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Could I get some help please. My 1996 4x4 f150 5.0 liter truck failed emissions test today. I bought it used about 2 months ago. The error codes were Po135, PO141,PO155. My mechanic tried changing the o2 sensor, it did not work. He has now told me that I must take it to the dealer. Also even though their were failure codes the check engine light was not lit up and when you turn the key to start the engine, all the dash board lights momentarily light up except the "check engine light". I'm worried that a previous owner disabled it before he sold it to me. Any ideas?
those codes indicate bank 1 O2 and you said mech. replaced it. these trucks have 3 O2 sensors, did he get the right one? did he clear the code after fix? disconnect battery for 10-15min and turn the headlights on, reconnect the battery and drive it around. I'd check to see if it has voltage and then read the codes again to see what you get.