P1108

P1108 is GM's BARO-to-MAP Sensor Comparison Too High — at key-on, the ECM compares the barometric pressure reading (from a discrete BARO sensor on older platforms, or from the MAP sensor sampled at KOEO on newer ones) against the MAP reading and looks for them to agree within a tight window (typically within 0.5V of each other since both are seeing atmospheric pressure at key-on). When they disagree by more than the threshold and MAP is reading HIGHER than BARO, the ECM sets P1108. This is almost always either a MAP sensor that's drifted high, a vacuum leak large enough to push MAP toward atmospheric at idle, or a discrete BARO sensor reading low. Cheapest-first ladder: (1) Scan live data at KOEO and compare MAP voltage to BARO voltage — they should both read 1.0-1.5V at sea level, lower at altitude (about 0.1V lower per 2,000 ft of elevation). If they disagree at KOEO, the higher-reading sensor is the suspect. (2) Smoke-test the intake for vacuum leaks — a torn intake duct or blown intake-manifold gasket will push MAP up at idle because manifold vacuum is bleeding to atmosphere. (3) Inspect the MAP vacuum port for carbon clog (high-mileage trucks) and the BARO sensor (if discrete) for moisture intrusion. (4) Verify altitude programming if the truck was recently shipped from sea level to a high-elevation market — some early LS PCMs need a BARO relearn. Don't replace the BARO sensor before smoke-testing the intake — the most common P1108 cause on high-mileage LS trucks is a torn intake manifold gasket allowing manifold pressure to rise toward atmospheric, not a failed sensor.

Vehicle-specific patterns: 1999-2007 Silverado / Tahoe / Suburban / Yukon / Escalade with the LS-based 4.8/5.3/6.0 (LM7, LR4, LQ4, LY5) — the early LS intake manifold gaskets are a known leak point at the front and rear cylinder ports, and the leak shows up as P1108 paired with lean trims and P0171/P0174. 2005-2010 Cobalt / G5 / HHR with the 2.2/2.4 Ecotec throws P1108 from a cracked plastic intake manifold (GM superseded the manifold under multiple PI bulletins). 2010+ Camaro / CTS / Traverse with the LFX/LLT 3.6L V6 throws P1108 less commonly but when it does, it's usually carbon buildup at the MAP port distorting the reading. 2006-2010 Silverado HD 6.6L LBZ/LMM Duramax uses a separate boost sensor and barometric pressure sensor — P1108 on the diesel typically points at a soot-fouled boost sensor reading high, not an intake leak. Estimated repair: $0 (relearn/recalibration after altitude change) to $650 (intake manifold gasket replacement on an LS truck).