The 1999–2007 Silverado and Sierra trucks use an electronic throttle control (ETC or TAC) system — there is no throttle cable. The throttle body is controlled entirely by the PCM using a motorized blade. When it fails or gets dirty, you get stalling, hesitation, and reduced power mode.
SYMPTOMS
- Reduced engine power message on the DIC
- Erratic idle — surging or stalling
- Slow throttle response — sluggish acceleration
- Check engine light with codes P0121, P0122, P0123, P2135
- Engine won’t rev past 1500–2000 RPM (fail-safe mode)
HOW THE TAC SYSTEM WORKS
- Throttle body has a motorized blade — no cable
- Accelerator pedal module sends position signal to PCM
- PCM commands the throttle motor to open/close
- Two TPS (Throttle Position Sensor) signals inside the throttle body for redundancy
- If the signals disagree — PCM goes into reduced power mode
COMMON FAILURES
Dirty Throttle Body
- Carbon buildup on the throttle plate and bore
- Causes rough idle and hesitation
- Clean with CRC throttle body cleaner — DO NOT use carb cleaner on drive-by-wire units
Failed Throttle Body
- TPS sensors inside the throttle body fail
- Motor inside fails
- Replace as a complete unit — sensors are not serviceable separately
Accelerator Pedal Sensor (APPS)
- The pedal module itself has sensors that fail
- Usually causes reduced power and P2138 code
- Easy to replace — 2 bolts, 1 connector
CLEANING vs REPLACEMENT
- Clean first: use throttle body cleaner, soft brush, Q-tip for plate edges
- If codes return after cleaning — throttle body needs replacement
- A-Premium and other aftermarket units work well as budget-friendly alternatives