Having an error with the O2 sensor sensor code should not trigger limp home mode. If the O2 sensor wire signal lead gets shorted to ground, it can blow circuits inside the ECU, which can cause all kinds of headaches. There are quite a few sensors which control the engine and transmission. If one of the sensors, such as the MAF, MAP, TPS or park sensor or speed sensors are sending improper signals to the computer, limp mode can activate or shut down the engine or not have the engine start. Failed fuel injectors, coil packs and worn out spark plugs can also cause it.
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