Code
U0121
AUDI
U — Network/User
A dead battery, A faulty ABS module, A problem with ABS module circuit, A problem with the CAN bus
Views:
UK: 37
EN: 87
RU: 107
AI status
Completed
Completed
100%
Causes
- Dead or very low vehicle battery / poor battery connections
- Blown fuse or faulty ABS module power supply
- ABS control module failure
- Open/short or high-resistance in ABS module wiring or connectors
- CAN bus wiring fault (short to ground/power, open, or missing termination)
- Faulty gateway or central electronics module interrupting bus
Symptoms
- ABS warning lamp illuminated (may also trigger traction/stability lights)
- Loss of ABS/traction control or limited braking system functionality
- Other modules/reporting devices show 'module not present' for ABS
- No ABS data available on diagnostic tool / ‘communication error’
- Possible multiple network-related warning lamps
What to check
- Read and record U0121 and any other network codes with a factory-level scanner
- Check battery voltage at rest and during cranking (>12.4 V at rest; >11.5 V while cranking recommended)
- Inspect battery terminals and ground straps for corrosion and tightness
- Check relevant fuses and relays for ABS and central electronics
- Scan for presence of ABS module on the CAN bus (module should appear in scanner)
- Measure CAN_H and CAN_L voltages at an accessible connector with ignition ON
Signal parameters
- Battery/resting supply voltage: typically 12.4–12.8 V (acceptable >12.0 V)
- Cranking voltage: should remain above ~11.5 V
- CAN idle voltages (approx): CAN_H ≈ 2.4–2.6 V, CAN_L ≈ 2.4–2.6 V
- CAN dominant state (message active): CAN_H ≈ 3.5 V, CAN_L ≈ 1.5 V
- CAN bus termination resistance: approx. 60 Ω measured between CAN_H and CAN_L (two 120 Ω resistors in parallel)
- Open or shorted CAN will typically show abnormal voltages or infinite/very low resistance
Diagnostic algorithm
- Connect a scan tool, read and save freeze frame and all U-codes. Note whether ABS module appears on network.
- Verify battery state of charge and charging system. If battery is low, charge or replace and retest—low voltage can cause network dropouts.
- Inspect and test fuses/relays feeding ABS module; restore power if faulty and re-scan.
- Visually inspect ABS module connector and harness for corrosion, loose pins, or mechanical damage. Repair as needed.
- With ignition ON, measure power and ground at ABS module connector pins to confirm proper supply.
- Measure CAN_H and CAN_L at the ABS connector with a multimeter (idle and while messages are sent). Compare to expected levels.
- Measure resistance between CAN_H and CAN_L with ignition OFF; confirm approx. 60 Ω. If not, isolate sections of the bus to find missing or extra terminator.
- If multimeter inconclusive, use a lab scope to view CAN differential signals while exercising network functions to find noise, collisions or bus-off events.
- Wiggle test harness while monitoring bus and scan tool for intermittent communication restoration. Repair wiring/clips as found.
- If wiring/power/grounds are good and bus signals look correct but module still not responding, consider replacing or reflashing the ABS module—only after confirming network/wiring integrity.
- After any repair, clear codes and perform functional tests and road test to confirm normal communication and ABS operation.
Likely causes
- Battery discharged or poor battery terminal connection
- Blown fuse or relay supplying ABS
- Damaged ABS module connector (corrosion, bent pins)
- CAN_H / CAN_L wiring shorted or open, missing 60 Ω termination
- Failed ABS module or internal CAN transceiver
Fault status
Status
Lost communication with ABS control module. Possible causes: dead battery, ABS module power/ground/fuse issue, damaged ABS harness or connector, or CAN bus fault.
