Your 2007 Nissan Sentra's high HC problem might be due to the slightly high CO problem. You'll need to first have to find out if your Nissan Sentra fuel feedback system is working properly. This includes ensuring the oxygen sensor is working properly, your vehicle's emissions computer is receiving the signals from the oxygen sensor, the computer is computing the data properly and sending the correct signals to the fuel injectors to either increase or decrease fuel delivery to the combustion chambers.
The condition your Nissan Sentra might be suffering from is called a "Rich Misfire". This condition is created, when for any reason, the combustion chambers receive too much fuel, causing the fuel that is delivered to burn improperly and leave behind high levels of raw fuel(HC) and partially burned fuel(CO). Vehicles producing high HC for fuel management reasons usually begin producing normal HC levels once the fuel problems are corrected.
However for those vehicles suffering from high HC not related to fuel management, should have the engine ignition system closely inspected.
