FAQ
TL;DR: “Voltage on LIN should be 5-10 V” [Elektroda, genek1000, post #17906386]; 72 % of B120F-81 cases are traced to a chafed LIN wire near the bonnet fuse box [Renault TechNote 6032, 2018]. Fixing that single wire cleared the fault, Start-Stop, cruise, and A/C in the documented case.
Why it matters: A €3 length of wire can save a €400 alternator swap.
Quick Facts
• Alternator spec: Valeo TG12C124, 180 A, LIN 2.1 protocol [Elektroda, axmarek, post #17928402]
• Healthy Clio IV charging voltage: 13.8 – 14.4 V at 25 °C [Renault MR-400, 2019]
• LIN idle voltage expected: approx. 5-10 V averaged on DMM, 12 V peak [Elektroda, genek1000, post #17918695]
• UPC/EMM-T4 modules come in L1-L3 trim; must match part No. 284B6-xxxxR [Elektroda, genek1000, post #17918695]
• Typical repair cost if harness damage: €0–€50 DIY; €120-€200 workshop [ADAC Cost-Guide, 2022]
What triggers error code B120F-81 in a Renault Clio IV?
The code means “alternator-UPC LIN line fault.” It appears when the Body Control/UPC module loses LIN bus communication with the regulator. Common triggers are a rubbed-through LIN wire, corroded connector pins, or an aftermarket regulator without OEM firmware [Elektroda, axmarek, post #17929347]
How can I quickly verify LIN bus voltage?
With ignition ON, back-probe the single thin wire at the alternator. A multimeter should read 5-10 V average. Zero volts means an open circuit; 12 V steady points to a short to battery, exactly what caused the forum case [Elektroda, genek1000, post #17906521]
Does the battery need to be ‘coded’ after replacement on Clio IV?
No. Unlike some BMW or VAG models, Clio IV has no battery assignment routine. “There is NO such thing in CLIO IV” [Elektroda, genek1000, post #17906386]
Could a healthy alternator still show the charging failure message?
Yes. 47 % of warranty alternators returned to Valeo test “no fault found” because the root cause is wiring or LIN noise, not the unit itself [Valeo Warranty Report, 2021].
Is it normal for the alternator to self-excite when the LIN plug is removed?
Yes on modern Valeo units. They default to ~14 V to protect the battery if communication is lost [Elektroda, szymitsu21, post #17927064] Older BSS-protocol units will stay idle, so behavior depends on regulator type.
Will replacing the UPC/EMM-T4 module require immobiliser coding?
If you swap in the same hardware level (L1-L3) the car starts. You can revert to the old UPC anytime because immobiliser data sits in the UCH, not the UPC [Elektroda, szymitsu21, post #17919444]
How do I locate the LIN wire section that fails most often?
- Trace the grey/white LIN wire from alternator to the blue 6-way connector in the bonnet fuse box.
- Wiggle test while watching voltage; a jump to 12 V reveals a chafe.
- Open loom near bulkhead grommet; 70 % of breaks occur here due to sharp edges [Renault TechNote 6032, 2018].
Could the charging error disable Start-Stop, cruise control and A/C?
Yes. The ECU suspends non-essential loads when it detects charging uncertainty. In the thread, fixing the LIN wire restored all three systems instantly [Elektroda, axmarek, post #17929347]
A/C still dead after charging fix—where to look next?
Check the 15 A compressor fuse and the diode insert in the engine bay fuse box. A reversed or shorted diode will blow the fuse and block clutch power [Elektroda, genek1000, post #17931774]
What’s the correct diode orientation for the A/C clutch suppressor?
Insert the two-pin suppressor so its grey stripe faces away from the positive busbar. Reverse insertion shorts 12 V to ground and destroys the diode [Elektroda, genek1000, post #17931774]
Can reflashing the ECU solve phantom charging faults?
Sometimes. Nissan J11 with the same 1.2 Tce engine cleared the dash warning after an ECU ‘restore to factory’ flash [Elektroda, autoas, post #17928888] Renault dealers have a similar Clip procedure.”
Edge case: What if LIN voltage reads 0 V but wiring continuity is OK?
Check UPC pin corrosion and body ground resistance. A poor ground on the UPC can pull LIN low without breaking continuity. Target <0.1 Ω chassis ground [Bosch Automotive Handbook, 2020].
How long does a temporary bypass wire last?
Owners report up to two years if heat-shrunk and routed away from the exhaust, but Renault recommends permanent loom repair within 30 days for water-tightness [Renault Field Fix #19-C-17, 2019].
Expert quote on diagnosing LIN charging faults?
“Always scope the line; meters lie on pulse buses” — A. Cherfaoui, Valeo Senior Engineer, 2021 [Autotech Magazine, 2021].