Repair difficulty: Medium
Diagnostic time: 1.5-4.0 hours
Similar codes
Workshop Manuals
Repair manuals for AUDI
11
6-speed manual gearbox 0B1, front-wheel drive — Workshop Manual (Edition 05.2014)
Workshop ManualAudi A3 (1997) – 1.6L 4-cylinder (2‑valve) Engine Mechanical Components Service Manual (AEH, AKL, APF) – Edition 07.2002
Workshop ManualAUDI A3 (2004) Workshop Manual — 2.0L FSI Turbo (4‑cyl, 4‑valve) Engine, Mechanics — Edition 03.2017
Workshop ManualAudi A3 2004 — Electrical System (Workshop Manual, Edition 02.2018)
Workshop ManualAudi A4 / A4 Cabriolet – 4.2 l V8 (5‑valve, timing chains) – Workshop Manual (Mechanics) – Edition 04.2007
Workshop ManualAudi A4 / A4 Cabriolet — Auxiliary Heater Workshop Manual (Edition 08.2004)
Workshop ManualAudi A4 / A4 Cabriolet (1.8T 4‑cyl turbo) — Motronic Injection & Ignition System Service Manual (Edition 01.2015)
Workshop ManualAudi A8 (2003) — Electrical System Workshop Manual (Edition 08.2014)
Workshop ManualAudi Q4 e-tron (Type F4) - Self-study Programme SSP 685
Workshop ManualAudi Q8 (2018) — Electrical System Workshop Manual (Edition 05.2019)
Workshop ManualAudi Servicing Manual — 7‑Speed Dual Clutch Transmission 0CJ / 0CL / 0CK / 0DN / 0DP / 0HL (Edition 05.2018)
Workshop ManualYour experience will help others
+100 karma for a short comment :)
Was this AI description helpful?
Your feedback helps improve AI descriptions.
👍 Like
0
👎 Dislike
0
Send to email
Code
U0121
Generic
U — Network/User
Lost Communication With Anti-Lock Brake System (ABS) Control Module A
Views:
UK: 39
EN: 124
RU: 152
AI status
Completed
Completed
100%
Causes
- Dead or very low vehicle battery / poor battery connections
- Blown fuse or faulty ABS module power supply
- ABS control module failure
- Open/short or high-resistance in ABS module wiring or connectors
- CAN bus wiring fault (short to ground/power, open, or missing termination)
- Faulty gateway or central electronics module interrupting bus
Symptoms
- ABS warning lamp illuminated (may also trigger traction/stability lights)
- Loss of ABS/traction control or limited braking system functionality
- Other modules/reporting devices show 'module not present' for ABS
- No ABS data available on diagnostic tool / ‘communication error’
- Possible multiple network-related warning lamps
What to check
- Read and record U0121 and any other network codes with a factory-level scanner
- Check battery voltage at rest and during cranking (>12.4 V at rest; >11.5 V while cranking recommended)
- Inspect battery terminals and ground straps for corrosion and tightness
- Check relevant fuses and relays for ABS and central electronics
- Scan for presence of ABS module on the CAN bus (module should appear in scanner)
- Measure CAN_H and CAN_L voltages at an accessible connector with ignition ON
Signal parameters
- Battery/resting supply voltage: typically 12.4–12.8 V (acceptable >12.0 V)
- Cranking voltage: should remain above ~11.5 V
- CAN idle voltages (approx): CAN_H ≈ 2.4–2.6 V, CAN_L ≈ 2.4–2.6 V
- CAN dominant state (message active): CAN_H ≈ 3.5 V, CAN_L ≈ 1.5 V
- CAN bus termination resistance: approx. 60 Ω measured between CAN_H and CAN_L (two 120 Ω resistors in parallel)
- Open or shorted CAN will typically show abnormal voltages or infinite/very low resistance
Diagnostic algorithm
- Connect a scan tool, read and save freeze frame and all U-codes. Note whether ABS module appears on network.
- Verify battery state of charge and charging system. If battery is low, charge or replace and retest—low voltage can cause network dropouts.
- Inspect and test fuses/relays feeding ABS module; restore power if faulty and re-scan.
