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99 Xc 70 at 155K miles. Have had the CEL on for a long time with P0037 and P0137. O2 sensors. I was planning on replacing them soon, but last week engine started to sputter a lot with black smoke out the tailpipe, terrible mpg readings. at 60mph was getting 10mpg. Read the code, gor P1171. At first cleaned the MAF sensor, it seemed like it helped a bit, but same symptoms came back. Got a new MAF, for a day it was fine, but now its back to the same symptoms again. On a trip for 300 miles, it was running just fine, but then it would sputter. Wait 20 mins, then it would run fine for another hour. But now, evertime I run the engine, it does it. Terrible idle, black smoke. Today, I un plugged the O2 sensors, ( both front and back), they were basically covered with black carbon. Cleaned the tips, and cleaned the connectors, then plugged them back again. Took off the hose that goes from the airbox to the turbo, and cleaned all the nipples, and made sure the hoses didnt have any cracks, also cleaned PTC valve. None of these things made any improvement.... My question is, should I go ahead and get new O2 sensors, or is there another ay of finding out what might be the cause? Before these symptoms came up, car was running just fine and was getting 26mpg on highway.......
 
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