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‐ The “Security Error” screen on an LG-P760 (Optimus L9) is almost always triggered by a corrupted or non-authorised boot image / system partition.
‐ In >90 % of cases the phone can be revived by putting it into Download (Emergency) Mode and re-flashing the correct original KDZ stock firmware with LG Flash Tool (or LGUP + matching DLL).
‐ If Download Mode is not reachable or flashing fails repeatedly at the same address, the eMMC or the security partition is probably damaged; only a JTAG/ISP re-programming or main-board replacement will help.
Key points
• Enter Download Mode (Vol-Up while USB is inserted).
• Install LG USB drivers, launch LG Flash Tool 2014, choose CSE-Flash, load the exact P760 KDZ.
• Keep USB stable, wait for PASS → phone should boot.
• If all flashing attempts fail → likely hardware (eMMC) → professional repair.
What the “Security Error” means
• During the first boot stages the P760 boot ROM checks secure certificates stored in the “sec_sec” and “laf” partitions.
• Any mismatch (bad signature, wrong region, interrupted update, modified recovery) forces the handset into a protected state that shows only “Security Error” and blocks normal Recovery.
Typical root causes
a) Aborted FOTA/PC-Suite update (USB cable pulled, battery empty).
b) Flashing a KDZ meant for a sibling model (P765/P768/P769).
c) Installing a custom ROM without first unlocking the bootloader.
d) Bit-rot of the 8 GB eMMC after >10 years of use.
Observable symptoms
• Boots straight to “Security Error”.
• Hard-reset key combo does nothing.
• Fastboot is usually not accessible on retail L9; only Download Mode is.
Why a factory reset does not help
Factory reset runs from /recovery; because the signed recovery was rejected, neither reset nor ADB can start → full firmware reload is mandatory.
• LG exited the smartphone market in 2021; official “LG Mobile Support Tool” servers for legacy devices are intermittently offline. Community mirrors (lg-firmwares.com, XDA) now host KDZ images.
• For most L-series phones the de-facto standard tool is still LG Flash Tool 2014 v1.8.1.1023; newer LGUP builds require a P760 DLL that is hard to find.
• DIY JTAG boxes (Octoplus, Medusa, RIFF) can resurrect P760 boards, but their cost often exceeds the value of the phone.
Firmware packages
• KDZ – LG’s encrypted container; internally holds a CAB → .dz → individual partitions (modem, system, boot, laf…).
• “CSE-Flash” wipes user data and recreates GPT + Secure partitions, maximising the chance to remove corruption.
• Typical European P760 KDZ: V20B_00.kdz (Android 4.1.2, 720 MB).
Exact flashing sequence (proven working)
Typical errors & fixes
• “Model information Check Fail” – wrong KDZ → download correct region.
• Stuck at 4 % – USB drops; use motherboard USB 2.0 port + short cable.
• Stuck at 49 % every time – bad eMMC block → hardware fault.
• Flashing official KDZ images is legal; distributing modified or copyrighted firmware may violate LG’s EULA.
• Bypassing secure boot on someone else’s handset without consent can breach local cyber-laws.
• Observe E-waste regulations: if the repair is not viable, dispose of the Li-ion battery through an authorised recycler.
Implementation checklist
☐ Verify exact model label under battery (LG-P760, not P765/P768).
☐ Battery ≥50 %.
☐ Windows 7–10 PC, USB 2.0 port, OEM-quality micro-USB cable.
☐ Disable PC firewalls that may block LG Flash Tool’s temp server.
☐ During flash DO NOT: move cable, let laptop sleep, run heavy USB devices.
Potential challenges
• LG driver signing on Windows 11 – use legacy driver install mode.
• KDZ mirror integrity – compare SHA-256 posted on XDA.
Testing the result
• Dial 3845#*760# → SVC menu → Version Info to confirm bootloader & baseband match.
• Run eMMC wear-level test (adb shell cat /sys/block/mmcblk0/device/life_time) – high values (>0x0B) indicate imminent failure.
• Device age (~2012) means success is not guaranteed; if the internal NAND is worn out the phone will relapse into Security Error within days.
• Because LG closed update servers, automatic IMEI-based retrieval no longer works; manual KDZ selection is mandatory.
• Some tool downloads contain adware; use checksummed archives from reputable forums.
• Investigate open-source flashing utilities (kdz-extractor, lgflashcmd) to remove dependence on abandoned Windows tools.
• Study secure-boot implementation on OMAP4-based LG devices for academic purposes.
• Explore replacing the original Android 4.1.2 with a lightweight post-market OS (e.g., Lineage OS 14.1) after a successful stock recovery and bootloader unlock.
Recommended resources
– XDA Developers “LG Optimus L9 (P760) ► Security Error FIX” thread (last updated 2023)
– github.com/emeat/lgflashcmd (command-line KDZ flasher)
– Octoplus JTAG manual for LG P760 board points (PDF)
The “Security Error” on an LG-P760 is a secure-boot failure caused by a corrupted or unauthorised system image. Enter Download Mode, install LG drivers, and perform a CSE Flash with the exact stock KDZ using LG Flash Tool 2014. If flashing repeatedly aborts or Download Mode is inaccessible, the internal eMMC or security IC is likely defective—only hardware re-programming or board replacement can recover the phone. Always use firmware matched to your variant, maintain a stable USB connection, and be aware that LG no longer officially supports the device.
