Many people encounter a confusing situation:
The Ethernet cable passes a cable tester, yet there is still poor internet connection.
This issue affects not only home users, but also distributors, and importers who later receive complaints such as:
“the cable tested fine, but the network doesn’t work well.”
PART 1 Quick Answer
Why does an Ethernet cable pass a test but still have poor internet?
Because most basic cable testers only confirm wire continuity, not whether the cable can reliably transmit data at the required speed.
A “pass”result means the wires are connected — not that the connection is stable or usable.
(Before troubleshooting cabling, external network issues such as ISP outages, faulty routers/modems, or incorrect network settings should first be ruled out.)
PART 2 Specific Technical Causes
1.Cable Test Results May Not Be Reliable
Most basic cable testers only detect wiring continuity, not transmission quality or stability.
They are typically designed to check:
* Whether wire pairs are connected in the correct standard
* Whether there are open circuits or short circuits
* Whether basic electrical continuity is maintained between both ends
However, they cannot detect:
* Signal integrity (SI) issues during high-frequency transmission
* Contact resistance fluctuations caused by oxidation or poor contact
* Crosstalk and impedance mismatch between twisted pairs
* Link stability under high bandwidth, long-distance transmission, or PoE power delivery
As a result, a “pass” result from a basic tester does not ensure the cable can support Gigabit or 10-Gigabit Ethernet.
Many hidden defects only appear after the network is live, during continuous data transmission or PoE operation.
2.Poor Termination of RJ45 Connectors or Keystone Jacks
In real-world cases, this is one of the most common causes of failure — which is often difficult to identify visually.
Typical issues include:
* Contact pins failing to fully pierce the conductor insulation, making contact only with the insulation or conductor surface and creating false continuity
* Crimping tools applying insufficient or mismatched pressure, resulting in weak pin contact
* Using connectors or modules incompatible with solid-core or stranded conductors, causing uneven contact force
In many cases, simply re-terminating the connector immediately resolves the issue.
3. Connector or Module Incompatibility with Cable Category
This issue becomes especially pronounced with higher-category cables such as Cat6, Cat6A, and Cat7.
Different cable categories have significantly different design parameters:
Lower-category cables (e.g., Cat5e) typically use thinner conductors (commonly 24AWG) and have looser structural tolerances
Higher-category cables use thicker conductors (commonly 23AWG or larger), tighter twist ratios, and stricter requirements for connector compatibility and termination precision
If connectors or modules are not designed for the specific cable category, even if they appear similar, several hidden issues may occur:
* Insufficient contact area between pins and conductors
* Mismatch between module slots and cable diameter, leading to uneven stress or loose connections after termination
* Excessive signal loss and crosstalk during high-frequency transmission, directly affecting network stability
4. Installation Environment and Physical Factors
Many failures do not appear immediately after installation, but develop gradually over time.
Common contributing factors include:
* Excessively small bending radius during installation, deforming internal twisted pairs
* Excessive pulling force during cable routing, stretching conductors or damaging insulation
* Temperature cycling in enclosed spaces such as cabinets or ceilings, accelerating contact oxidation and insulation aging
* Continuous vibration from HVAC systems or industrial equipment, causing micro-movement at contact points
These factors lead to the following typical performance issues:
Normal operation at lower speeds (e.g., 100 Mbps), but frequent disconnections or speed drops at Gigabit or higher speeds
Passing short continuity tests but failing after long-term full-load operation
PART 3 Issues Originating from the Cable Itself
In some cases — particularly when manufacturing quality control is insufficient — the cable itself may be the root cause of failure.
While these issues occur less frequently, they are usually more difficult to diagnose.
1. Conductor Material Issues
* Copper-clad aluminum (CCA) conductors instead of solid copper
* Inconsistent copper purity
* Conductor diameter below specification
2. Manufacturing Process Issues
* Uneven twist rates
* Poor pair separation
* Inconsistent insulation thickness
3. Insulation and Shielding Problems
* Deformation under temperature changes
* Moisture ingress after long-term use
4. Other Low-Frequency Causes
* Poor shielding or external electromagnetic interference (EMI)
* Cable runs exceeding the recommended maximum length (100 meters)
* Environmental exposure to heat, humidity, or use of non-aging-resistant PVC/PE jackets
PART 4 Core Conclusion
A “Pass” result from a cable tester is only a basic screening outcome — it does not guarantee network performance.
A truly stable Ethernet connection depends on the entire termination system, not just the cable itself.
When post-installation issues occur, repeatedly replacing cables is often ineffective.
In many cases, checking RJ45 connectors, keystone jacks, and termination quality leads to faster and more accurate problem resolution.