U0604: Lost Communication With Fuel Pressure Sensor B

What it means

The vehicle bus reports loss of communication with smart fuel pressure sensor B (typically common-rail or low-pressure side). The ECM loses its actual rail-pressure feedback for closed-loop control.

At a glance

• Usually lights the warning light of the system that lost communication
• Affects emissions; the car can fail an emissions test
• The engine or gearbox may go into limp mode

How it works

Fuel pressure sensor B reports a separate pressure channel over the vehicle network for engine fuel control. U0604 sets when communication with sensor B is lost, so the controller cannot use that channel’s pressure feedback and may enter a fail-safe strategy.

Common causes

Most likely first.

1. Bus wiring open or shorted
2. Module without power or ground
3. Module failed

Symptoms

• Multiple warning lights may illuminate
• Affected modules may go into limp mode or shut down
• Specific functions inoperative depending on module
• Diagnostic tools may show communication faults

How to diagnose it

1. Save freeze-frame data and scan all modules for fuel-pressure, engine-control, and network codes to distinguish a sensor B communication fault from broader bus trouble.
2. Check live data for sensor B specifically and compare it with sensor A or other available fuel-pressure readings; note whether only B is absent.
3. Inspect sensor B’s connector and accessible wiring for looseness, corrosion, fuel contamination, chafing, or heat damage.
4. Test sensor B’s specified power, ground, and network wiring to the manufacturer’s specification, checking whether other devices on the same bus are also offline.
5. If wiring and power are intact, test sensor B communication and confirm its configuration before investigating controller failure.

Common fixes

• Repair a loose, corroded, contaminated, or damaged sensor B connector or harness.
• Restore the sensor’s specified power or ground.
• Repair shared network wiring if multiple devices have lost communication.
• Replace fuel pressure sensor B if testing confirms that it is not communicating.
• Correct a controller or configuration fault only when vehicle-specific diagnosis identifies it.

Can I keep driving?

Do not rely on the vehicle for normal driving if it stalls, loses power, or has unstable fuel delivery; have it moved safely for diagnosis. Missing sensor B feedback can cause reduced power or stalling, so avoid high-speed and heavy-load driving.