CAT Engine 1004 Fault Code: Engine Oil Pressure short to ground

What does CAT Engine error code 1004 mean?

CAT code 1004 points to the Engine Oil Pressure Sensor circuit reporting a voltage below normal, specifically a signal that looks shorted to ground. The sensor is a variable DC voltage device, normally outputting between 0.2 and 4.42 VDC depending on actual oil pressure, and the ECM compares that signal against an rpm-based oil pressure map stored in the Personality Module.

The sensor gets 5.0 volts DC supply from the ECM (J1/P1 pin -63), with its return through pin -6 and its signal through pin -10 on many C-series engines. When the ECM sees the signal drop below 0.2 volts for the qualifying time, it treats this as a short to ground rather than a real low-pressure reading.

Because the ECM cannot trust this signal, it marks oil pressure as invalid data and substitutes a default value (600 kPa / 87 psi in some ECM software) instead of using the sensor. The engine keeps running normally since this code by itself does not trigger a derate, but you lose the ECM's ability to catch a genuinely low oil pressure condition until the sensor circuit is fixed.

What triggers a CAT Engine 1004 code?

The ECM logs 1004 when it reads an oil pressure signal voltage below 0.2 volts for at least 0.24 seconds (one description) or for more than 8 seconds (another description), and only after the ECM has been powered for at least 2 seconds. The diagnostic is disabled whenever atmospheric pressure is less than 68 kPa (10 psi) or displayed gauge oil pressure is 0 kPa (0 psi), until the code becomes valid again. On some ECM software, this code also requires no active 262 code and no active 168 code before it can go active.

Common causes of 1004

• Damaged, corroded, or abraded connectors and wiring on the oil pressure sensor circuit
• Improper engine oil level, either too low or too high
• Actual oil leakage from the engine or lines
• Incorrect sensor installed for this application
• Damaged 5-volt supply wire feeding the sensor
• Defective engine oil pressure sensor
• A problem elsewhere in the engine wiring harness, including pinch points or intermittent connections
• Signal wire shorted to ground, or a short between the analog sensor common and the signal wire
• Open circuit in the 5-volt supply wire, or a short between the 5-volt supply and the signal wire
• Defective ECM

How to troubleshoot CAT Engine 1004: first checks

1. 1Check actual engine oil level and condition on the dipstick before doing any electrical diagnosis; rule out a real low-oil condition first
2. 2Inspect the oil pressure sensor connector and mating ECM connector pins for corrosion, moisture, bent pins, or an incomplete connection
3. 3Visually trace the harness between the sensor and ECM for abrasion, pinch points, or chafing against brackets or moving parts
4. 4With a meter, verify the sensor is getting its 5.0 VDC supply and check the signal wire for a short to ground or a short to the sensor common line
5. 5Confirm the sensor connector terminals match the expected wiring (supply, return, signal) and that the correct sensor part is installed
6. 6If wiring and supply voltage check good, swap in a known-good oil pressure sensor to isolate a sensor failure from a wiring or ECM issue

How the code clears

No separate clearing step is listed beyond fixing the underlying wiring, sensor, or ECM issue. Once the signal is restored to a normal operating range and stays above the short-to-ground threshold, the ECM should stop logging the code on the next qualifying cycle. Since the pressure sensors are calibrated by the ECM during the first five seconds after keyswitch ON (only if the engine is not cranked during that window), cycling the key without cranking can help ensure a clean calibration after a repair.

Affected models and serial ranges

1004 appears in our records across 10 CAT Engine models. Match your machine by model and serial number.