CAT Engine 4603 Fault Code: Fuel Pressure Sensor - After Fuel Filter : Voltage Above Normal
Also called Fuel Pressure Sensor - After Fuel Filter - Voltage Above Normal, Fuel Pressure Sensor - After Fuel Filter voltage above normal, Fuel Pressure Sensor - After Fuel Filter:Voltage Above Normal
Fuel Pressure Sensor - After Fuel Filter : Voltage Above Normal · ai-assisted, editor-reviewed · Last updated 2026-07-13
TL;DR
CAT code 4603 (SPN 460 / FMI 3) means the ECM sees the secondary fuel filter outlet pressure sensor signal voltage go above 4.8 VDC for more than 8 seconds. This is an open-circuit type fault on the signal wire, not a real fuel pressure reading. Affects C13, C15, C175, C18, C27, and C32 engines.
Medium severity. The engine will keep running using a default sensor value, so this is not an immediate shutdown fault. But fuel pressure monitoring after the filter is effectively blind until it's fixed, so it should be diagnosed within the shift rather than ignored.
What does CAT Engine error code 4603 mean?
Fault code 4603 is set when the CAT ECM detects that the signal voltage coming from the fuel pressure sensor located after the secondary fuel filter is higher than the normal operating range. The ECM continuously applies a pull-up voltage to this signal circuit so it can tell the difference between a working sensor, an open circuit, and a short circuit. When that pull-up voltage shows up as an out-of-range high signal for a sustained period, the ECM flags it as voltage above normal rather than trusting it as a real pressure value.
In practical terms, this almost always points to a wiring or connector problem on the signal circuit, or a failed sensor, rather than an actual fuel pressure event. The ECM cannot get a valid pressure reading after the fuel filter while this code is active, so it substitutes a default value and logs the fault. That means any fuel pressure related diagnostics or protections tied to that sensor are running blind until the fault clears.
This sensor is part of the automatic self-calibration routine the ECM runs on pressure sensors whenever the ECM is powered and the engine has been off for at least 5 seconds. If wiring or connector issues are present at power-up, they can affect this calibration as well as normal running readings.
What triggers a CAT Engine 4603 code?
The ECM sets this code when the signal voltage from the secondary fuel filter outlet pressure sensor is greater than 4.8 VDC for more than 8 seconds. For the code to become active, there must be no active 262 code and no active 168 code present at the same time.
Common causes of 4603
- Intermittent electrical problem in the sensor circuit
- Damaged connectors or wiring at the sensor or ECM connector
- A problem within the wiring harness itself, such as chafing or a pinched section
- A faulty fuel pressure sensor (after the fuel filter)
- A faulty ECM, though this is listed as a possible cause and should be considered only after wiring and sensor checks are exhausted
How to troubleshoot CAT Engine 4603: first checks
- Check that no 262 or 168 codes are active, since either of those will block 4603 from becoming active and should be diagnosed first if present
- Visually inspect the sensor connector and ECM connector for corrosion, bent pins, moisture intrusion, or loose terminals
- Trace the wiring harness between the sensor and the ECM looking for chafed insulation, pinch points, or damaged sections
- Check that terminal 1 (supply), terminal 2 (sensor return), and terminal 3 (signal) at the sensor connector are wired and seated correctly
- With the connector disconnected and engine off, verify the pull-up voltage is present on the signal circuit at the harness side, confirming the wiring back to the ECM is not open or shorted to ground
- If wiring and connectors check out, swap in a known-good fuel pressure sensor to isolate a faulty sensor from a harness or ECM issue
How the code clears
No separate clearing step is listed beyond fixing the root cause. Once the wiring, connector, or sensor issue is repaired and the signal voltage returns to a normal range for the required time, the ECM should stop logging the active fault. Confirm with an electronic service tool that the code is no longer active and, if needed, clear the logged code through the service tool after the repair is verified.
Affected models and serial ranges
4603 appears in our records across 6 CAT Engine models. Match your machine by model and serial number.
| Model | Serial ranges |
|---|---|
| C13 | Serial range not listed in source records |
| C15 | Serial range not listed in source records |
| C175 | Serial range not listed in source records |
| C18 | Serial range not listed in source records |
| C27 | Serial range not listed in source records |
| C32 | Serial range not listed in source records |
Frequently asked questions
What does CAT fault code 4603 mean?
It means the ECM detected the signal voltage from the fuel pressure sensor located after the secondary fuel filter reading above normal, specifically above 4.8 VDC for more than 8 seconds. It's an electrical fault on the signal circuit, not necessarily a true high fuel pressure condition.
Is CAT code 4603 serious enough to stop working?
Not immediately. The ECM substitutes a default value for the sensor and the engine keeps running, but fuel pressure monitoring after the filter is not reliable while the code is active. It should be diagnosed promptly rather than run indefinitely.
What usually causes fault code 4603 on CAT engines?
The most common causes are damaged connectors or wiring, a problem within the harness, a faulty sensor, an intermittent electrical connection, or in rarer cases a faulty ECM.
Why won't code 4603 go active even though something seems wrong?
The code requires that there are no active 262 codes and no active 168 codes at the same time. If either of those codes is active, 4603 will not become active until they clear.
Which CAT engines use this fault code?
This code applies to the C13, C15, C175, C18, C27, and C32 engine models.
Can a bad ECM cause code 4603?
Yes, a faulty ECM is listed as one of the possible causes, but it's typically the least likely cause and should be checked only after wiring, connectors, and the sensor itself have been ruled out.
Does the sensor need automatic calibration and can that affect this code?
Yes. The ECM automatically calibrates pressure sensors, including this one, whenever it is powered and the engine has been off for at least 5 seconds. Wiring or connector problems present during that calibration window can contribute to abnormal readings being logged.