If your vehicle is showing "service power steering" message displayed on the dashboard, here are the most likely causes and the parts you'll need to fix each one.
Electric Power Steering (EPS) systems draw massive amounts of electrical current. A weak battery, failing alternator, or loose terminal will often trigger a power steering warning message before other electrical symptoms appear due to voltage drops.
If your vehicle uses an electro-hydraulic steering system, low fluid caused by a leaking hose or pump will trigger a warning message and result in heavy steering.
The electric motor that provides steering assist can overheat, wear out, or fail internally over time. This triggers the service message and immediately disables power assist.
The EPS system relies on sensors to know the exact position of the steering wheel. If the sensor fails or loses calibration (very common after a recent wheel alignment or suspension work), the system disables itself for safety.
A power surge, electrical short, or an overworked EPS motor can blow the high-amp fuse or relay protecting the power steering circuit, cutting power to the system entirely.
Catastrophic internal mechanical or electronic failure within the rack and pinion assembly itself will trigger a permanent fault code and require a complete replacement of the rack.