Question: I bought a 2005 F-150 with the 5.4-liter engine. I knew when I bought it that the catalytic converter had been taken out, and the check engine light was on. I had the OBD codes checked and it showed a bad O2 sensor. I expected that, but I also have codes P0155 and P1151. What are those codes?

Answer: Diagnostic Trouble Codes P0155 and P1151 both refer to an oxygen sensor failure. One deals with an open or short in the heater circuit; the other the sensor circuit. Current O2 sensors include a built-in electric heater. An O2 sensor can’t function (measure exhaust oxygen content) until it's hot. The heater circuit brings the sensor up to operating temperature on a cold start quicker than heat from the engine's exhaust gas on its own. This gets the entire system up and running sooner, therefore reducing emissions and increasing fuel economy. Today's powertrains use two or four oxygen sensors, mounted before and after the catalytic converter(s). The primary O2 sensor measures exhaust gas oxygen content and sends a signal to the powertrain control module to determine a rich or lean condition. The secondary O2 also measures oxygen content, but this signal enables the PCM to verify the functionality of the catalytic converter. My guess is that one or more of your oxygen sensors have been removed or damaged with the cat extraction. Unless you're an off-roader and/or live on a farm, you're going to need a state inspection sticker. And in most states, you’ll fail without the cat, and with the check engine light on (DTCs stored).


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