U1720

Code

Generic U — Network/User

CAN Bus message invalid or corrupted

Brand: Generic

Type: U — Network/User

Views: UK: 25 EN: 27 RU: 19

AI status

Completed

ready

Completed 100%

Page language: EN

Causes

Corroded, loose, or damaged connectors or wiring on the CAN high/low wires
Failed or malfunctioning control module sending corrupted frames
Missing, open, or incorrect termination resistor(s) on the CAN bus
Low or unstable battery/vehicle voltage during communication
High electromagnetic interference (EMI) or grounding issue causing bit errors
Software mismatch, corrupted firmware, or protocol configuration error

Symptoms

Communication-related trouble codes and MIL illumination
Intermittent or permanent loss of operation of systems on the affected CAN network (e.g., ABS, instrument cluster, transmission)
Modules not present or unavailable on a diagnostic scan tool
Random or repeated communication timeouts, bus-off conditions, or chatter
No-crank or no-start when essential modules lose messages

What to check

Read all network-related trouble codes and note freeze-frame/test data; check for multiple modules reporting errors
Visually inspect CAN wiring, connectors, and grounds for damage, corrosion, or loose pins
Measure battery voltage and charging system health; ensure stable supply during tests
Measure DC resistance between CAN_H and CAN_L with ignition off (approx. 60 ohm if two 120Ω terminators in parallel) and to vehicle ground/ignition voltage to detect shorts
Use a scan tool to verify which modules are present and whether any report bus-off or error counters
Capture CAN waveforms with a dual-channel oscilloscope to look for noise, missing recessive levels, clipped signals, or incorrect bit timing

Signal parameters

Recessive idle voltage: CAN_H ≈ 2.5 V, CAN_L ≈ 2.5 V (both lines near mid-supply)
Dominant bus state: CAN_H ≈ 3.5 V, CAN_L ≈ 1.5 V (differential ~2.0 V)
Typical differential voltage when dominant: ~1.5–2.5 V
Termination resistance across CAN_H and CAN_L: approx. 60 Ω (two 120 Ω terminators in parallel) measured with ignition off
Common bus speeds: 125 kbps, 250 kbps, 500 kbps (verify vehicle-specific baud rate)

Diagnostic algorithm

Step 1 — Scan and document: Read all codes on every module, note if multiple modules report the same U1720 or related network faults.
Step 2 — Battery & grounds: Verify battery voltage >12.4 V (at rest) and good engine/chassis grounds; charge if low and retest.
Step 3 — Visual inspection: Inspect CAN connectors, junctions, and harness routing for damage, exposed conductors, or water intrusion; repair as needed.
Step 4 — Termination check: With ignition off, measure resistance between CAN_H and CAN_L at a convenient connector; expect ~60 Ω. If not, locate missing/open/shorted terminator.
Step 5 — Isolation: If termination and wiring appear normal, disconnect non-critical modules one at a time (ignition off) and clear codes, then cycle ignition to see if the fault clears to identify a faulty node.
Step 6 — Voltage/waveform test: With ignition on, use a multimeter and then an oscilloscope across CAN_H/CAN_L to check recessive/dominant voltages and waveform quality; look for clipped signals, excessive noise, or repeated error frames.
Step 7 — Repair/replace: Repair damaged wiring or connectors. If a specific module is identified as corrupting the bus, reprogram or replace the module per manufacturer procedure.
Step 8 — Reprogramming: If software mismatch or corrupted firmware is suspected, update or reflash module software and clear codes.
Step 9 — Verify: After repair, clear codes, perform smoke/road test and verify no reoccurrence. Re-scan all modules.

Likely causes

Poor connector or wiring at a nearby module (most common)
Open or shorted CAN_H or CAN_L wiring between modules
Faulty module intermittently driving the bus with invalid frames
Incorrect/missing termination (should be ~120 ohm total or two 60 ohm in parallel)
Low battery or poor chassis ground affecting transceiver operation

Fault status

⚠️ Status

CAN bus message received but invalid or corrupted; indicates data frame errors, bit timing/voltage problems, or a node transmitting malformed CAN messages.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 1.0-3.0 hours