U0043 DTC: CAN Bus Communication Bus (+) Low

Diagnostic Trouble Code (DTC) U0043 indicates that the vehicle has detected a low-voltage condition on the CAN Bus (+) communication circuit. Depending on the manufacturer, this may be described as CAN High Low Voltage, CAN Bus (+) Low, or a communication bus fault.

What Does U0043 Mean?

Modern vehicles use the Controller Area Network (CAN) bus to allow modules such as the ECM, TCM, ABS, BCM, SRS, and instrument cluster to exchange data. When the CAN High (CAN+) line remains lower than its expected voltage, communication becomes unstable or stops completely, causing U0043 to be stored.

The problem may be intermittent or permanent depending on whether the fault is caused by damaged wiring, a short circuit, excessive network load, or a faulty control module.

Affected Systems

• Engine Control Module (ECM/PCM)
• Transmission Control Module (TCM)
• ABS Module
• Body Control Module (BCM)
• SRS Airbag Module
• Instrument Cluster
• Power Steering Module
• All CAN Bus Communication Networks

Typical CAN Bus Circuit Diagram

Battery +12V
│
▼ ECM / Gateway Module
│
├── CAN High (CAN+)
├── CAN Low (CAN−)
│
├──────── ABS
├──────── BCM
├──────── TCM
├──────── SRS
└──────── Instrument Cluster

Typical CAN Connector Pinout

Pin 16 → Battery Power (B+)

Pin 4 → Chassis Ground

Pin 5 → Signal Ground

Pin 6 → CAN High (CAN+)

Pin 14 → CAN Low (CAN−)

Note: The pinout above represents the standard OBD-II DLC connector. Internal module connector pin numbers differ between manufacturers.

Common Symptoms

• Check Engine Light ON.
• Multiple warning lights on the dashboard.
• No communication with one or more modules.
• ABS warning light.
• Airbag warning light.
• Transmission shifting problems.
• Intermittent starting or no-start condition.
• Loss of scan tool communication.

Possible Causes

• Short to ground on the CAN High circuit.
• Damaged CAN bus wiring.
• Corroded connectors.
• Water intrusion into a control module.
• Faulty control module pulling the network voltage low.
• Poor power supply or ground connection.
• Incorrect aftermarket electronic equipment installation.
• Damaged CAN gateway module.

Driving Risk Level

Risk Level: HIGH

Depending on which modules lose communication, the vehicle may experience limited performance, disabled safety systems, transmission problems, or even a no-start condition.

Diagnostic Flow Chart

Scan All Modules
↓ Read Communication Codes
↓ Inspect CAN Wiring
↓ Measure CAN Voltage
↓ Check 60 Ω Network Resistance
↓ Disconnect Suspected Modules
↓ Locate Fault
↓ Repair & Verify

Oscilloscope Analysis

Normal CAN High
_/‾\_/‾\_/‾\_/‾\_

Normal CAN Low
‾\_/‾\_/‾\_/‾\_/‾

U0043 (CAN High Low Voltage)
____________________

Short to Ground
____________________

Short to Battery
‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾

Waveform Analysis

• Normal: CAN High and CAN Low mirror each other with stable differential communication.
• U0043: CAN High remains below its expected operating voltage, causing communication failures.
• Short to Ground: CAN High stays near 0 V with no valid data frames.
• Short to Battery: CAN High remains near battery voltage and communication stops.

Oscilloscope Setup

• Channel A → CAN High (Pin 6).
• Channel B → CAN Low (Pin 14).
• Ground → Battery Negative.
• Voltage Scale → 1 V/div.
• Time Base → 20–100 µs/div.
• Compare both channels simultaneously for mirrored waveforms.

Scanner Live Data

• CAN Bus Status
• Network Communication State
• ECM Communication Status
• ABS Communication Status
• BCM Communication Status
• TCM Communication Status
• SRS Communication Status
• Gateway Module Status
• Battery Voltage
• Ignition Voltage

CAN Bus Voltage Test (Typical Values)

Test Item	Typical Value
Battery Voltage	12.0–12.8 V (Engine OFF)
Charging Voltage	13.5–14.8 V (Engine Running)
CAN High (Idle)	≈2.5 V
CAN High (Communication)	≈2.5–3.5 V
CAN Low (Idle)	≈2.5 V
CAN Low (Communication)	≈2.5–1.5 V
CAN Network Resistance (Ignition OFF)	Approximately 60 Ω

Professional Diagnostic Tips

• Scan every control module instead of checking only the ECM.
• Record all communication codes before clearing them.
• Measure battery voltage before diagnosing the CAN network.
• Inspect connectors for corrosion, water intrusion, or bent terminals.
• If CAN High is low, disconnect modules one at a time until communication returns.
• Check aftermarket alarms, GPS trackers, radios, or remote starters connected to the CAN network.
• Verify CAN termination resistance before replacing any control module.

