CAT 28 Fault Code: Throttle Sensor out of calibration
Also called Throttle Position out of Calibration, Throttle Position out of calibration, Throttle Sensor Calibration, Throttle sensor calibration
Throttle Sensor out of calibration · ai-assisted, editor-reviewed · Last updated 2026-07-13
TL;DR
CAT code 28 (SPN 91 / FMI 13) means the Engine Control Module (ECM) has detected the accelerator pedal position sensor signal is out of its expected calibration range, either on voltage supply, signal frequency, or duty cycle. It applies to C11, C13, C15, and C18 engines and points to a problem with the pedal sensor, its wiring, or the ECM connection.
High severity. The engine can lose normal throttle response or default to a fixed rpm behavior when this code is active, which affects drivability and productivity. It is not an immediate safety shutdown, but it should be diagnosed before continuing normal operation, especially in machines relying on precise throttle or PTO control.
What does CAT error code 28 mean?
Code 28 relates to the accelerator pedal position sensor circuit on C11, C13, C15, and C18 engines. This sensor tells the ECM where the pedal is by sending a pulse width modulated (PWM) signal, expressed as a duty cycle percentage, that varies with pedal travel. The ECM translates that duty cycle into a pedal position reading used to manage engine rpm and fueling.
The ECM watches for three separate failure conditions tied to this code: a low 8-volt supply to the sensor, a signal frequency outside the expected range, or a duty cycle outside the expected range. Any of these tells the ECM that the pedal signal cannot be trusted, so it sets code 28 and can substitute a default value rather than follow an erratic pedal signal.
This sensor is fixed to the pedal assembly and cannot be adjusted. If it reads out of calibration, either the sensor, its wiring, or the whole pedal assembly needs to be replaced, not adjusted.
What triggers a CAT 28 code?
The ECM sets this code when any of the following are detected after the ECM has been powered for three seconds: the 8 volt sensor supply drops below 7.5 VDC for more than one second; the accelerator pedal position sensor signal frequency is below 150 Hz or above 1050 Hz for more than two seconds; or the sensor duty cycle is below 5 percent or above 95 percent for more than two seconds.
Common causes of 28
- ECM connection or connector issue, including corrosion or a loose pin
- Accelerator pedal position sensor failure
- Wiring harness fault between the accelerator pedal position sensor and the ECM
- ECM internal failure
- Accelerator pedal assembly fault (the assembly cannot be recalibrated if out of range, it must be replaced as a unit)
- Throttle position sensor fault as a general wiring or sensor issue reported separately for the throttle pedal assembly
How to troubleshoot CAT 28: first checks
- Inspect the ECM connector at pin P1-4 and the accelerator pedal position sensor connector terminal A for corrosion, looseness, or damage
- Check the wiring harness between the accelerator pedal position sensor and the ECM for chafing, pinches, or opens
- Verify the 8 volt supply to the sensor is present and steady, watching for a drop below 7.5 VDC
- With the pedal cycled from low idle to fully depressed, check that the sensor duty cycle moves through its expected range of 10 to 22 percent at low idle up to 75 to 90 percent at full pedal, translated by the ECM to 3 to 100 percent pedal position
- If PTO functions are in use, confirm the PTO configuration setting (remote switches, remote throttle, or cab switches) since the accelerator pedal signal is ignored in some PTO modes, which can be mistaken for a sensor fault
- If sensor and wiring check out, suspect the pedal assembly itself, since it cannot be adjusted and must be replaced as a complete unit if out of calibration
How the code clears
No separate clearing step is listed. Once the underlying cause, whether the sensor, harness, connector, or pedal assembly, is repaired or replaced and the signal returns to its normal duty cycle and frequency range, the code should clear on its own after the ECM re-evaluates the input.
Affected models and serial ranges
28 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 code 28 mean?
It means the ECM has detected the accelerator pedal position sensor signal is out of its normal calibration range, either from a low sensor supply voltage, a signal frequency outside 150 Hz to 1050 Hz, or a duty cycle outside 5 percent to 95 percent.
Can I recalibrate the accelerator pedal position sensor myself?
No. The sensor and pedal assembly require no adjustment and cannot be calibrated in the field. If the assembly is out of calibration, the entire pedal assembly must be replaced.
Which engines does code 28 affect?
It applies to CAT C11, C13, C15, and C18 engines equipped with the ECM-controlled accelerator pedal position sensor.
Will this code affect PTO operation?
It can. If the PTO configuration is set to remote switches or remote throttle, the ECM ignores the accelerator pedal signal entirely during PTO mode, so a pedal sensor issue may not show up the same way during PTO operation as it does during normal driving.
Is it safe to keep operating with code 28 active?
It is not recommended. An out-of-range pedal signal means the ECM cannot reliably read operator throttle input, which can cause unpredictable engine response. Diagnose and repair before continuing normal operation.
What parts commonly cause code 28?
The accelerator pedal position sensor, its wiring harness, the ECM connector, or the pedal assembly itself are the most common causes. In rare cases the ECM itself has failed.