P2104

P2104 means the ECM has commanded the throttle into forced-idle mode because it detected a fault somewhere in the TAC system serious enough that it doesn't trust the driver's pedal input anymore. The car will start and idle, but stepping on the gas does nothing — the blade stays at its idle stop and the ECM ignores the pedal sensors entirely. This is almost never the primary fault; P2104 is a consequence code that rides along with whatever actually triggered limp mode (P2101, P2118, P2135, P0120-series, etc.). Pull all stored codes before you touch anything — the companion code tells you which subsystem the ECM doesn't trust. Cheapest-first: read the freeze-frame data on the companion code to see what the engine was doing when limp engaged, then run the throttle-body clean if you haven't recently. Backprobe TPS-A and TPS-B at the throttle body and APP-D and APP-E at the pedal — all four sensors have to agree within their correlation window or the ECM defaults to forced idle. A single backed-out pedal-sensor pin (super common on GM trucks where the pedal harness runs under carpet and gets stepped on) will pop P2104 along with P2135.

Vehicle-specific patterns: 2007-2014 GM full-size trucks (Silverado / Sierra / Tahoe / Yukon) with the 5.3L / 6.0L is the dominant P2104 platform — corrosion in the accelerator-pedal-position (APP) harness connector causes intermittent voltage drops on the pedal sensors and the ECM forces idle within seconds; the fix is repinning or replacing the pedal-sensor pigtail and reseating the connector with dielectric grease. 2005-2009 Subaru Legacy / Outback / Forester with the EJ25 throws P2104 from internal cracking of the throttle body's plastic gear-train where the motor connects to the blade shaft; replacement throttle body required. 2008-2012 Honda Accord / Pilot / Odyssey J35 sees P2104 with P2122 / P2127 from a worn pedal-position sensor where the wiper element flat-spots at the rest position. Never clear P2104 without verifying the companion code is also resolved — clearing it alone will let the car drive for 30 seconds before the underlying fault re-asserts and forces idle again, which is dangerous if you're pulling out into traffic.