Toggle Side Panel

Shopping Cart

No products in the basket.

Shopping Cart

No products in the basket.

Close search
Home / DTC / U0121 — A dead battery, A faulty ABS module, A problem with ABS module circuit, A problem with the CAN bus

U0121 — A dead battery, A faulty ABS module, A problem with ABS module circuit, A problem with the CAN bus

Detailed page for trouble code U0121.

34,332codes
59brands
11,841generic
22,491specific
All codes P-codes (Powertrain) B-codes (Body) C-codes (Chassis) U-codes (Network/User) Tesla Alerts
Reset
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

Brand: AUDI
Type: U — Network/User
AI status
Completed
ready
Completed 100%
Page language: EN

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

  1. Connect a scan tool, read and save freeze frame and all U-codes. Note whether ABS module appears on network.
  2. Verify battery state of charge and charging system. If battery is low, charge or replace and retest—low voltage can cause network dropouts.
  3. Inspect and test fuses/relays feeding ABS module; restore power if faulty and re-scan.
  4. Visually inspect ABS module connector and harness for corrosion, loose pins, or mechanical damage. Repair as needed.
  5. With ignition ON, measure power and ground at ABS module connector pins to confirm proper supply.
  6. Measure CAN_H and CAN_L at the ABS connector with a multimeter (idle and while messages are sent). Compare to expected levels.
  7. 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.
  8. If multimeter inconclusive, use a lab scope to view CAN differential signals while exercising network functions to find noise, collisions or bus-off events.
  9. Wiggle test harness while monitoring bus and scan tool for intermittent communication restoration. Repair wiring/clips as found.
  10. 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.
  11. 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

U0100 U0073 U0122 U0140 U0155
Repair manuals

Manual library for AUDI

1,160

Browse 1,160 AUDI manuals: repair procedures, diagnostics, wiring diagrams, component locations, service data and Labor Times by year, model and trim.

AUDI

Your experience will help others
+100 karma for a short comment :)
Keep it short and practical. Up to 500 characters.
Send to email
Code

U0121

Generic U — Network/User

Lost Communication With Anti-Lock Brake System (ABS) Control Module A

Brand: Generic
Type: U — Network/User
AI status
Completed
ready
Completed 100%
Page language: EN

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

  1. Connect a scan tool, read and save freeze frame and all U-codes. Note whether ABS module appears on network.
  2. Verify battery state of charge and charging system. If battery is low, charge or replace and retest—low voltage can cause network dropouts.
  3. Inspect and test fuses/relays feeding ABS module; restore power if faulty and re-scan.
  4. Visually inspect ABS module connector and harness for corrosion, loose pins, or mechanical damage. Repair as needed.
  5. With ignition ON, measure power and ground at ABS module connector pins to confirm proper supply.
  6. Measure CAN_H and CAN_L at the ABS connector with a multimeter (idle and while messages are sent). Compare to expected levels.
  7. 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.
  8. If multimeter inconclusive, use a lab scope to view CAN differential signals while exercising network functions to find noise, collisions or bus-off events.
  9. Wiggle test harness while monitoring bus and scan tool for intermittent communication restoration. Repair wiring/clips as found.
  10. 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.
  11. 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

U0100 U0073 U0122 U0140 U0155
7,097

The library contains 7,097 repair and diagnostic manuals. Choose a brand to open the full manual tree by year, model and trim.

AUDI
1,160
DAEWOO
75
FIAT
26
GENESIS
207
HUMMER
138
HYUNDAI
371
ISUZU
86
LAND ROVER
320
LEXUS
263
LINCOLN
166
MERCEDES-BENZ
856
MERCURY
296
MITSUBISHI
406
PORSCHE
136
TESLA
58
TOYOTA
2,042
TRIUMPH
131
VOLKSWAGEN
139
VOLVO
221
Your experience will help others
+100 karma for a short comment :)
Keep it short and practical. Up to 500 characters.
Send to email
Code

U0121

HYUNDAI U — Network/User

Lost Communication With Anti-Lock Brake System (ABS) Control Module

Brand: HYUNDAI
Type: U — Network/User
AI status
Completed
ready
Completed 100%
Page language: EN

