How to Diagnose Intermittent Network Drops from EMI
A technician is troubleshooting a user's desktop computer that intermittently loses network connectivity. The issue occurs only when the building's central air conditioning (HVAC) system turns on. The technician has already replaced the Ethernet cable, the NIC (network interface card), and the switch port. The computer is connected to a surge protector and the power cabling is properly routed. Which of the following is the MOST likely cause of the intermittent connectivity?
Quick Answer
The strongest clue in this question is timing correlation: the network drops precisely when the HVAC system turns on, and that correlation survives even after replacing the cable, the NIC, and the switch port, which eliminates a faulty component as the cause since a new cable, new NIC, and new switch port would not all fail in sync with an unrelated appliance. Large motors, like those in HVAC compressors and blowers, draw a heavy inrush of current when they start, which generates a strong, brief electromagnetic field. If nearby Ethernet cabling is unshielded and running close to that motor or its power wiring, that field can induce enough noise into the twisted-pair cable to corrupt the data signal momentarily, causing exactly the kind of brief connectivity loss described, without any component actually failing. This is a physical-layer interference problem rather than a hardware defect, which is why swapping parts does not resolve it. Whenever network problems correlate tightly with a physical event elsewhere in the building, such as heavy equipment cycling on or off, and hardware replacement does not fix the issue, shift your thinking from defective components toward environmental interference, particularly electromagnetic interference from motors or other high-current devices located near the cabling.
⚠ Common exam trap
Candidates often assume a hardware failure (NIC, cable, switch port) is the cause, but the question explicitly states those have been replaced, forcing you to consider an external environmental factor like EMI.
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
✓
Electromagnetic interference (EMI) from the HVAC motor
The HVAC motor generates a strong electromagnetic field when it starts, which can induce current in nearby unshielded Ethernet cables, corrupting the data signal and causing intermittent connectivity. This is a classic example of electromagnetic interference (EMI), and the fact that the issue correlates precisely with the HVAC system turning on, combined with the replacement of the cable, NIC, and switch port, points directly to an external interference source.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Electromagnetic interference (EMI) from the HVAC motor
Why this is correct
HVAC motors generate electromagnetic interference that couples onto unshielded Ethernet cabling, corrupting frames and causing intermittent link loss synchronised with compressor startup. Replacing cable, NIC, and switch port rules out those components, leaving EMI as the remaining cause.
- ✗
IP address conflict due to DHCP lease expiration
Why it's wrong here
A DHCP lease conflict produces connectivity loss at lease renewal or address assignment, unrelated to HVAC activation, and would recur on a fixed schedule rather than coinciding with compressor startup. It is the right diagnosis when duplicate addresses or exhausted scopes cause repeated failures.
- ✗
Faulty switch port
Why it's wrong here
The technician already replaced the switch port, so it cannot be the remaining cause; HVAC activation points to electrical interference on the power circuit, not the port. A faulty switch port would produce constant link loss on that port regardless of HVAC state, making it the right diagnosis only when swapping ports has not been performed.
- ✗
DNS server unavailability
Why it's wrong here
DNS unavailability prevents name resolution while IP connectivity persists, so pings to addresses would still succeed and the fault would not track HVAC cycling. It is the correct diagnosis when users can reach resources by IP but not by hostname.
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Same concept, more angles
5 more ways this is tested on 220-1101
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A user reports that their desktop computer intermittently loses network connectivity, especially when the nearby office printer starts printing. The link light on the NIC occasionally turns off during these events. The Ethernet cable runs near a fluorescent light fixture and a power cable. The technician has already replaced the Ethernet cable with a known-good Cat6 shielded cable, but the issue persists. Which of the following should the technician do NEXT?
medium- A.Move the computer to a different electrical outlet.
- B.Replace the network interface card (NIC).
- ✓ C.Ensure the Ethernet cable is not routed near sources of electromagnetic interference (EMI).
- D.Update the NIC driver from the manufacturer's website.
