---
name: Fault Injection
slug: fault-injection
category: Quality
description: Fault Injection adds and configures faults on Simulink, Simscape, and System Composer signal ports for robustness and safety validation. Use it to simulate stuck, noise, gain, or offset failures and test fault effects.
github: "https://github.com/matlab/simulink-agentic-toolkit/tree/main/skills-catalog/verification-validation-and-test/inject-faults"
language: HTML
stars: 963
forks: 94
install: "npx degit https://github.com/matlab/simulink-agentic-toolkit/tree/main/skills-catalog/verification-validation-and-test/inject-faults ~/.claude/skills/inject-faults"
installs_to: ~/.claude/skills/inject-faults
source_path: skills-catalog/verification-validation-and-test/inject-faults/SKILL.md
collection_size: 24
category_size: 1354
collection_url: "https://dirskills.com/collections/matlab/simulink-agentic-toolkit"
added: 2026-08-21T05:15:18.737Z
last_synced: 2026-08-21T05:15:18.737Z
canonical_url: "https://dirskills.com/skills/fault-injection"
---

# Fault Injection

Fault Injection adds and configures faults on Simulink, Simscape, and System Composer signal ports for robustness and safety validation. Use it to simulate stuck, noise, gain, or offset failures and test fault effects.

**Install:**

```bash
npx degit https://github.com/matlab/simulink-agentic-toolkit/tree/main/skills-catalog/verification-validation-and-test/inject-faults ~/.claude/skills/inject-faults
```

## README

# Fault Injection

Use `evaluate_matlab_code` with the `Simulink.fault` API to programmatically inject faults into Simulink models. Faults override signal values at block ports to simulate hardware failures, sensor faults, and actuator malfunctions.

## When to Use

- Injecting sensor stuck-at faults, noise, or bias for safety analysis (Simulink, Simscape, System Composer)
- Validating FMEA failure modes through simulation
- Testing controller robustness against actuator or sensor failures
- Adding fault placeholders (without behavior) for FMEA traceability before defining fault logic
- Configuring fault trigger conditions (timed, conditional, always-on)
- Creating fault scenarios for automated testing
- Comparing nominal vs. faulted behavior
- Linking faults to FMEA failure mode cells
- Generating fault specification reports
- Exporting models with embedded fault behavior for teams without Simulink Fault Analyzer
- Registering custom fault behavior libraries
- Synchronizing (also called **resync** / **sync** / **synchronize faults** / **update fault references**) fault information from referenced models, subsystems, or linked libraries into a parent model

## When NOT to Use

- Creating FMEA spreadsheets or safety analysis documents -> use `manage-safety-analysis`
- Building or editing model structure -> use `building-simulink-models`
- Running standard (non-faulted) simulations -> use `simulating-simulink-models`
- Writing pass/fail tests -> use `testing-simulink-models`

## Probe

```matlab
hasFA = ~isempty(which('Simulink.fault.addFault'));
fprintf('Simulink Fault Analyzer available: %d\n', hasFA);
```

## Key Concepts

- **Fault**: A modification applied to a block port (inport or outport) that overrides the signal value.
- **Fault without Behavior**: A placeholder that documents a failure point but cannot be simulated.
- **Fault with Behavior**: A fault with an associated behavior model that can be simulated (see `references/fault-behaviors.md`).
- **Fault Model**: The `.slx` file containing fault behavior subsystem(s).
- **Trigger**: When the fault activates (Always On, Timed, Conditional, Manual).
- **Fault Info File**: `<ModelName>_faultInfo.xml` stores all fault metadata alongside the model.

## Critical Constraints

1. **Faults attach to block PORTS, not blocks.** You must get the port handle via `get_param(blockPath, 'PortHandles')`.
2. **NEVER directly edit `_faultInfo.xml` files.** Always use MATLAB APIs to modify fault definitions.
3. **Fault names must be valid MATLAB identifiers** (no spaces, slashes, or special characters). Use camelCase or PascalCase.
4. **The fault behavior model name (first arg to `addBehavior`) must also be a valid MATLAB identifier.**
5. **NEVER use `Simulink.fault.deleteAll` or `Simulink.fault.deleteFault` to toggle faults on/off for simulation.** Deleting faults destroys fault objects, their behavior models (.slx files), and their traceability links permanently. Use `Simulink.fault.injection(model, false)` to disable ALL fault injection for a nominal run, and `Simulink.fault.injection(model, true)` to re-enable. Use `Simulink.fault.enable(modelElement, false)` to disable faults on specific elements.
6. **Not supported:** Faults inside Stateflow charts, fault models, or observer models.

