I bought a 1999 Ford Contour last Saturday and on Wednesday, the check engine light came on when I was driving home that day after work. I ran the code (thankfully there was only one) and it came up with a P1151 which seems to be unique to Ford and the description is: Lack of HO2S-21 Switch, Sensor Indicates Lean. When I looked it up online, it said one of the causes may be oil overfill. Which brings me to my question: Would overfilling the transmission cause this code? Or does it just mean the engine oil? The reason I ask is because I had just added some transmission fluid to it that day at lunch time because is leaking and was a little low and may have added a bit too much.
no, the transmission’s fluid is its own and does not go through the engine. what that code means is that the oxygen (O2) sensor is sending a signal which is never changing. See, in order to keep the catalytic converter working efficiently, the engine has to vary between slightly rich and slightly lean. therefore, the O2 sensor should be sending back an oscillating electrical signal varying between two voltage levels. Your car’s PCM is saying its sensor isn’t, and is indicating the engine is running always lean.
this is one of those situations- especially given the age of the car- the O2 sensor could simply have failed or is fouled.