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P1516 GM — Throttle Actuator Control (TAC) Module — Throttle Actuator Position Performance

Severe

Quick answer

P1516 means the throttle blade on your GM truck or car isn’t where the computer commanded it — the throttle actuator control system flagged the mismatch, usually with a “Reduced Engine Power” message. Connector problems and a dirty throttle body cause many of these; do the cleaning and relearn before condemning parts.

What it means

GM’s electronic throttle has no cable: the accelerator pedal sends a request, the throttle actuator control (TAC) system drives a motor in the throttle body, and two position sensors on the throttle shaft report where the blade actually is. P1516 sets when commanded and actual positions disagree — the computer said “open to here” and the blade either didn’t arrive, arrived late, or moved erratically. Because a throttle it can’t trust is a safety issue, the response is dramatic: the dash announces “Engine Power Reduced” and the engine is restricted to a crawl, sometimes barely above idle.

This is the famous Reduced Engine Power code on GM trucks and SUVs — Silverado, Sierra, Tahoe, Yukon, Suburban, TrailBlazer — plus plenty of sedans of the same era. The drama of the limp mode makes people fear the worst, but the causes run mundane: a throttle bore so dirty the blade physically drags, and above all, electrical connection problems. The throttle body connector suffers documented pin fretting — micro-vibration wears the terminal plating until the low-voltage sensor signals turn noisy — and corroded engine grounds produce the same confusion. A genuinely failed throttle body motor or sensor pair is real, but it’s the third suspect, not the first.

One GM-specific trap on the way out: after cleaning or replacing the throttle body, or disconnecting the battery, the computer must relearn the throttle’s idle position. Skip the relearn and you can manufacture a fresh complaint — surging idle, stalling, or the same code back — and conclude, wrongly, that the new part is bad.

P1516 symptoms: what you'll notice

  • “Engine Power Reduced” (or “Reduced Engine Power”) message with the engine restricted to very low power — the signature presentation.
  • Check engine light, often accompanied by other throttle/TAC codes.
  • Stalling, surging or erratic idle before or between limp-mode episodes.
  • Intermittent strikes — fine for days, then limp mode out of nowhere — typical of connector and ground problems.

Common causes

Ordered from most to least likely.

  1. 1.

    Throttle body connector pin fretting or poor contact

    The famous cause — vibration wears the terminal plating until the 5-volt sensor signals go noisy. Unplugging, inspecting and reseating is free.

  2. 2.

    Dirty throttle body binding the blade

    Carbon buildup makes the blade drag and overshoot — a ten-dollar cleaning.

  3. 3.

    Corroded or loose engine grounds

    The TAC system reads small voltages; bad grounds make good sensors look erratic.

  4. 4.

    Failed throttle body (motor or position sensors)

    Real, and the common parts ending — but only after connector, cleaning and grounds.

  5. 5.

    Wiring damage between throttle body and computer

    Chafed or rodent-eaten harness sections produce maddening intermittents.

  6. 6.

    Moisture freezing in the throttle area in extreme cold

    A documented GM pattern in sub-zero weather — suspect it when the code only sets on frigid mornings.

How to fix it: diagnosis, step by step

Cheapest and most likely checks first.

  1. 1 Read all codes and the freeze frame

    P1516 rarely travels alone — companions like P2135 (sensor disagreement) or pedal-sensor codes shift suspicion along the chain. Freeze frame tells you cold/hot, idle/load, which separates the frozen-moisture and connector stories.

  2. 2 Inspect and reseat the throttle body connector

    Engine off, unplug the throttle body connector and look hard: bent terminals, green corrosion, and the subtle gray dust of fretting wear. Reseat it firmly (a thin film of dielectric grease helps), clear codes, and drive. This free step resolves a famous share of P1516s — and if the code returns intermittently anyway, wiggle-test the harness while watching throttle data.

  3. 3 Clean the throttle body

    Remove the intake duct and clean the bore and blade edges with throttle body cleaner and a soft cloth (key off; don’t force the blade against the motor). Heavy carbon at the blade’s resting edge is exactly what makes position control ragged at small openings.

  4. 4 Perform the idle relearn

    After any cleaning, battery disconnect or part replacement: key on without starting for a few seconds, start and idle in park with accessories off until the idle settles, then cycle the key. Some models need a scan-tool relearn instead — check your procedure. Skipping this step is the top cause of “the new throttle body is bad too.”

  5. 5 Check engine grounds

    Locate and clean the main engine-to-frame and engine-to-body grounds (wire-brush the contact faces, retorque). Cheap insurance, and a documented fix for erratic TAC behavior on the trucks.

  6. 6 Replace the throttle body if condemned

    If position data is still erratic with a clean bore, a good connector and solid grounds, the throttle body assembly (motor + sensors, serviced as a unit on most of these) has earned replacement. Use AC Delco/OEM — this is a control component, not a place for the bargain bin — and do the relearn after.

Parts & tools you may need

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Related codes

Frequently asked questions

What does code P1516 mean?
P1516 means the throttle blade on your GM truck or car isn’t where the computer commanded it — the throttle actuator control system flagged the mismatch, usually with a “Reduced Engine Power” message. It’s serious — diagnose it promptly to avoid expensive damage.
Can I drive with P1516 and Reduced Engine Power?
Only to get somewhere safe. Limp mode caps the engine at a crawl — merging, hills and intersections become genuinely risky, and the fault can strike again without warning. Treat the first Reduced Engine Power event as the end of that trip, not an inconvenience to drive through.
Why does the message come and go?
That intermittency is itself a clue. A failed throttle body fails consistently; a fretted connector pin or marginal ground misbehaves with vibration, temperature and humidity — fine for a week, limp mode at a stoplight. Codes that come and go argue for the electrical-connection diagnosis before any parts.
Do I need a relearn after cleaning or replacing the throttle body?
Yes — this is the step that separates fixed trucks from repeat customers. The computer learns the blade’s rest and idle positions; cleaning or replacement changes them. The basic procedure is an idle relearn (key on, start, idle in park until stable, key cycle), with some models requiring a scan tool. Skip it and you get surging idle or the code right back.
Is it the gas pedal sensor instead?
Usually not for this specific code — P1516 points at the throttle body end of the system (actuator position), while pedal-position trouble sets its own codes (P2122-family). If pedal codes appear together with P1516, diagnose the shared items first: power, grounds and the harness.
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