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

  1. Connect a scan tool, read and save freeze frame and all U-codes. Note whether ABS module appears on network.
  2. Verify battery state of charge and charging system. If battery is low, charge or replace and retest—low voltage can cause network dropouts.
  3. Inspect and test fuses/relays feeding ABS module; restore power if faulty and re-scan.
  4. Visually inspect ABS module connector and harness for corrosion, loose pins, or mechanical damage. Repair as needed.
  5. With ignition ON, measure power and ground at ABS module connector pins to confirm proper supply.
  6. Measure CAN_H and CAN_L at the ABS connector with a multimeter (idle and while messages are sent). Compare to expected levels.
  7. 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.
  8. If multimeter inconclusive, use a lab scope to view CAN differential signals while exercising network functions to find noise, collisions or bus-off events.
  9. Wiggle test harness while monitoring bus and scan tool for intermittent communication restoration. Repair wiring/clips as found.
  10. 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.
  11. 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

U0100 U0073 U0122 U0140 U0155
371

Browse 371 HYUNDAI manuals: repair procedures, diagnostics, wiring diagrams, component locations, service data and Labor Times by year, model and trim.

HYUNDAI

Your experience will help others
+100 karma for a short comment :)
Keep it short and practical. Up to 500 characters.
Send to email
Code

U0121

LAND ROVER U — Network/User

Lost communication with the ABS control module

Brand: LAND ROVER
Type: U — Network/User
AI status
Completed
ready
Completed 100%
Page language: EN

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

  1. Connect a scan tool, read and save freeze frame and all U-codes. Note whether ABS module appears on network.
  2. Verify battery state of charge and charging system. If battery is low, charge or replace and retest—low voltage can cause network dropouts.
  3. Inspect and test fuses/relays feeding ABS module; restore power if faulty and re-scan.
  4. Visually inspect ABS module connector and harness for corrosion, loose pins, or mechanical damage. Repair as needed.
  5. With ignition ON, measure power and ground at ABS module connector pins to confirm proper supply.
  6. Measure CAN_H and CAN_L at the ABS connector with a multimeter (idle and while messages are sent). Compare to expected levels.
  7. 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.
  8. If multimeter inconclusive, use a lab scope to view CAN differential signals while exercising network functions to find noise, collisions or bus-off events.
  9. Wiggle test harness while monitoring bus and scan tool for intermittent communication restoration. Repair wiring/clips as found.
  10. 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.
  11. 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

U0100 U0073 U0122 U0140 U0155
320

Browse 320 LAND ROVER manuals: repair procedures, diagnostics, wiring diagrams, component locations, service data and Labor Times by year, model and trim.

LAND ROVER

Your experience will help others
+100 karma for a short comment :)
Keep it short and practical. Up to 500 characters.
Send to email
Code

U0121

OPEL U — Network/User

CAN-Bus no Communication with ABS/TC

Brand: OPEL
Type: U — Network/User
AI status
Completed
ready
Completed 100%
Page language: EN

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

  1. Connect a scan tool, read and save freeze frame and all U-codes. Note whether ABS module appears on network.
  2. Verify battery state of charge and charging system. If battery is low, charge or replace and retest—low voltage can cause network dropouts.
  3. Inspect and test fuses/relays feeding ABS module; restore power if faulty and re-scan.
  4. Visually inspect ABS module connector and harness for corrosion, loose pins, or mechanical damage. Repair as needed.
  5. With ignition ON, measure power and ground at ABS module connector pins to confirm proper supply.
  6. Measure CAN_H and CAN_L at the ABS connector with a multimeter (idle and while messages are sent). Compare to expected levels.
  7. 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.
  8. If multimeter inconclusive, use a lab scope to view CAN differential signals while exercising network functions to find noise, collisions or bus-off events.
  9. Wiggle test harness while monitoring bus and scan tool for intermittent communication restoration. Repair wiring/clips as found.
  10. 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.
  11. 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

U0100 U0073 U0122 U0140 U0155
Your experience will help others
+100 karma for a short comment :)
Keep it short and practical. Up to 500 characters.
Send to email

Report

Harassment or bullying behavior
Contains mature or sensitive content
Contains misleading or false information
Contains abusive or derogatory content
Contains spam, fake content or potential malware

Block Member?

Please confirm you want to block this member.

You will no longer be able to:

  • See blocked member's posts
  • Mention this member in posts
  • Invite this member to groups
  • Message this member
  • Add this member as a connection

Please note: This action will also remove this member from your connections and send a report to the site admin. Please allow a few minutes for this process to complete.

Report

You have already reported this .