Power supply failure

If the power supply fits the system, it will work (assuming the problem is indeed the PS and not the mainboard, it should solve the problem).

Replacing your Dell OptiPlex 7040 power supply with one from an OptiPlex 3040 might be possible, but it depends heavily on the form factor of both your specific PCs (Mini Tower, Small Form Factor, or Micro).
⚠️ Compatibility Factors
Dell OptiPlex power supplies are often proprietary and non-standard. The two most critical factors for compatibility are the form factor and the main motherboard connector.
 * Form Factor (Size/Shape): The power supply must physically fit in the case and align with the mounting screws.
   * If both your 7040 and 3040 are Mini-Tower (MT) models, they often use a TFX form factor power supply (and sometimes a slightly larger, proprietary one). A MT PSU from a 3040 may fit a 7040 MT.
   * If both are Small Form Factor (SFF) models, they use a small, proprietary PSU (often 180W). A 3040 SFF PSU is often reported to be compatible with a 7040 SFF.
   * If the models are different (e.g., 7040 MT and 3040 SFF), they are unlikely to be compatible due to major differences in size, shape, and mounting.
 * Motherboard Connector: Both the OptiPlex 7040 and 3040 typically use a Dell proprietary 8-pin connector for the motherboard's main power.
   * Crucially, they both generally belong to the same generation (SkyLake/Kaby Lake platforms) that use this specific 8-pin connector (plus a standard 4-pin CPU connector), which increases the likelihood of electrical compatibility.
✅ Conclusion
If your OptiPlex 7040 and the donor OptiPlex 3040 share the same form factor (e.g., both are SFF or both are MT), the power supply is very likely to be a direct physical and electrical replacement. Dell often uses the same power supply unit (PSU) for corresponding form factors across models in the same generation.
You can see a general guide on how to replace a PSU in this model range here: Dell OptiPlex 3040 5040 7040 Power Supply Upgrade. This video demonstrates the physical process of changing the power supply in a similar model, confirming the potential interchangeability of PSUs within this OptiPlex generation.

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