FAQ
TL;DR: One key cause is lost ECU communication (1 critical path). “Immo will not turn off the car when it is on.” Diagnose wiring/ABS‑ESP links before blaming keys or ECU. [Elektroda, grala1, post #16689847]
Why it matters: This FAQ helps Peugeot 308 owners fix F527/U1308 no‑start issues by prioritizing the actual communication and power faults, not the immobilizer.
Quick Facts
- Typical fault codes: F527 (coded immobilizer) and U1308 (no communication with engine ECU). [Elektroda, bugija, post #16689451]
- Network layout: BSI talks to the engine ECU through the ABS/ESP controller on the CAN lines. [Elektroda, grala1, post #16689888]
- Safety first: Disconnect the battery before unplugging any controller; start checks at ABS/ESP plugs and BSI–ABS wiring. [Elektroda, grala1, post #16689928]
- Test shortcut: You can bridge the two CAN lines at the ABS/ESP plug to bypass the ABS/ESP for a comms test. [Elektroda, grala1, post #16702330]
- Mechanical finding: Two cylinders lacked compression in one case; engine work restored compression. [Elektroda, bugija, post #16746431]
What does the F527 “coded immobilizer” fault actually mean on a 308?
F527 appears when the BSI sees the key transponder but the engine ECU remains blocked. In the thread, keys were recognized and the immobilizer deactivated, yet the engine ECU didn’t communicate. Treat it as a communication/power path problem, not a key issue. [Elektroda, bugija, post #16689451]
Will the immobilizer shut off a running engine?
No. An experienced member stated, “Immo will not turn off the car when it is on.” Focus on why the engine ECU isn’t reachable or powered, because that blocks starting and triggers related injection/ignition errors. [Elektroda, grala1, post #16689847]
Why am I also seeing U1308 “no communication with engine ECU”?
U1308 flags that the network cannot reach the engine ECU. In this platform, that points toward power loss, wiring faults, or a dead ECU on the communication side. Diagnose the network path before replacing modules. [Elektroda, bugija, post #16689451]
How does the BSI talk to the engine ECU on a Peugeot 308?
On these cars the BSI communicates with the engine ECU via the ABS/ESP controller over the CAN bus. A break at ABS/ESP can isolate the ECU, so verify this link early in diagnosis. [Elektroda, grala1, post #16689888]
Where should I start my physical checks for F527/U1308?
Start at the ABS/ESP connector and the engine ECU connector. Look for corrosion, moisture, or damaged locks; a “rotten connector” can break CAN continuity and power. Clean and reseat thoroughly. [Elektroda, dafit906, post #16689511]
Do I need to disconnect the battery before unplugging modules?
Yes. Disconnect the battery before unplugging BSI, ABS/ESP, or the ECU. Then check continuity BSI↔ABS/ESP and ABS/ESP↔ECU, and only reconnect once checks finish. This prevents controller damage. [Elektroda, grala1, post #16689928]
How do I bridge the CAN lines at ABS/ESP to test communication?
With battery disconnected, place two jumpers to connect the CAN‑H and CAN‑L pairs so the BSI talks directly to the engine ECU, bypassing ABS/ESP. Reconnect and scan; restored comms confirm an ABS/ESP path fault. [Elektroda, grala1, post #16702330]
What is CAN bus, and why does it matter here?
CAN bus is a two‑wire network that lets modules share data. In this 308, ABS/ESP sits in the path between BSI and the engine ECU, so a fault there can block starting even with a recognized key. [Elektroda, grala1, post #16702330]
Can I diagnose this with only a multimeter?
Yes. Use a multimeter for continuity checks across BSI–ABS/ESP–ECU pins and to verify 12 V supplies and grounds at ABS/ESP. Advanced tools (Lexia/DiagBox) help, but continuity and power checks come first. [Elektroda, bugija, post #16689862]
I cloned the ECU and the car still won’t start—what next?
If a replacement ECU with copied data acts the same, the fault is likely wiring, power, ABS/ESP path, or mechanical timing. Return to network and power diagnostics before more parts swaps. [Elektroda, bugija, post #16689451]
Could mechanical issues mimic an immobilizer problem?
Yes. One case showed two cylinders with no compression, causing no‑start despite electronics work. Fix the engine, set timing correctly, clear codes, then re‑check electronics. [Elektroda, bugija, post #16746431]
What about timing and variable valve timing (VVT) faults?
A specialist reported hard starts and low power traced to timing/VVT phasing. Once corrected, behavior improved. “Something was wrong with the timing and variable phase system,” they noted. [Elektroda, bugija, post #18942425]
Can a sensor connector cause random stalling and no‑start?
Yes. A loose or dirty crankshaft position sensor plug caused intermittent stalls and no‑starts on a related model. Cleaning and refitting the plug fixed it. Check this edge case early. [Elektroda, szakallo13, post #19437190]
How do I perform a quick 3‑step comms check before replacing parts?
- Disconnect battery; inspect and reseat ABS/ESP and ECU connectors.
- Verify power/ground at ABS/ESP; continuity BSI↔ABS/ESP↔ECU.
- Bridge CAN at ABS/ESP; scan for ECU comms.
If comms return, focus on ABS/ESP path. [Elektroda, grala1, post #16689928]
Why does my scan show many injection/ignition errors with F527?
When the ECU is offline, dependent modules log cascading faults. These are secondary to the main communication loss. Restore ECU communication first, then clear and rescan to see what remains. [Elektroda, bugija, post #16689451]
Which engine is in the VIN VF34C5FWC55235805 mentioned?
A moderator identified it as the 5FW 1.6 VTi engine. This matters for timing/VVT checks and sensor pinouts during diagnostics. [Elektroda, ALIBABA I, post #16754180]
I know that lexi has the option of a multimeter and an oscilloscope, but for this you also need additional accessories
