CAT 21 Fault Code: 5 Volt Supply voltage low
Also called 5 Volt Sensor Supply #1 voltage high, 5 Volt Sensor Supply #1 voltage low, 5 Volt Supply voltage high, 5V or 8V Sensory Supply Fault, 8 Volt Supply voltage high, 8 Volt Supply voltage low
5 Volt Supply voltage low · ai-assisted, editor-reviewed · Last updated 2026-07-13
TL;DR
CAT code 21 (SPN 678 / FMI 3) means the Engine Control Module has detected that its 5-volt or 8-volt sensor supply rail is out of range, either too high or too low, for more than a brief period after the ECM has been powered up. This supply feeds the accelerator pedal position sensor and several engine pressure sensors, so the fix depends on which supply and which sensor circuit is involved.
Medium severity. The engine will typically still run, but sensor readings feeding the ECM (pedal position, boost, oil pressure, fuel pressure) can become unreliable or default to a safe value, which can affect performance or trigger a derate. Diagnose within the shift rather than ignoring it, especially if it affects throttle response.
What does CAT error code 21 mean?
This is a family of related low-voltage electrical faults, all logged under CAT code 21, involving the ECM's internal 5-volt or 8-volt regulated sensor supply circuits. The ECM uses these regulated supplies to power several sensors so their signals stay accurate and consistent.
The 8-volt supply feeds the accelerator pedal position sensor, which sends the ECM a pulse-width modulated signal representing pedal position, expressed as a duty cycle between 10 to 22 percent at low idle and 75 to 90 percent at full pedal. The ECM translates that into a pedal position of 3 to 100 percent.
The 5-volt supply feeds several pressure sensors at once: boost pressure, atmospheric pressure, engine oil pressure, injection actuation pressure, and fuel pressure (if equipped) and, on some configurations, the intake valve actuator oil pressure sensor. Because one supply rail feeds multiple sensors, a short or fault anywhere in that shared wiring can pull the whole rail out of range and affect several readings at once.
What triggers a CAT 21 code?
The ECM sets this code under any of the following conditions, each requiring the ECM to have been powered for more than three seconds (at least three seconds for the 5-volt versions): - 8-volt supply above 8.5 VDC for more than one second (8-volt high) - 8-volt supply below 7.5 VDC for more than one second (8-volt low) - 5-volt supply above 5.16 VDC for more than one second (5-volt high) - 5-volt supply below 4.84 VDC for more than one second (5-volt low) The nominal 5-volt supply is specified as 5.0 VDC plus or minus 0.2 VDC.
Common causes of 21
- ECM connection issues, including corroded, loose, or damaged pins at the ECM connector
- Accelerator pedal position sensor or its wiring harness (for the 8-volt supply versions)
- Wiring problems between the accelerator pedal position sensor and the ECM, including chafed or shorted wiring
- A failed pressure sensor on the shared 5-volt rail: boost, atmospheric, engine oil pressure, injection actuation pressure, fuel pressure, or intake valve actuator oil pressure sensor depending on configuration
- Sensor harness faults on the 5-volt side, most often a short circuit to ground or a short circuit to another voltage source
- A faulty accelerator pedal assembly (the sensor is not separately adjustable if the pedal assembly itself is out of calibration; the whole assembly must be replaced in that case)
- ECM internal failure
How to troubleshoot CAT 21: first checks
- Inspect the ECM connector for corrosion, bent pins, or loose contact before replacing any sensor
- For 8-volt supply codes, check the accelerator pedal position sensor connector and wiring, since terminal A carries the +8 VDC supply from the ECM's P1-4 connector pin
- For 5-volt supply codes, disconnect one sensor at a time on the shared 5-volt rail, waiting 15 seconds after each disconnect, and watch whether the code for that specific sensor goes active. This isolates which sensor or branch is causing the short
- Visually trace the harness for chafe points, pinched wiring, or contact with ground or another voltage source, since a short circuit to ground or to another voltage source is the most likely cause of the 5-volt fault
- Confirm the accelerator pedal assembly is not miscalibrated; the sensor itself has no adjustment, so if the pedal assembly is out of range the whole assembly needs replacement, not just the sensor
- If the machine uses PTO functions with remote switches or remote throttle configuration, note that the accelerator pedal sensor is ignored in that mode, which can affect how the fault presents during PTO operation
How the code clears
No separate clearing step is listed beyond fixing the underlying wiring, connector, or sensor fault. Once the supply voltage is back within its normal range and stays there, the code should stop being active. Cycling the ECM power after a repair is standard practice to confirm the fault does not reset.
Affected models and serial ranges
21 appears in our records across 4 CAT models. Match your machine by model and serial number.
| Model | Serial ranges |
|---|---|
| C11 | Serial range not listed in source records |
| C13 | Serial range not listed in source records |
| C15 | Serial range not listed in source records |
| C18 | Serial range not listed in source records |
Frequently asked questions
What does CAT fault code 21 mean?
It means the ECM has detected that either its 5-volt or 8-volt internal sensor supply voltage is outside the acceptable range, either too high or too low, for longer than one second after the ECM has been powered up for more than three seconds. This supply feeds either the accelerator pedal position sensor or a group of engine pressure sensors, depending on which version of the code is active.
Will code 21 cause a derate or shutdown?
The engine will generally keep running, but affected sensor readings can become inaccurate or default to a substitute value, which can affect pedal response or engine performance since pedal position, boost, oil pressure, and fuel pressure readings may all be tied to the same supply rail.
How do I find which sensor is causing the 5-volt supply fault?
Disconnect the pressure sensors on the 5-volt rail one at a time, waiting 15 seconds after each disconnection, and check whether the code for that sensor goes active. That tells you which sensor or branch has the short.
Is this an accelerator pedal problem or a pressure sensor problem?
It depends on which version of code 21 is active. The 8-volt supply versions point to the accelerator pedal position sensor, its harness, or the pedal assembly. The 5-volt supply versions point to one of several pressure sensors sharing that rail: boost, atmospheric, engine oil pressure, injection actuation pressure, fuel pressure, or intake valve actuator oil pressure sensor.
Can I adjust or calibrate the accelerator pedal position sensor?
No. The sensor itself requires no adjustment. If the pedal assembly is out of calibration, the entire accelerator pedal assembly must be replaced rather than adjusted.
Does PTO mode affect how this code shows up?
Yes, if the vehicle is using dedicated PTO functions configured for remote switches or remote throttle, the ECM ignores the accelerator pedal position sensor while in PTO mode, which can change how the fault behaves during that operation. In cab switches configuration, the cab pedal can still be used or ignored depending on how customer parameters are programmed.
What is the most likely root cause of this code?
For the 8-volt versions, it is most often the pedal sensor, its harness, or wiring back to the ECM. For the 5-volt versions, it is most often a short circuit to ground or to another voltage source somewhere in the shared pressure sensor harness, though ECM connection issues and outright ECM failure are also listed as possible causes for both.