Mikuni STVA C28 Repair Service

The C28 fault code is a widespread issue affecting Suzuki GSX-R 600, 750, and 1000 models fitted with Mikuni throttle bodies. This fault indicates a problem with the Secondary Throttle Valve Actuator (STVA) system, which controls the secondary throttle plates to deliver smooth and responsive power. When the STVA system fails, the FI warning light appears on the dashboard and the bike’s performance becomes noticeably reduced.

Riders commonly report poor throttle response, hesitation under acceleration, uneven idle, or the bike feeling sluggish or unresponsive at low RPM. In some cases, the GSX-R may enter a limp or reduced-power mode, making it difficult to ride normally. The C28 fault can appear intermittently or remain constant until the issue is professionally repaired.

• FI warning light illuminated on the dashboard

• Poor throttle response or delayed acceleration

• Hesitation or flat spots during throttle input

• Uneven or unstable idle speed

• Reduced engine performance or limp mode activation

• Intermittent loss of power that worsens over time

• Secondary throttle plates not moving during ignition self-test

• Engine feels sluggish or unresponsive at low RPM

If your Suzuki GSX-R or Suzuki GSR is showing a C28 fault code or STVA fault, it’s likely affected by a common issue within the Mikuni throttle body system used across many models. This fault has been widely reported by riders and is most common on the following bikes:

• Suzuki GSX-R 600 K3 – K6 (2003–2006)

• Suzuki GSX-R 750 K3 – K6 (2003–2006)

• Suzuki GSX-R 1000 K3 – K6 (2003–2006)

• Suzuki GSX-R 1000 K7 – K8 (2007–2008) – occasionally affected by similar STVA issues

• Suzuki GSR 600 (2006–2011)

• Suzuki GSR 750 (2011–2016)

These models all share a similar Secondary Throttle Valve Actuator (STVA) design, which can trigger the C28 fault code when the actuator system fails to operate correctly.

Do you have an STVA fault but your bike is not listed? No problem — please get in touch to see how we can help. Our technicians can often repair and test STVA systems from a wide range of Suzuki motorcycles.

At VCUS, we provide professional testing and repair services for Suzuki GSX-R and GSR Secondary Throttle Valve Actuators (STVA) affected by the C28 fault code. With over 18 years of experience in electronic control unit diagnostics and repair, we use advanced testing methods to identify and resolve the underlying fault — restoring full throttle control and eliminating the FI warning light. By repairing your original STVA, we retain all existing ECU coding, avoiding the need for costly reprogramming or setup.

Follow the simple steps below to have your Suzuki GSX-R or GSR STVA professionally repaired by VCUS:

• Remove the STVA unit from your motorcycle.

• Complete our TEST FORM and select a convenient day for Free-of-Charge collection. Please ensure the STVA is securely packaged in a sturdy box. Alternatively, you can send it directly to us at:

  VCUS
  Vehicle Control Unit Solutions
  27 St Michaels Square
  Bramcote, Nottingham
  NG9 3HG

• Once we receive your STVA, our technicians will test, diagnose, and repair the fault. You’ll receive a detailed test report, lifetime warranty, and invoice via email — with payment required only after the repair is completed.

• Your repaired actuator will be returned via For Next Day Delivery.

• Refit your fully functional STVA to your motorcycle — no additional programming or calibration required.