U0618

Code

Generic U — Network/User

Lost Communication With Engine Coolant Bypass Valve D

Brand: Generic

Type: U — Network/User

Views: UK: 29 EN: 39 RU: 30

AI status

Completed

ready

Completed 100%

Page language: EN

Open, shorted or damaged wiring between engine control module (ECM/PCM) and bypass valve or valve driver module
Poor or corroded connector terminals at the valve, harness or control module
Faulty coolant bypass valve (internal electronics, driver or solenoid)
Faulty module that controls or monitors the valve (ECM, BCM or valve control module)
CAN/LIN bus fault, missing ground, or battery power issue affecting communication
Software error or calibration mismatch after module replacement or update

Malfunction Indicator Lamp (MIL) or DTC stored
Coolant temperature control issues (slow warm-up, overheating, or heater performance issues) depending on the valve function
Reduced HVAC or engine temperature regulation
No other obvious drivability symptoms if only the bypass valve communication is lost
Related subsystem warnings if the controlling module isolates or limits functions

Scan tool: read freeze frame data, pending/stored U0618 and related U-codes
Check for other communication codes (U0100, U0121, etc.) that indicate broader bus issues
Visual inspection of wiring and connectors to coolant bypass valve D for damage, corrosion, or disconnection
Verify battery voltage and ground integrity at engine and control modules
Inspect fuses and relays supplying the valve and control module
Listen for valve activation during commanded tests (if accessible) or use an oscilloscope/scan tool to observe commands

No CAN/LIN messages from the valve or associated driver node when scanned (module appears offline)
CAN bus idle differential voltage ~2.5 V per side (typical) — look for missing or corrupted CAN frames
LIN bus idle ~battery/ground levels (if LIN used) — lack of expected response to master frames
Open circuit or short to ground/power on valve control pin (infinite or near-zero resistance)
PWM command to valve (when present) typical duty cycle 0–100% at manufacturer-specific frequency — no change when commanded if communication lost

Use a scan tool to confirm U0618 is active and note any related codes. Record freeze frame data and readiness status.
Perform a visual inspection of the valve D connector and harness for damage, corrosion, bent pins, or moisture. Repair or reseat if needed.
Check battery voltage and grounds. Verify fuses and power supply to the valve and controlling module are present and within spec.
With ignition ON, use the scan tool to command the valve (if supported). Observe for any response or change in live data. If no response, probe wiring for expected control signals.
Backprobe connector: check for proper supply voltage, ground and reference signals. Measure continuity to the controlling module. Repair any open/shorts.
If communication uses CAN/LIN, monitor the bus with the scope or scan tool. Look for missing frames, bus errors, or a module that drops off the network when other modules are present.
Swap or bench-test the coolant bypass valve (per manufacturer procedure) or substitute a known-good valve to verify valve electronics and coil resistance if applicable.
If wiring and valve are good, inspect or test the controlling module (ECM/BCM/valve module) for a faulty transceiver or internal failure. Reprogram or replace only with correct manufacturer guidance.
After repairs, clear codes and perform a test drive or functional test to ensure the code does not return and system operates normally.

Disconnected or corroded connector at coolant bypass valve D
Broken wire or chafed harness to valve driver
Internal failure of the coolant bypass valve (open circuit or failed electronics)
Faulty or intermittent bus transceiver in control module (ECM/BCM)
Blown fuse or poor battery/ground causing module to drop off the network