## Workflow

1. **Understand the model.** Use `model_overview` and `model_read` to identify fragile signal paths -- single-point actuators, sensor feedback, computed quantities with wide fan-out.
2. **Pre-flight check.** Query compiled port properties to confirm signal compatibility. If the model is not already compiled, ask the user before compiling. See `references/preflight-and-adding-faults.md`.
3. **Get port handles.** Faults attach to port handles, not block paths. Use `get_param(blk, 'PortHandles')`.
4. **Add fault.** Call `Simulink.fault.addFault(portHandle, Name=..., Description=...)`. See `references/preflight-and-adding-faults.md`.
5. **Add behavior.** Call `addBehavior(fault, modelName, FaultBehavior='mwfaultlib/...')`. See `references/fault-behaviors.md`.
6. **Configure trigger.** Set `fault.TriggerType` and the corresponding properties (e.g., `StartTime` for Timed, `Conditional` for Conditional, `TriggerActive` for Manual). See `references/triggers-and-conditionals.md`.
7. **Save.** Call `Simulink.fault.save('ModelName')`.
8. **Simulate.** Enable injection with `Simulink.fault.injection(mdl, true)` then `sim(...)`. See `references/simulation-workflows.md`.

## Guardrails

### Always
- Use `model_read` to identify target blocks before injecting faults
- Check compiled port info before creating faults (see `references/preflight-and-adding-faults.md`)
- Add behavior BEFORE setting trigger properties
- Save with `Simulink.fault.save('ModelName')` after any fault changes
- Verify fault is active before running faulted simulation with `activate(fault)`
- Disable fault injection after analysis with `Simulink.fault.injection(mdl, false)`
- Verify test input signals are non-zero at fault injection points
- Check bus compatibility before choosing a `mwfaultlib` behavior

### Never
- Use the `Simulink.fault.Fault()` constructor -- use `Simulink.fault.addFault()` instead
- Pass library block paths (e.g., `mwfaultlib/Stuck-at-Ground`) as the fault model name argument to `addBehavior` -- use the `FaultBehavior` name-value pair
- Assume fault simulation is off -- verify before each nominal run
- Violate any item in the Critical Constraints section

### Ask First
- Before modifying the design model for any reason (adding blocks, changing sample times, reconnecting signals)
- Before enabling faults on a model that may be shared (it modifies model behavior)
- Before deleting existing faults (the behavior model .slx is also deleted)
- Before enabling `fault.Persistent = true` -- it makes the fault irreversible during simulation

## Troubleshooting

| Issue | Cause | Fix |
|-------|-------|-----|
| `Assertion failed` in `Simulink.fault.Fault()` | Do not use the `Fault()` constructor directly | Use `Simulink.fault.addFault(portHandle, Name=..., Description=...)` instead |
| `Fault name must follow MATLAB variable naming conventions` | Name contains spaces or special chars | Use camelCase/PascalCase with no spaces (e.g., `SensorStuckAtZero`) |
| `Invalid name for a fault model` | Behavior model name has invalid characters | Use a simple MATLAB identifier (e.g., `StuckAtZero`, not `Stuck-at-Zero`) |
| `Unable to add fault... only to block input or output ports` | Passed a block path/handle instead of port handle | Use `ph = get_param(blockPath, 'PortHandles'); ph.Outport(1)` |
| `Target signal is a continuous-time signal` | Fault Analyzer cannot inject on continuous signals going to continuous blocks | See Continuous-Time Signal Decision Tree in `references/advanced-operations.md` |
| `Output dimensions do not match` or `Data type mismatch` | Fault constant size/type ≠ target signal | See "Vector and Matrix Fault Values" in `references/fault-behaviors.md` |
| Bus signal fault fails with non-Ground behavior | Most `mwfaultlib` behaviors are scalar-only | Use `Stuck-at-Ground` for full bus, or use `BusElement` to target individual elements |
| Multiple faults on same port but only one fires | Only one fault per port is active at a time | Use `activate(fault)` to select, or use different `StartTime` values |
| Fault exists but simulation runs nominally | Fault injection not enabled or fault not active | Call `Simulink.fault.injection('Model', true)` and `activate(f)` |

## Reference Files

| File | Contents |
|------|----------|
| `references/preflight-and-adding-faults.md` | Port inspection, compile checks, adding faults, port handle resolution, fault object properties/methods |
| `references/fault-behaviors.md` | Built-in behaviors, custom behaviors, bus faults, vector/matrix faults, discovery, data inport source |
| `references/triggers-and-conditionals.md` | Trigger types, conditional creation, conditional/symbol object properties |
| `references/simulation-workflows.md` | Running simulations, nominal vs. faulted comparison, selective fault isolation |
| `references/advanced-operations.md` | FMEA linking, reports, export, deletion, continuous-time workarounds, synchronizing faults from referenced models/subsystems/libraries |
| `references/test-case-integration.md` | Adding fault sets to Simulink Test cases (R2024a+) |

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Copyright 2026 The MathWorks, Inc.

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