U0603

Lost Communication With Navigation Control Module

U0603 is a generic OBD-II network diagnostic trouble code: Lost Communication With Navigation Control Module. It is logged by the engine control unit when the network monitor detects that a specific fault threshold has been exceeded — typically resulting in the malfunction-indicator lamp (MIL / check-engine light) being illuminated.

Code
U0603
Group
Network
System
Network
Severity
Warning (MIL on, possible limp mode)
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What U0603 means

DTC U0603 is a network communication code set when the powertrain or engine control module loses valid serial communication with Fuel Pressure Sensor A over the vehicle data bus. On systems where this sensor reports its value as a bus message rather than a direct analog signal, the controller expects a continuous data stream describing fuel rail or supply pressure; U0603 is stored when those messages are missing or corrupted for a calibrated number of cycles.

Fuel pressure feedback is critical for accurate fuel delivery, injection timing, and rail pressure control. Losing this data prevents closed-loop fuel pressure regulation, so the ECU may adopt a default strategy, derate power, or enter limp mode to protect the engine and fuel system. This is a communication fault, not necessarily a mechanical fuel pressure problem, so the sensor and rail may be physically healthy while the data link has failed.

Typical root causes are electrical: open or shorted bus wiring, loss of sensor power or ground, connector corrosion, or a failed sensor or interface module. Because the fault degrades driveability, diagnosis should confirm the sensor is present on the network and verify its power and ground before any replacement, ruling out harness and connector issues first.

Common causes

Most-frequently reported root causes when U0603 is logged.

  • 1
    Open, short, or high-resistance fault in the data bus wiring to Fuel Pressure Sensor A.
  • 2
    Failed fuel pressure sensor or its bus interface module.
  • 3
    Loss of power or ground supply to the fuel pressure sensor circuit.
  • 4
    Corroded, loose, or damaged connector at the fuel pressure sensor.
  • 5
    Chafed or damaged wiring harness routed near the engine or fuel rail.
  • 6
    Blown fuse supplying the sensor or its host module.
  • 7
    Internal software fault in the module reporting fuel pressure data on the bus.

Symptoms drivers notice

Check engine light illuminated with U0603 stored.
Hard starting, rough running, or hesitation due to compromised fuel pressure control.
Reduced engine power or limp mode.
Fuel pressure value unavailable or frozen on a scan tool.
Possible additional fuel system or communication DTCs stored alongside U0603.

How to diagnose U0603

A typical diagnostic flow when this code is present.

  1. 1
    Retrieve all stored DTCs from every module and note any related fuel system or network codes.
  2. 2
    Check live data to confirm whether the fuel pressure sensor is reporting on the bus or reads as missing.
  3. 3
    Inspect the sensor connector and harness for corrosion, chafing, heat damage, or loose terminals.
  4. 4
    Verify power and ground at the sensor connector with a multimeter and the wiring diagram.
  5. 5
    Measure bus integrity (approximately 60 ohms across CAN-H and CAN-L) to rule out wiring or termination faults.
  6. 6
    Attempt communication with the sensor or host module; if absent, confirm supply before condemning it.
  7. 7
    Replace the fuel pressure sensor only after wiring, power, and ground have been confirmed good.

Related network codes

Frequently asked questions

What does U0603 actually mean?

It is a lost-communication code for Fuel Pressure Sensor A, indicating the controller is no longer receiving valid fuel pressure data over the bus.

Is U0603 a mechanical fuel pressure problem?

Not necessarily; it is primarily a communication fault, so the sensor and fuel rail may be fine while the data link or wiring has failed.

Can I drive with U0603 present?

It is not advisable, because loss of fuel pressure feedback can cause poor running or limp mode and may leave the engine in a protective default strategy.

Is U0603 related to the navigation system?

No. Lost communication with the navigation control module is code U0163; U0603 concerns the fuel pressure sensor.

Disabling U0603 in software

RaceTune can permanently disable U0603 — and any other OBD-II diagnostic trouble code — on every ECU family we support. The monitor is disabled inside the ECU itself, so the fault stops being logged: the warning light stays off and the engine never enters limp mode for this code. The change is tied to your exact software version.

Permanent
The monitor is disabled in the ECU itself — not just cleared. It cannot return.
Tailored to your file
Each patch is matched to your specific software version — never a one-size-fits-all file.
Reversible
The original file is always preserved. Reflash the stock to return the ECU to factory state.

Software modifications affect emissions compliance and are not road-legal in many jurisdictions. RaceTune service files are intended for motorsport, off-road, and export use.

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