EN

Severity: Moderate Code type: Generic (all makes) ✱

P2108 Code: Throttle Actuator Control Module Performance

The throttle control module isn't performing as expected – check its power, data link and throttle body.

Unverified: this manufacturer-specific definition comes from a single source or varies by model. Confirm it with a brand-specific scan tool.

At a glance

Severity: 3/5 – Moderate3/5

Severity: Moderate

Meaning
Throttle Actuator Control Module Performance
System
Speed & Idle Control · Powertrain
Safe to drive?
With caution
Typical repair cost
$80–$1,600
DIY difficulty
Hard

Where P2107 reports a processor that fails its self-test, P2108 says the throttle control module as a whole isn't performing correctly: its output doesn't produce the expected throttle response, its internal voltages drift, or its conversation with the main engine computer breaks down. The exact criteria differ a lot between manufacturers, so treat the definition as a broad one. On vehicles with a separate throttle actuator control (TAC) module, the data link and that module's own power and grounds are prime suspects. On vehicles where throttle control sits inside the PCM, supply voltage quality and a throttle body that makes the control loop work too hard are the usual culprits, with the module itself further down the list.

Can I keep driving? Usually drivable with reduced power, but the car can drop into limp mode unexpectedly. Keep trips short, avoid heavy traffic and highway merges, and have the charging system and wiring checked soon.

How to read P2108

How to read P2108P12213084
  1. P Powertrain
  2. 2 Generic (SAE)
  3. 1 Fuel & air metering
  4. 08 Specific fault

Symptoms

  • Intermittent reduced power or limp mode
  • Uneven idle or brief hesitation that comes and goes
  • Throttle or reduced-power warning together with the check engine light
  • Communication codes such as U0100 or low-voltage codes stored alongside
  • Symptoms worse with high electrical load or on rough roads
  • Lights flickering or a whining alternator on some vehicles

Possible causes

Ranked by how often each cause is behind this code (editorial estimate).

  1. Poor supply voltage quality – weak battery, alternator ripple or corroded ground 26%
  2. Sticking or worn throttle body forcing the control loop outside its limits 22%
  3. Faulty data link wiring between PCM and a separate TAC module 16%
  4. Internal fault in the throttle control module or PCM 16%
  5. Software needing an update 12%
  6. Moisture or loose pins in the module connector 8%

Diagnosis step by step

Diagnostic flow: P21081Identify how your vehicle's throttlecontrol is built2Check charging voltage and ripple3Test module power and ground underload4Check the data link on two-modulesystems5Rule out a lazy throttle body6Update or replace the module

  1. Identify how your vehicle's throttle control is built

    Check the service information: is there a separate throttle actuator control module, or is it integrated in the PCM? That decides whether the data link and a second module are on your test list at all.

    Tools: OBD-II scanner

  2. Check charging voltage and ripple

    At idle with headlights and blower on, charging voltage should stay steady around 13.5–14.7 V. Switch the meter to AC volts across the battery; more than about 0.5 V AC suggests failing alternator diodes that can upset module performance.

    Tools: Multimeter

  3. Test module power and ground under load

    Back-probe the module's power and ground pins and measure voltage drop to the battery posts while the engine runs and the throttle moves. Losses above a few tenths of a volt on either side need repair.

    Tools: Multimeter

  4. Check the data link on two-module systems

    With both modules unplugged and ignition off, check the serial data wires between PCM and TAC module for continuity and for shorts to ground or voltage. Wiggle the harness at the module while watching for communication dropouts in live data.

    Tools: Multimeter, OBD-II scanner

  5. Rule out a lazy throttle body

    Compare commanded and actual throttle angle during quick pedal inputs. If the actual angle lags or overshoots, clean the bore and plate with the ignition off, relearn, and see whether the performance code stays away.

    Tools: OBD-II scanner, Soft brush

  6. Update or replace the module

    Once power, grounds, wiring and throttle body are confirmed good, check for a software update. Replace the module only as a last step, and program or relearn it as the manufacturer requires.

    Tools: OBD-II scanner

Where the parts are

Charging system and control modules – P21081234567
Charging system and control modules
  1. 1Battery
  2. 2Alternator
  3. 3Engine computer (ECM)
  4. 4CAN bus
  5. 5Transmission
  6. 6Wheel speed sensor
  7. 7High-voltage battery

Repair options & costs

RepairDIY?Estimated cost
Repair grounds or replace battery Yes $80–$300
Replace alternator No $400–$900
Repair data link or module connector No $120–$400
Clean or replace throttle body No $90–$900
Replace and program throttle control module or PCM No $400–$1,600

Rough US estimates for parts and labor; shop rates vary by region. Not a quote.

Notes by brand

Chevrolet

Many GM trucks and SUVs of the GMT800 generation used a separate throttle actuator control module that talks to the PCM over a dedicated data link. On those vehicles P2108 more often traces back to that module, its connector or its power and ground than to the PCM.

GMC

GMC trucks and SUVs of the GMT800 era share Chevrolet's separate throttle actuator control module. Check its power, grounds and the data link to the PCM before replacing either computer.

Emissions / smog test impact

Will fail

P2108 keeps the check engine light on, so an OBD-based emissions test fails. Monitors reset after module work, so drive normally for several days before retesting.

Frequently asked questions

What's the difference between P2107 and P2108?

P2107 points at the processor failing its internal self-test. P2108 is broader: the module isn't delivering the expected result, which can be caused by supply voltage, wiring, the throttle body or the module itself.

Can a bad alternator cause P2108?

Yes. Electrical ripple from failing diodes or unstable charging voltage can disturb module performance checks. Testing the charging system is quick and should come before any module replacement.

Does my car have a separate throttle control module?

Most current vehicles integrate throttle control into the PCM. Some older designs, notably certain GM trucks, used a separate module; the service information for your vehicle tells you which you have.

Sources