- Visually inspect ABS module connector and harness for corrosion, loose pins, or mechanical damage. Repair as needed.
- With ignition ON, measure power and ground at ABS module connector pins to confirm proper supply.
- Measure CAN_H and CAN_L at the ABS connector with a multimeter (idle and while messages are sent). Compare to expected levels.
- Measure resistance between CAN_H and CAN_L with ignition OFF; confirm approx. 60 Ω. If not, isolate sections of the bus to find missing or extra terminator.
- If multimeter inconclusive, use a lab scope to view CAN differential signals while exercising network functions to find noise, collisions or bus-off events.
- Wiggle test harness while monitoring bus and scan tool for intermittent communication restoration. Repair wiring/clips as found.
- If wiring/power/grounds are good and bus signals look correct but module still not responding, consider replacing or reflashing the ABS module—only after confirming network/wiring integrity.
- After any repair, clear codes and perform functional tests and road test to confirm normal communication and ABS operation.
Likely causes
- Battery discharged or poor battery terminal connection
- Blown fuse or relay supplying ABS
- Damaged ABS module connector (corrosion, bent pins)
- CAN_H / CAN_L wiring shorted or open, missing 60 Ω termination
- Failed ABS module or internal CAN transceiver
Fault status
Status
Lost communication with ABS control module. Possible causes: dead battery, ABS module power/ground/fuse issue, damaged ABS harness or connector, or CAN bus fault.
Repair difficulty: Medium
Diagnostic time: 1.5-4.0 hours
Similar codes
Workshop Manuals
Available brands with manuals
2
AUDI 11
6-speed manual gearbox 0B1, front-wheel drive — Workshop Manual (Edition 05.2014)
Workshop ManualAudi A3 (1997) – 1.6L 4-cylinder (2‑valve) Engine Mechanical Components Service Manual (AEH, AKL, APF) – Edition 07.2002
Workshop ManualAUDI A3 (2004) Workshop Manual — 2.0L FSI Turbo (4‑cyl, 4‑valve) Engine, Mechanics — Edition 03.2017
Workshop ManualAudi A3 2004 — Electrical System (Workshop Manual, Edition 02.2018)
Workshop ManualAudi A4 / A4 Cabriolet – 4.2 l V8 (5‑valve, timing chains) – Workshop Manual (Mechanics) – Edition 04.2007
Workshop ManualAudi A4 / A4 Cabriolet — Auxiliary Heater Workshop Manual (Edition 08.2004)
Workshop ManualAudi A4 / A4 Cabriolet (1.8T 4‑cyl turbo) — Motronic Injection & Ignition System Service Manual (Edition 01.2015)
Workshop ManualAudi A8 (2003) — Electrical System Workshop Manual (Edition 08.2014)
Workshop ManualAudi Q4 e-tron (Type F4) - Self-study Programme SSP 685
Workshop ManualAudi Q8 (2018) — Electrical System Workshop Manual (Edition 05.2019)
Workshop ManualAudi Servicing Manual — 7‑Speed Dual Clutch Transmission 0CJ / 0CL / 0CK / 0DN / 0DP / 0HL (Edition 05.2018)
Workshop ManualLAND ROVER 3
Land Rover Defender 300Tdi — Workshop Manual (1996 model year)
Workshop ManualLand Rover Defender Workshop Manual Supplement & Body Repair Manual (1999 & 2002 MY)
Workshop ManualLand Rover Range Rover — Electrical Library (LRL 0453ENG, 2002)
Workshop ManualYour experience will help others
+100 karma for a short comment :)
Was this AI description helpful?
Your feedback helps improve AI descriptions.
