep, they’re expensive alright, so it is important that the mechanic has proven the CAT is causing the trouble before replacing it and not replace it needlessly. If replacement is needed, I would first see if the clean out trick worked before installing a new one. It may take a couple of cleanings to get things working again.