Thanks for the update
The code specifically is:
DTC P0420: CATALYST SYSTEM EFFICIENCY BELOW THRESHOLD
1. Perform basic diagnostic procedure. See BASIC DIAGNOSTIC PROCEDURE. If DTC P0420 is only DTC set, go to next step. If any DTCs other than P0420 are present, diagnose and repair those DTCs first. Do not diagnose DTC P0420.
2. Check for exhaust leaks or air suction caused by loose or missing nuts and bolts, or damage to exhaust system. Check especially between cylinder head and front exhaust pipe, between front exhaust pipe and front catalytic converter, and between front and rear catalytic converters. If damage is found, repair or replace exhaust system. If no damage is found, go to next step.
3. Separate rear catalytic converter from exhaust pipe. If there is damage at catalytic converter, replace front converter. If there is no damage to catalytic converter, go to next step.
4. Disconnect rear HO2S harness connector. Measure resistance between ECM harness connector B135, terminal No. 19 and rear HO2S harness connector terminal No. 3. See Fig. 10 and Fig. 11. If resistance is less than one ohm, go to next step. If resistance is greater than one ohm, repair open between ECM and rear HO2S harness connectors.
5. Check sealed wire. If sealed wire is not properly connected, repair sealed wire and test system operation. If sealed wire is properly connected, contact authorized Subaru dealership because probable cause is deterioration of multiple parts. Clear memory and perform verification procedure. See CLEARING DIAGNOSTIC TROUBLE CODES and VERIFICATION PROCEDURE
What I would do is definitely number 2 above. Check your exhaust system really good for leaks, air suction etc. This may be the noise you are hearing.
Make sure your O2 sensors are secure and nothing is loose on the exhaust as it says.
The cost of $850 is close. Depending on what they are replacing. May be just a tad high but again, I am not sure what all they are replacing and the area you are in.
I would get a few more estimates.I wouldn't worry about just Subaru parts for the cat.
Again, before replacing anything I would do as number 2 says above.
Let me know.
Monday, October 26th, 2009 AT 9:53 PM