In real-world electronics, fixed resistors can fail after stress or aging. What is the most common failure mode observed for fixed resistors when they are overstressed or reach end-of-life?

Introduction / Context:
Fixed resistors are widely used as passive current limiters and voltage dividers. Understanding how they typically fail helps technicians diagnose faults quickly and helps designers choose proper power ratings and derating margins to improve reliability.

Given Data / Assumptions:

• We consider common fixed resistor technologies such as carbon film, metal film, metal oxide, and wirewound.
• The stressors include excessive power dissipation, overvoltage transients, thermal cycling, and environmental factors.
• We focus on the usual failure mode, not rare corner cases.

Concept / Approach:
When a resistor is overstressed (P_actual > P_rated or sustained high temperature), the resistive element can crack, burn, or separate from end caps. The typical electrical result is loss of continuity (open circuit), since the film or wire breaks or lead termination fails. Although resistance drift upward can precede failure, the final catastrophic state is usually open rather than short.

Step-by-Step Solution:
Identify stress: excess power causes temperature rise.Thermal damage: resistive film or wire weakens, micro-cracks form.Separation: conductor path breaks or solder/end-cap joint fails.Electrical symptom: measured resistance goes to extremely high value (open).

Verification / Alternative check:
In-circuit, the open resistor stops current flow. Out-of-circuit DMM measurement reads very high/OL. Visual inspection often shows discoloration or cracking. Reliability literature and field repair experience consistently report “goes open” as the dominant end state for overheated fixed resistors.

Why Other Options Are Wrong:

• Slowly over time: Descriptive of drift, not the predominant ultimate failure mode.
• By increasing their value: Upward drift can occur, but the most common failure is open.
• Increase then open: Possible, but the single most typical mode asked by the question is simply “open.”

Common Pitfalls:

• Confusing temporary thermal coefficient changes with permanent damage.
• Expecting shorts: shorts are rare in fixed resistors; opens dominate.

Final Answer:
by becoming an open circuit