CAT Engine 1903 Fault Code: Engine Speed Sensor Voltage Above Normal
Also called Engine Speed Open/Short to + Batt, Engine Speed Open/Short to +Batt, Engine Speed Sensor voltage above normal, Engine Speed open/short to + batt, Engine Speed open/short to +batt
Engine Speed Sensor Voltage Above Normal · ai-assisted, editor-reviewed · Last updated 2026-07-13
TL;DR
CAT code 1903 means the ECM sees the primary engine speed/timing sensor signal wire disconnected, shorted to +Battery, or reading supply voltage above normal. Affects C15 and C27 engines. The ECM will try to switch to a secondary engine speed/timing sensor if equipped; if both sensors fail, the engine shuts down.
High severity. This code can lead to engine misfires and full engine shutdown if both the primary and secondary engine speed/timing sensors fail. Even before shutdown, erratic injector firing can cause rough running and unpredictable performance, so it should be diagnosed promptly rather than run indefinitely.
What does CAT Engine error code 1903 mean?
CAT code 1903 covers a family of related fault descriptions: engine speed open or short to +Battery, and engine speed sensor voltage above normal. All of them point to a problem with the signal circuit feeding the ECM from the primary engine speed/timing sensor.
The ECM supplies the engine speed/timing sensor with 12.5 ± 1.0VDC. As the timing reference gear on the rear of the left camshaft rotates past the sensor's pickup, it generates a pulse signal. The ECM counts these pulses to determine engine rpm, and reads the unique tooth pattern to determine crankshaft position and locate the No. 1 cylinder. This lets the ECM fire each injector in the correct order and at the correct time, adjusting timing and duration based on rpm and load.
If the primary sensor's signal wire is disconnected, shorted to +Battery, or the supply voltage reads above normal, the ECM logs this code. On engines with a secondary engine speed/timing sensor, the ECM will automatically switch to that secondary signal to keep running. If both sensors are lost or abnormal, the ECM has no valid timing reference, so it terminates the injector firing signals and the engine shuts down.
Common causes of 1903
- Damaged or corroded connectors and/or wiring in the sensor circuit
- Problem within the engine harness
- Faulty or damaged engine speed/timing sensor, including a damaged sliphead
- Intermittent problem in the circuit or connections
- Faulty ECM (listed as a possible cause in several versions of this code)
How to troubleshoot CAT Engine 1903: first checks
- Inspect the connectors at the primary engine speed/timing sensor and along the harness for corrosion, damage, pushed-back pins, or loose connections.
- Visually and physically check the harness routing for chafing, pinched wiring, or contact with hot or moving engine components that could cause a short to +Battery.
- Check the engine speed/timing sensor itself and its sliphead for physical damage.
- Look for intermittent conditions by wiggling connectors and harness sections while watching for the code to reappear or clear on a diagnostic scan tool.
- Confirm whether the engine has a secondary engine speed/timing sensor and whether the ECM has defaulted to it, which would explain continued running despite the code.
- If wiring, connectors, and the sensor all check out, consider the ECM as a possible cause, since it is listed among the possible causes for this code.
How the code clears
No separate clearing step is listed. Repair the faulty connector, wiring, sensor, or ECM as applicable, and the ECM should stop logging the code once the underlying open, short, or abnormal voltage condition is corrected. Confirm with a diagnostic scan tool that the code does not return after the repair and after clearing logged codes.
Affected models and serial ranges
1903 appears in our records across 2 CAT Engine models. Match your machine by model and serial number.
| Model | Serial ranges |
|---|---|
| C15 | Serial range not listed in source records |
| C27 | Serial range not listed in source records |
Frequently asked questions
What does CAT code 1903 mean?
It means the ECM detected a problem with the primary engine speed/timing sensor signal circuit: either the wire is open or shorted to +Battery, or the sensor's supply voltage is reading above normal. It affects C15 and C27 engines.
Will the engine shut down with code 1903 active?
Not necessarily right away. If the engine has a secondary engine speed/timing sensor, the ECM will default to that signal to keep running. The engine only shuts down if both the primary and secondary engine speed/timing sensors fail or are abnormal.
Can I keep driving or operating with this code active?
It is not recommended. Engine misfires and erratic injector solenoid firing are listed responses to this fault, and there is a real risk of complete engine shutdown if the secondary sensor also fails. Get it diagnosed as soon as possible.
What is the difference between 1903 and code 190-08?
If the engine timing gear is installed backward, the ECM generates a different code, 190-08, for an abnormal engine speed signal, and the ECM will not fire the injectors. That is a distinct condition from 1903, which relates to the primary sensor's wiring or supply voltage.
What parts commonly cause code 1903?
Possible causes listed include damaged connectors and wiring, a problem within the harness, a faulty or damaged engine speed/timing sensor or sliphead, an intermittent connection issue, or in some cases a faulty ECM.
Where is the engine speed/timing sensor located?
It reads a timing reference gear mounted on the rear of the left camshaft. As the gear's teeth pass the sensor's pickup, it generates the pulse signal the ECM uses to determine engine rpm and crankshaft position.
Does code 1903 always mean a bad sensor?
No. Wiring and connector problems are listed as possible causes just as often as a faulty sensor, and an intermittent issue can also trigger the code. A faulty ECM is also listed as a possible cause in some versions of this fault.