This iMac overheating repair in Stockport began with a frustrating fault: the computer became extremely slow after the customer logged in. Our testing and monitoring revealed a pattern. As the iMac became hot, the macOS process kernel_task showed around 98% CPU usage, and the machine became difficult to use.
A full strip-down, internal dust clean and fresh thermal paste on both the CPU and GPU resolved the slowdown. These workshop photographs show the build-up we found and the difference after cleaning.
The fault: an iMac that became slow after login
The reported problem was poor performance after signing in. That symptom alone does not identify the cause, so we tested the iMac and monitored its behaviour as it warmed up. The important clue was the connection between rising heat, high kernel_task activity and the slowdown.
- Device: Apple iMac.
- Reported fault: severe slowdown after login.
- Finding during testing: approximately 98% CPU usage attributed to kernel_task when the iMac became hot.
- Repair: full strip-down, dust removal and new CPU and GPU thermal paste.
- Outcome: the reported slowdown was resolved.
Why was kernel_task showing high CPU usage?
Seeing kernel_task near the top of Activity Monitor can make the problem look like a runaway application. However, it also has a role in managing processor temperature: macOS can reduce the CPU resources available to demanding processes when thermal conditions require it.
As Apple explains in its kernel_task guidance, this activity can be a response to heat. The process does not itself cause those overheating conditions. In this case, monitoring the iMac pointed us towards investigating the cooling system.
Inside this iMac overheating repair in Stockport
With the iMac opened, we found substantial dust inside the machine. The photographs show deposits around the internal components and a thick accumulation around the removed cooling fan’s outlet. This was more than dust on the outside of the casing.
Dust can restrict airflow through a cooling system. Removing that build-up was one part of the repair; we also renewed the thermal paste on the processor and graphics chip to address heat transfer to their cooling assemblies.
The repair: a full clean and fresh CPU and GPU thermal paste
We fully stripped the iMac down to carry out the internal service. We removed the accumulated dust and applied fresh thermal paste to both the CPU (processor) and GPU (graphics processor), before reassembling the machine.
The photographs below show the fan outlet and internal area after cleaning. The thermal-paste work formed part of the same repair, although it is not shown in these pictures.
The result: the slowdown was resolved
Following the clean and thermal-paste replacement, the problem that brought this iMac into the workshop was fixed. This case shows why observing when a fault appears matters: the slowdown after login was connected to the machine getting hot.
It does not mean every slow iMac needs the same treatment. Storage, software and other hardware faults can produce similar symptoms. Diagnosis helps establish which repair is appropriate for the individual machine.
Is your iMac slow or getting too hot?
Tell us what happens, when it starts and whether it changes as the computer warms up. At AC Computer Warehouse, we offer a free diagnosis and a quote before repair work starts. You can read more about our computer repair services in Stockport or ask us about your iMac repair.
Visit us at Unit 13 Kingston Mill, Chestergate, Stockport SK3 0AL, with free parking outside, or call 0161 282 2548. Our repairs include a six-month warranty.
Frequently asked questions
Can overheating make an iMac slow after login?
Yes. In this repair, the slowdown appeared as the iMac became hot, alongside high kernel_task activity. Other faults can cause similar symptoms, so the behaviour needs to be investigated before choosing a repair.
Does high kernel_task CPU usage mean the processor has failed?
No. High kernel_task activity alone does not establish that the processor has failed. It can be part of macOS managing thermal conditions. On this iMac, an internal clean and fresh CPU and GPU thermal paste resolved the reported problem.
Will cleaning fix every slow iMac?
No. Cleaning helped resolve this particular heat-related fault. A slow iMac can have a different underlying problem, which is why we diagnose the machine and explain the proposed work before going ahead.