Why C: The issue persists even after replacing the cable with a known-good Cat6 shielded cable, indicating that the problem is not the cable itself but the environment. The Ethernet cable runs near a fluorescent light fixture and a power cable, both of which are common sources of electromagnetic interference (EMI). Since the link light turns off when the printer starts, the EMI from the printer or nearby sources is likely inducing noise on the cable, disrupting the signal. The next logical step is to reroute the cable away from these EMI sources, as shielding alone may not be sufficient if the interference is strong or the shield is not properly grounded.
Variation 2. A user reports that their desktop computer intermittently loses network connectivity when the overhead fluorescent lights in the office are turned on. The link light on the network interface card (NIC) turns off during these events. The technician replaced the existing unshielded twisted pair (UTP) Cat5e cable with a shielded Cat6 cable, but the problem continues. Which of the following should the technician do NEXT to resolve the issue?
medium- A.Replace the network switch
- ✓ B.Move the Ethernet cable away from the fluorescent light fixtures
- C.Replace the network interface card (NIC)
- D.Check the grounding of the electrical system in the building
Why B: The problem is electromagnetic interference (EMI) from the fluorescent lights, which generates a strong magnetic field that induces noise in the Ethernet cable. Even with shielded Cat6 cable, the interference can still couple into the signal if the cable is routed too close to the light fixtures, especially if the shielding is not properly grounded. Moving the cable away from the fixtures physically separates the cable from the EMI source, which is the most direct and effective next step.
Variation 3. A user reports that their desktop computer intermittently loses network connectivity for a few seconds at a time. The technician observes that the link light on the NIC turns off briefly and then back on. The technician has already replaced the Ethernet cable and the switch port. The issue persists. Which of the following should the technician check NEXT?
medium- A.Update the NIC driver
- ✓ B.Check for electromagnetic interference near the cable
- C.Replace the network interface card (NIC)
- D.Configure the NIC to use a static IP address
Why B: The link light turning off indicates a physical layer issue, and since the cable and switch port have already been replaced, the next likely cause is electromagnetic interference (EMI) from nearby sources such as power cables, fluorescent lights, or large motors. EMI can corrupt the signal on unshielded twisted-pair (UTP) cabling, causing the NIC to momentarily lose link. Checking for and rerouting cables away from interference sources is the appropriate next step before replacing hardware.
Variation 4. A user reports that their desktop computer loses network connectivity for a few seconds every time they operate a large power tool (e.g., an electric saw) in the same room. The technician has already replaced the Ethernet cable, tested the switch port with another device (which works fine), and updated the NIC driver. Which of the following is the MOST likely cause of the intermittent disconnects?
medium- ✓ A.Electromagnetic interference (EMI) from the power tool affecting the Ethernet cable
- B.Radio frequency interference (RFI) from the power tool's motor
- C.An IP address conflict with the power tool's network interface
- D.The DHCP lease is being forced to renew due to the power tool's operation
Why A: The large power tool generates significant electromagnetic interference (EMI) when operating, which can induce noise in the unshielded Ethernet cable, causing temporary signal degradation and loss of link. Since the technician has already ruled out the cable, switch port, and NIC driver, EMI from the tool is the most likely remaining cause.
Variation 5. A user reports that their desktop computer loses network connectivity intermittently. When the problem occurs, the link light on the network interface card (NIC) is off. The technician has replaced the Ethernet cable and tried a different port on the switch, but the issue persists. The technician notices that the desktop is located in a cubicle next to a large metal filing cabinet. Which of the following is the MOST likely cause of the intermittent connectivity?
medium- ✓ A.Electromagnetic interference (EMI) from the metal cabinet
- B.Faulty router causing periodic DHCP lease failures
- C.Incorrect static IP address configuration on the desktop
- D.Duplicate IP address conflict on the network
Why A: The link light being off indicates a loss of physical layer connectivity. While EMI can cause intermittent link drops, a large metal filing cabinet is a passive object and does not generate electromagnetic interference. The actual cause is more likely a faulty NIC, a damaged cable, or EMI from a nearby electrical source such as a motor or fluorescent light. Therefore, none of the given options accurately describes the most likely cause.
JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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