👍 Like
0
👎 Dislike
0
Send to email
Code
U0121
HYUNDAI
U — Network/User
Lost Communication With Anti-Lock Brake System (ABS) Control Module
Views:
UK: 26
EN: 98
RU: 76
AI status
Completed
Completed
100%
Causes
- Dead or very low vehicle battery / poor battery connections
- Blown fuse or faulty ABS module power supply
- ABS control module failure
- Open/short or high-resistance in ABS module wiring or connectors
- CAN bus wiring fault (short to ground/power, open, or missing termination)
- Faulty gateway or central electronics module interrupting bus
Symptoms
- ABS warning lamp illuminated (may also trigger traction/stability lights)
- Loss of ABS/traction control or limited braking system functionality
- Other modules/reporting devices show 'module not present' for ABS
- No ABS data available on diagnostic tool / ‘communication error’
- Possible multiple network-related warning lamps
What to check
- Read and record U0121 and any other network codes with a factory-level scanner
- Check battery voltage at rest and during cranking (>12.4 V at rest; >11.5 V while cranking recommended)
- Inspect battery terminals and ground straps for corrosion and tightness
- Check relevant fuses and relays for ABS and central electronics
- Scan for presence of ABS module on the CAN bus (module should appear in scanner)
- Measure CAN_H and CAN_L voltages at an accessible connector with ignition ON
Signal parameters
- Battery/resting supply voltage: typically 12.4–12.8 V (acceptable >12.0 V)
- Cranking voltage: should remain above ~11.5 V
- CAN idle voltages (approx): CAN_H ≈ 2.4–2.6 V, CAN_L ≈ 2.4–2.6 V
- CAN dominant state (message active): CAN_H ≈ 3.5 V, CAN_L ≈ 1.5 V
- CAN bus termination resistance: approx. 60 Ω measured between CAN_H and CAN_L (two 120 Ω resistors in parallel)
- Open or shorted CAN will typically show abnormal voltages or infinite/very low resistance
Diagnostic algorithm
- Connect a scan tool, read and save freeze frame and all U-codes. Note whether ABS module appears on network.
- Verify battery state of charge and charging system. If battery is low, charge or replace and retest—low voltage can cause network dropouts.
- Inspect and test fuses/relays feeding ABS module; restore power if faulty and re-scan.
- Visually inspect ABS module connector and harness for corrosion, loose pins, or mechanical damage. Repair as needed.
- With ignition ON, measure power and ground at ABS module connector pins to confirm proper supply.
- Measure CAN_H and CAN_L at the ABS connector with a multimeter (idle and while messages are sent). Compare to expected levels.
- Measure resistance between CAN_H and CAN_L with ignition OFF; confirm approx. 60 Ω. If not, isolate sections of the bus to find missing or extra terminator.
- If multimeter inconclusive, use a lab scope to view CAN differential signals while exercising network functions to find noise, collisions or bus-off events.
- Wiggle test harness while monitoring bus and scan tool for intermittent communication restoration. Repair wiring/clips as found.
- If wiring/power/grounds are good and bus signals look correct but module still not responding, consider replacing or reflashing the ABS module—only after confirming network/wiring integrity.
- After any repair, clear codes and perform functional tests and road test to confirm normal communication and ABS operation.
Likely causes
- Battery discharged or poor battery terminal connection
- Blown fuse or relay supplying ABS
- Damaged ABS module connector (corrosion, bent pins)
- CAN_H / CAN_L wiring shorted or open, missing 60 Ω termination
- Failed ABS module or internal CAN transceiver
Fault status
Status
Lost communication with ABS control module. Possible causes: dead battery, ABS module power/ground/fuse issue, damaged ABS harness or connector, or CAN bus fault.
Repair difficulty: Medium
Diagnostic time: 1.5-4.0 hours
Similar codes
Your experience will help others
+100 karma for a short comment :)
Was this AI description helpful?
Your feedback helps improve AI descriptions.
