Recheck your Pnnnn code. A P0135 is for the front O2 sensor, not the rear. The front sensor is on the down pipe and the rear sensor is at the back of the CAT. Use your scan tool on the front O2 sensor and see if it is fluctuating nicely.

Usually, the O2 sensor messages are a result of running too rich. A vacuum leak will cause this. What is your gas mileage, anything below 16mpg is a suspected air leak or the front O2 sensor.

Remember, the rear O2 sensor is only checking the efficiency of the CAT and doesn't fluctuate much. Clearing the code on an efficient engine will delay the CEL for up to a week.

Klaus
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Volvoless. Sold the R. There is now a void...