MOSFET failures are one of the most common causes of dead or unstable graphics cards. Unlike core or VRAM faults, MOSFET issues usually affect power delivery, which makes them critical to diagnose early in the troubleshooting process.
This guide focuses on real-world symptoms, measurement methods, and how to isolate faulty MOSFETs on GPU VRM circuits.
Understanding the Role of MOSFETs in GPU
Modern GPUs use multi-phase VRM (Voltage Regulator Module) designs. Each phase consists of:
- High-side MOSFET (switches 12V input)
- Low-side MOSFET (grounds switching node)
- Driver IC
- Inductor (coil)
These components work together to generate stable VCore (typically 0.8V–1.2V). Any MOSFET failure disrupts this switching process.
Common MOSFET Failure Symptoms
1. GPU Completely Dead (No Power)
- No fan spin
- No LED activity
- System does not detect GPU
Typical cause: Shorted high-side MOSFET pulling 12V rail down.
2. Short Circuit on 12V Rail
- PSU shuts down instantly
- Sparks or heating near VRM area
Diagnosis:
- Check resistance between 12V and ground
- Inject low voltage (1V) and observe heat
3. No VCore Output
- GPU powers on (fans spin)
- No display
- VCore = 0V
Possible causes:
- Blown low-side MOSFET
- Driver IC not switching gate
- Missing enable signal
4. Intermittent Boot / Random Shutdown
- GPU works sometimes
- Crashes under load
- System restarts randomly
Typical cause:
- Weak MOSFET (partial failure)
- Thermal instability
5. Overheating MOSFET
- One phase gets abnormally hot
- Burn marks or discoloration
Check:
- Gate voltage waveform
- Compare temperature with other phases
6. Burnt or Cracked MOSFET
- Visible damage
- Blown package
This is a direct failure and requires replacement.
---Step-by-Step MOSFET Testing
1. Resistance Test (Power OFF)
- Drain → Source resistance
- Gate → Source leakage
2. Diode Mode Testing
- Check body diode behavior
- Compare readings with known good phase
3. Gate Signal Check (Power ON)
- Use oscilloscope
- Look for switching waveform
4. Thermal Detection
- Use IPA or thermal camera
- Inject voltage if needed
Real Case Insight
In one GTX 960 repair case, the card showed:
- 12V present
- No VCore
- Enable signal OK
On probing the VRM:
- Gate signal was present
- One low-side MOSFET had abnormal resistance
After replacing the MOSFET, VCore returned and the card POSTed normally.
---Repair Recommendations
- Always replace MOSFET with same spec or equivalent
- Check driver IC before replacing multiple MOSFETs
- Inspect nearby capacitors and resistors
Quick Fault Mapping
- No power → High-side MOSFET short
- No VCore → Low-side MOSFET or driver
- Random shutdown → Weak MOSFET
- Overheating → Switching imbalance