👍 Like
0
👎 Dislike
0
Send to email
Code
U0121
LAND ROVER
U — Network/User
Lost communication with the ABS control module
Views:
UK: 11
EN: 60
RU: 54
AI status
Completed
Completed
100%
Causes
- Dead or very low vehicle battery / poor battery connections
- Blown fuse or faulty ABS module power supply
- ABS control module failure
- Open/short or high-resistance in ABS module wiring or connectors
- CAN bus wiring fault (short to ground/power, open, or missing termination)
- Faulty gateway or central electronics module interrupting bus
Symptoms
- ABS warning lamp illuminated (may also trigger traction/stability lights)
- Loss of ABS/traction control or limited braking system functionality
- Other modules/reporting devices show 'module not present' for ABS
- No ABS data available on diagnostic tool / ‘communication error’
- Possible multiple network-related warning lamps
What to check
- Read and record U0121 and any other network codes with a factory-level scanner
- Check battery voltage at rest and during cranking (>12.4 V at rest; >11.5 V while cranking recommended)
- Inspect battery terminals and ground straps for corrosion and tightness
- Check relevant fuses and relays for ABS and central electronics
- Scan for presence of ABS module on the CAN bus (module should appear in scanner)
- Measure CAN_H and CAN_L voltages at an accessible connector with ignition ON
Signal parameters
- Battery/resting supply voltage: typically 12.4–12.8 V (acceptable >12.0 V)
- Cranking voltage: should remain above ~11.5 V
- CAN idle voltages (approx): CAN_H ≈ 2.4–2.6 V, CAN_L ≈ 2.4–2.6 V
- CAN dominant state (message active): CAN_H ≈ 3.5 V, CAN_L ≈ 1.5 V
- CAN bus termination resistance: approx. 60 Ω measured between CAN_H and CAN_L (two 120 Ω resistors in parallel)
- Open or shorted CAN will typically show abnormal voltages or infinite/very low resistance
Diagnostic algorithm
- Connect a scan tool, read and save freeze frame and all U-codes. Note whether ABS module appears on network.
- Verify battery state of charge and charging system. If battery is low, charge or replace and retest—low voltage can cause network dropouts.
- Inspect and test fuses/relays feeding ABS module; restore power if faulty and re-scan.
- Visually inspect ABS module connector and harness for corrosion, loose pins, or mechanical damage. Repair as needed.
- With ignition ON, measure power and ground at ABS module connector pins to confirm proper supply.
- Measure CAN_H and CAN_L at the ABS connector with a multimeter (idle and while messages are sent). Compare to expected levels.
- Measure resistance between CAN_H and CAN_L with ignition OFF; confirm approx. 60 Ω. If not, isolate sections of the bus to find missing or extra terminator.
- If multimeter inconclusive, use a lab scope to view CAN differential signals while exercising network functions to find noise, collisions or bus-off events.
- Wiggle test harness while monitoring bus and scan tool for intermittent communication restoration. Repair wiring/clips as found.
- If wiring/power/grounds are good and bus signals look correct but module still not responding, consider replacing or reflashing the ABS module—only after confirming network/wiring integrity.
- After any repair, clear codes and perform functional tests and road test to confirm normal communication and ABS operation.
Likely causes
- Battery discharged or poor battery terminal connection
- Blown fuse or relay supplying ABS
- Damaged ABS module connector (corrosion, bent pins)
- CAN_H / CAN_L wiring shorted or open, missing 60 Ω termination
- Failed ABS module or internal CAN transceiver
Fault status
Status
Lost communication with ABS control module. Possible causes: dead battery, ABS module power/ground/fuse issue, damaged ABS harness or connector, or CAN bus fault.
Repair difficulty: Medium
Diagnostic time: 1.5-4.0 hours
Similar codes
Workshop Manuals
Repair manuals for LAND ROVER
3
Land Rover Defender 300Tdi — Workshop Manual (1996 model year)
Workshop ManualLand Rover Defender Workshop Manual Supplement & Body Repair Manual (1999 & 2002 MY)
Workshop ManualLand Rover Range Rover — Electrical Library (LRL 0453ENG, 2002)
Workshop ManualYour experience will help others
+100 karma for a short comment :)
Was this AI description helpful?
Your feedback helps improve AI descriptions.
