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I just got a code P0139 tonight. Code is for slow secondary o2 sensor response. The thing is I'm running all the stock cats. Only exhaust mod is TurboXS axle-back. Any ideas? This is on an 04 WRX with ~15000 miles.
 

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That is a time to activity code. It doesn't have to do with the cats. More likely, there is a problem in the sensor heater circuit. Have the dealer fix it under warranty.
 

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Sorry, OBDII speak, it is an emissions thing. The ecu monitors the oxygen sensors to see how long they take to begin returning useful data after a "cold" startup (ect and iat within 10 degrees F). O2 sensors have to be hot to work so most have heaters on them now to heat up faster than they would just from the exhaust. Subaru figured out how long it should take for the sensor to start working under various conditions and programmed the ecu to turn on the MIL if it takes longer than planned.
 

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Nope. A laundry list of parameters have to be met for the MIL to come on. If your scanner has the feature, check to see if the P0139 test has run since code cleared. If it has run and passed, then the problem is intermittent. If it han't run, you haven't learned anything more. Typically, it takes a week or two for the average driver to run thru all the conditions for all the different OBDII tests to run. Oxygen sensor and EVAP DTCs are some of the most persistent ones.
 
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