820-02098 No Backlight Repair – UP800 and QP800 - QP803 Replaced
Detailed board-repair case note for a 2021 14-inch MacBook Pro A2442, logic board 820-02098, with water damage and no backlight. UP800 was replaced first; after the backlight failed again and QP803 was found shorted, all four LUXE MOSFETs were replaced as a set.
- Model: 14-inch MacBook Pro A2442 (2021)
- Board: 820-02098
- Fault: Water damage, dim image only, no backlight
- USB-C meter: 20V / 0.23A
- Final fix: UP800 + QP800/QP801/QP802/QP803
Case Summary
| Device | MacBook Pro 14-inch |
|---|---|
| Model | A2442 |
| Manufacture year | 2021 |
| Logic board | 820-02098 |
| Fault | Water damage, no backlight, dim image only |
| Previous assessment | Apple Store recommended logic board replacement |
| Final repair | Replaced UP800 first, then QP800 / QP801 / QP802 / QP803 |
| Result | Backlight restored and MacBook fully functional |
Final result: 14-inch MacBook Pro backlight restored and working normally.
Initial Symptom
This 2021 14-inch MacBook Pro A2442 came in after water damage. The machine powered on and produced an image, but there was no backlight; only a very dim image could be seen on the LCD. The screen was confirmed good. The customer had already been told by Apple that the logic board needed to be replaced.
The visible liquid damage looked small and was concentrated around the LUXE backlight power area near QP800.
Measurements
| Check | Reading / observation |
|---|---|
| USB-C meter | 20V / 0.23A |
| PPVOUT_LUXE | 0V |
| LCD image | Present, but dim only |
| LCD panel | Confirmed good |
| Second failure | QP803 shorted to ground |
820-02098 logic board: the failed parts had to be identified among thousands of components.
Backlight Circuit Overview
The 820-02098 uses a LUXE 4-switch buck-boost backlight stage: UP800 (RAA209100), LP800, and four MOSFETs QP800/QP801/QP802/QP803. In this case PPBUS_AON was 12.0V, while the backlight rail needed to be slightly higher, so the stage was operating mainly in boost mode.
| FET | Main boost-mode role |
|---|---|
| QP800 | Input-side high-side FET, mostly on |
| QP801 | Input-side low-side FET, mostly off |
| QP802 | Output-side synchronous rectifier, switching |
| QP803 | Output-side low-side boost switch, switching |
When QP803 turns on, current builds in LP800. When QP803 turns off and QP802 turns on, the inductor releases energy into PPVOUT_LUXE, raising the output to 15V. QP803 is therefore one of the most stressed FETs in boost operation.
Water damage around the backlight power section.
Repair Timeline
- Confirmed dim image only and confirmed the LCD panel was good.
- Found local water damage in the LUXE backlight area.
- Replaced UP800 first because it is the controller for all four gate drives.
- Backlight returned and worked for about 10 minutes.
- Backlight failed again; QP803 was then found shorted to ground.
- Replaced QP803, but the backlight still did not return.
- Replaced UP800 again, still no backlight.
- Replaced all four MOSFETs: QP800, QP801, QP802 and QP803.
- Backlight returned and the MacBook remained fully functional.
Why UP800 Was Replaced First
At the first inspection there was no confirmed shorted MOSFET; the only clear symptom was no backlight with local liquid damage in the LUXE section. Since UP800 controls all four gate-drive outputs, it was a reasonable first suspect. Replacing UP800 restored the backlight immediately, which proved that the stage was still capable of operating, but the 10-minute failure showed that another latent fault remained.
Why All Four MOSFETs Were Replaced
After QP803 failed short, replacing only the visibly failed FET was not enough. In a 4-switch synchronous buck-boost stage, the MOSFETs work as one power stage. A remaining FET can be partly damaged, gate-leaky, or fail only under switching stress while still looking acceptable in basic diode-mode tests. After liquid damage and a load-related failure, replacing the complete set was the safer repair.
UP800 and the four LUXE MOSFETs removed during the final successful repair.
Key Lesson
If an 820-02098 / A2442 no-backlight case has local LUXE water damage and UP800 replacement restores backlight only temporarily, suspect latent MOSFET damage. If QP803 later shorts, do not assume QP803 alone is the full fault; replacing QP800/QP801/QP802/QP803 as a set may be required.
Searchable Repair Notes
Useful search terms: 820-02098 no backlight, A2442 no backlight, UP800 RAA209100, QP803 short to ground, PPVOUT_LUXE no backlight, QP800 QP801 QP802 QP803 replacement.