CAN Resistance Test

Ignition OFF
Measure between DLC Pin 6 and Pin 14

Normal Network
≈ 60 Ω

120 Ω
One terminating resistor missing or open circuit

0–10 Ω
Short circuit in CAN wiring

Infinite Resistance
Open CAN circuit

Common Repair Mistakes

• Replacing the ECM before testing the CAN network.
• Ignoring low battery voltage.
• Checking only one module instead of the entire network.
• Skipping resistance measurements.
• Installing a replacement module without confirming it is causing the fault.
• Clearing DTCs before saving Freeze Frame data.

Recommended Diagnostic Tools

• Professional OBD-II Scanner
• Digital Multimeter (DMM)
• Two-Channel Oscilloscope
• CAN Bus Breakout Box
• CAN Bus Analyzer
• Battery Load Tester
• Factory Wiring Diagrams

Common Repairs

• Repair damaged CAN High wiring.
• Repair short-to-ground faults.
• Repair damaged connectors.
• Replace corroded terminals.
• Repair poor power or ground connections.
• Replace the faulty control module pulling CAN High low.
• Repair or replace the CAN Gateway module if required.

Estimated Repair Cost

Repair	Estimated Cost
Network Diagnostic	$80–$180
Connector Repair	$50–$200
CAN Wiring Repair	$100–$400
Gateway Module Replacement	$250–$900
Control Module Replacement	$300–$1,500+

Typical Fault Example

Normal
CAN High = Active Communication
CAN Low = Active Communication

U0043
CAN High Voltage Drops
Communication Becomes Intermittent
Multiple U-Codes May Be Stored

How to Diagnose DTC U0043 Step by Step

1. Connect a professional scan tool and perform a complete vehicle network scan. Record all stored U-codes and Freeze Frame data.
2. Verify battery voltage and charging system operation before diagnosing the CAN network.
3. Inspect the CAN High wiring for damaged insulation, pinched wires, corrosion, or loose connectors.
4. Measure CAN High voltage with the ignition ON. Compare the reading with manufacturer specifications.
5. Turn the ignition OFF and measure resistance between DLC Pin 6 (CAN High) and Pin 14 (CAN Low). A healthy network is typically close to 60 Ω.
6. If the resistance or voltage is incorrect, disconnect one control module at a time while monitoring the CAN network. If communication returns after disconnecting a specific module, that module or its wiring may be causing the fault.
7. Inspect CAN connectors for moisture, bent pins, corrosion, or poor terminal contact.
8. Repair or replace damaged wiring, connectors, or the faulty control module as required.
9. Clear all DTCs and perform a road test. Confirm that U0043 does not return and verify communication with every control module.

Related DTC Codes

• U0001 – High Speed CAN Communication Bus
• U0002 – High Speed CAN Communication Bus Performance
• U0003 – High Speed CAN Communication Bus (+) Open
• U0040 – Vehicle Communication Bus
• U0041 – CAN Bus Communication Bus Performance
• U0042 – CAN Bus Communication Bus (+) Open
• U0043 – CAN Bus Communication Bus (+) Low
• U0073 – Control Module Communication Bus Off

Frequently Asked Questions (FAQ)

Can I drive with DTC U0043?

It depends on which control module is affected. Some vehicles remain drivable, while others may experience transmission problems, disabled ABS or SRS functions, reduced engine performance, or even a no-start condition.

What is the most common cause of U0043?

The most common causes are a short to ground on the CAN High circuit, damaged wiring, corroded connectors, or a faulty control module pulling the CAN bus voltage low.

Can a weak battery cause U0043?

Yes. Low battery voltage or unstable charging system operation can cause communication errors and multiple U-codes.

Why are several U-codes stored together?

Because a single CAN network fault can interrupt communication with multiple control modules at the same time.

Should I replace the ECM first?

No. Always verify battery condition, CAN voltage, CAN resistance, wiring integrity, and network communication before replacing any control module.

OEM Notes

• GM: Inspect the High-Speed CAN network and isolate any module pulling CAN High voltage below specification.
• Ford: Check gateway module operation and inspect wiring near the left kick panel and engine compartment.
• Toyota/Lexus: Verify CAN junction connectors and inspect for corrosion or water intrusion.
• Volkswagen/Audi: Gateway module faults are common sources of multiple communication DTCs.

Safety Notes

• Turn the ignition OFF before measuring CAN network resistance.
• Never inject external voltage into CAN communication circuits.
• Use high-impedance test equipment to avoid damaging control modules.
• Disconnect the battery when performing major wiring repairs.

Technical Notes

• U0043 is a generic communication code, but the exact diagnostic procedure varies by manufacturer.
• Normal CAN High and CAN Low signals should mirror each other during communication.
• Always diagnose the complete CAN network instead of focusing on a single module.
• Consult the factory wiring diagram for connector locations, splice points, and network topology.

Conclusion

DTC U0043 indicates that the CAN High communication line is operating below its expected voltage. The fault is commonly caused by damaged wiring, a short to ground, connector corrosion, or a control module that is loading the network. A systematic diagnosis using a scan tool, multimeter, and oscilloscope will accurately identify the root cause and restore reliable CAN communication.