1998 Subaru Legacy CEL on. Code states 2nd converter/O2 sen

Notice any hesitation when taking off? If so you may need a new ECM, Here's the tech bulliten...
HESITATION ON ACCELERATION - REPLACE ECM TECHNICAL SERVICE BULLETIN Reference Number(s): 11-53-98, Date of Issue:  January 5, 1999 Related Ref Number(s): 11-53-98 ARTICLE BEGINNING HESITATION ON ACCELERATION Model(s): 1997-98 Subaru Legacy, Impreza, and Forester Bulletin No.: 11-53-98 Date: January 5, 1999 APPLICABILITY 1997-98 Legacy, Impreza, and Forester Manual Transmission vehicles with 2.5L & 2.2L engines SERVICE INFORMATION In the event you encounter a customer complaint of a slight engine hesitation between 1500 - 2500 rpm's when the engine is cold or hot, perform the following: Eliminate all external influences, such as a incorrect or dirty air filter, loose or cracked intake duct, dirty fuel filter, low fuel pressure, PVC system, or low engine vacuum that would indicate an external leak or an internal engine component. Check for trouble codes and repair as necessary. Verify the fuel quality. If all external components are confirmed to be operating within vehicle specifications, the hesitation may be caused by the ignition control logic in the ECM. Under certain low rpm driving patterns, the ignition control system can pick up engine vibrations through the knock sensor and may retard the ignition timing. This ignition timing is learned by the ECM and placed in memory. Note: This area of the memory can not be viewed by using the Select Monitor. When the vehicle is driven under these conditions, the timing may be retarded and could cause the engine to hesitate on acceleration. To confirm this condition, road test the vehicle while viewing the Knock Sensor Signal on the Select Monitor. If you duplicate the hesitation, and the reading on the monitor is around -10 Degrees, you will need to change the ECM to correct the concern. NOTE: Some vehicles (shown on the following two charts) were produced with two different emission systems during the same model year. To identify which system you have, look at the Vehicle Identification Number. Refer to the chart to identify which system you have on the 98MY Forester and 97MY Impreza vehicles and choose the correct part number from the chart on the following page.

Have never noticed any hesitation. The car runs well and starts good. Once the engine light comes back on the drivability does decrease. The overall power is not there. When the light is not on it accelerates strongly right up to red line and drives just fine. After the light is on for a few days, you notice a decrease in power. It is still drivable, but you can drive it like it's a race car any more. I don't seem to see any drop in fuel economy and have been averaging 24 mpg. The engine light codes have always pointed to the O2 sensor in "bank 2", never anything else. I realize that the root of the problem could be something else, like the ECM, but I have never noticed any of the hesitation that it speaks of in the tech bulletin. Lack of power? Yes. Hesitation? No. Any further help would be appreciated. Thanks! b

You need to hook this up to a scan tool and monitor the 02 sensors, this will tell you if its lean or rich, and help finding out what can be done. Could be as simple as a vacuum leak, or possibly it needs a new MAF, this in conjuntion with 02's controls air/fuel mixture.
You may have a vacuum leak, get a can of spray gumout, spray the vac lines and maniflod area, when the idle changes, youve found the leak! You want the engine running and spray on the outside but do direct the stream onto the hoses, if it is a vacuum leak, the engine idle will change speed, then you have detected a leak, repair the vacuum leak and see how it runs!