👍 Like
0
👎 Dislike
0
Send to email
Code
U0121
OPEL
U — Network/User
CAN-Bus no Communication with ABS/TC
Views:
UK: 4
EN: 27
RU: 24
AI status
Completed
Completed
100%
Causes
- Dead or very low vehicle battery / poor battery connections
- Blown fuse or faulty ABS module power supply
- ABS control module failure
- Open/short or high-resistance in ABS module wiring or connectors
- CAN bus wiring fault (short to ground/power, open, or missing termination)
- Faulty gateway or central electronics module interrupting bus
Symptoms
- ABS warning lamp illuminated (may also trigger traction/stability lights)
- Loss of ABS/traction control or limited braking system functionality
- Other modules/reporting devices show 'module not present' for ABS
- No ABS data available on diagnostic tool / ‘communication error’
- Possible multiple network-related warning lamps
What to check
- Read and record U0121 and any other network codes with a factory-level scanner
- Check battery voltage at rest and during cranking (>12.4 V at rest; >11.5 V while cranking recommended)
- Inspect battery terminals and ground straps for corrosion and tightness
- Check relevant fuses and relays for ABS and central electronics
- Scan for presence of ABS module on the CAN bus (module should appear in scanner)
- Measure CAN_H and CAN_L voltages at an accessible connector with ignition ON
Signal parameters
- Battery/resting supply voltage: typically 12.4–12.8 V (acceptable >12.0 V)
- Cranking voltage: should remain above ~11.5 V
- CAN idle voltages (approx): CAN_H ≈ 2.4–2.6 V, CAN_L ≈ 2.4–2.6 V
- CAN dominant state (message active): CAN_H ≈ 3.5 V, CAN_L ≈ 1.5 V
- CAN bus termination resistance: approx. 60 Ω measured between CAN_H and CAN_L (two 120 Ω resistors in parallel)
- Open or shorted CAN will typically show abnormal voltages or infinite/very low resistance
Diagnostic algorithm
- Connect a scan tool, read and save freeze frame and all U-codes. Note whether ABS module appears on network.
- Verify battery state of charge and charging system. If battery is low, charge or replace and retest—low voltage can cause network dropouts.
- Inspect and test fuses/relays feeding ABS module; restore power if faulty and re-scan.
- Visually inspect ABS module connector and harness for corrosion, loose pins, or mechanical damage. Repair as needed.
- With ignition ON, measure power and ground at ABS module connector pins to confirm proper supply.
- Measure CAN_H and CAN_L at the ABS connector with a multimeter (idle and while messages are sent). Compare to expected levels.
- Measure resistance between CAN_H and CAN_L with ignition OFF; confirm approx. 60 Ω. If not, isolate sections of the bus to find missing or extra terminator.
- If multimeter inconclusive, use a lab scope to view CAN differential signals while exercising network functions to find noise, collisions or bus-off events.
- Wiggle test harness while monitoring bus and scan tool for intermittent communication restoration. Repair wiring/clips as found.
- If wiring/power/grounds are good and bus signals look correct but module still not responding, consider replacing or reflashing the ABS module—only after confirming network/wiring integrity.
- After any repair, clear codes and perform functional tests and road test to confirm normal communication and ABS operation.
Likely causes
- Battery discharged or poor battery terminal connection
- Blown fuse or relay supplying ABS
- Damaged ABS module connector (corrosion, bent pins)
- CAN_H / CAN_L wiring shorted or open, missing 60 Ω termination
- Failed ABS module or internal CAN transceiver
Fault status
Status
Lost communication with ABS control module. Possible causes: dead battery, ABS module power/ground/fuse issue, damaged ABS harness or connector, or CAN bus fault.
Repair difficulty: Medium
Diagnostic time: 1.5-4.0 hours
Similar codes
Your experience will help others
+100 karma for a short comment :)
Was this AI description helpful?
Your feedback helps improve AI descriptions.
👍 Like
0
👎 Dislike
0
Send to email