P0340

Hi, this is Bo, an auto expert on RepairPal's Car Genius team! A P0340 code on your 2012 GMC Sierra 1500 means the engine’s computer isn’t happy with the camshaft position sensor signal—or lack thereof. You've already thrown a good chunk of the parts catalog at it, so let’s dig deeper. Unplugging the alternator and clearing the code points to electrical interference or a voltage irregularity from the alternator itself. A failing alternator or one with a bad diode can produce "dirty" voltage, causing signal interference for sensitive sensors like the camshaft position sensor. Use a multimeter to check the alternator output for AC voltage ripple—it should be close to zero. If it's not, you might need an alternator replacement. Also, inspect wiring around the cam sensor for chafing or corrosion, especially grounds. A weak ground can cause more drama than a soap opera. If all else fails, the engine control module (ECM) may have an issue, but rule out everything else first. If you need any further advice or want help finding a trusted RepairPal shop in your area, please feel free to call us at 877-913-1554.