- A
Replace the ONT immediately.
Why wrong: Replacing the ONT is premature; the issue is likely with the physical connection or signal, not the ONT itself.
- B
Check the power supply to the ONT.
Why wrong: If the LOS light is on, the ONT has power; a power issue would show no lights at all.
- C
Inspect and clean the fiber connector.
Dirty or damaged connectors are a common cause of signal loss in fiber; cleaning often restores the link.
- D
Reboot the router connected to the ONT.
Why wrong: Rebooting the router will not fix a missing optical signal; the LOS light indicates a physical layer issue upstream.
Question 377 of 1,020
Fiber ONT Red LOS Light Troubleshooting
This 220-1201 practice question tests your understanding of internet connection types. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
A technician is troubleshooting a fiber optic connection where the ONT (Optical Network Terminal) has a red 'LOS' (Loss of Signal) light. The fiber cable appears intact. What should the technician check next?
Quick Answer
The answer is to inspect and clean the fiber connector. A red LOS light on the ONT means the device is detecting no optical signal, and even if the fiber cable looks intact from the outside, microscopic dust, oil, or scratches on the connector end-face are the most frequent culprits for signal loss. This scenario directly tests your understanding of fiber optic physical layer troubleshooting for the CompTIA A+ Core 1 220-1201 exam, where a common trap is assuming a visually intact cable is fully functional—the real issue is often contamination at the termination point. A useful memory tip is “Red LOS, clean the gloss,” reminding you that a loss of signal almost always starts with a dirty connector before you suspect a broken cable.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Inspect and clean the fiber connector.
A red LOS (Loss of Signal) light on the ONT indicates that no light is being received from the optical line terminal (OLT) at the central office. Since the fiber cable appears intact, the most likely cause is a dirty or damaged connector at the ONT or the wall jack, which can attenuate the optical signal below the receiver threshold. Cleaning the fiber connector with a proper one-click cleaner or inspection scope is the standard next step before considering hardware replacement.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Replace the ONT immediately.
Why it's wrong here
Replacing the ONT is premature; the issue is likely with the physical connection or signal, not the ONT itself.
- ✗
Check the power supply to the ONT.
Why it's wrong here
If the LOS light is on, the ONT has power; a power issue would show no lights at all.
- ✓
Inspect and clean the fiber connector.
Why this is correct
Dirty or damaged connectors are a common cause of signal loss in fiber; cleaning often restores the link.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
Reboot the router connected to the ONT.
Why it's wrong here
Rebooting the router will not fix a missing optical signal; the LOS light indicates a physical layer issue upstream.
Common exam traps
Common exam trap: answer the scenario, not the keyword
A common misconception is that a red LOS light means the ONT is faulty or that rebooting the router will fix the fiber link, when in fact the LOS indicator is purely optical and requires cleaning or verifying the physical connection first.
Trap categories for this question
Command / output trap
If the LOS light is on, the ONT has power; a power issue would show no lights at all.
Detailed technical explanation
How to think about this question
In GPON (Gigabit Passive Optical Network) systems, the ONT uses a burst-mode receiver that expects a continuous downstream signal at 1490 nm from the OLT. Even microscopic dust or oil on the fiber endface can cause back-reflection or insertion loss exceeding the typical -28 dBm receiver sensitivity, triggering LOS. Real-world scenarios often involve contamination from construction dust or improper handling during installation, making physical inspection with a fiber microscope the definitive diagnostic step before escalating to OLT-side troubleshooting.
KKey Concepts to Remember
- Read the scenario before looking for a memorised answer.
- Find the constraint that changes the correct option.
- Eliminate answers that are true in general but not in this case.
TExam Day Tips
- Watch for words such as best, first, most likely and least administrative effort.
- Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A practitioner preparing for the 220-1201 exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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FAQ
Questions learners often ask
What does this 220-1201 question test?
Internet Connection Types — This question tests Internet Connection Types — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Inspect and clean the fiber connector. — A red LOS (Loss of Signal) light on the ONT indicates that no light is being received from the optical line terminal (OLT) at the central office. Since the fiber cable appears intact, the most likely cause is a dirty or damaged connector at the ONT or the wall jack, which can attenuate the optical signal below the receiver threshold. Cleaning the fiber connector with a proper one-click cleaner or inspection scope is the standard next step before considering hardware replacement.
What should I do if I get this 220-1201 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Last reviewed: Jul 4, 2026
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