Question: I was on a long drive with my 2009 Sierra with the 5.3-liter, and the MIL light came on. The code is a P3400 for cylinder deactivation. It's running fine.
Answer: DTC P3400 tells us the PCM has seen an engine performance problem while running in V-4 mode. The PCM determines this by seeing Mass Air Flow (MAF), Manifold Absolute Pressure (MAP), and/or Throttle Position Sensor inputs out of their acceptable ranges while running on four cylinders. This can be caused by a mechanical engine problem, an inaccurate sensor, and air intake or vacuum leaks. I don't see anything published on the DTC from GM, but there have been instances where aftermarket intake systems have triggered the code. If you have one installed, reinstall the OEM setup. If it's a K&N filter, follow up by cleaning off the element side of the MAF Sensor with an electrical contact spray you'll find at Radio Shack. The filter oil can collect on the MAF element and throw the readings out of whack. If you haven't touched the intake on the truck, bring it in to GMC service. There may be a non-published software update to fix the problem that you won't find anywhere else. TT
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