The fault code 28 / PID 91-13 – Throttle Position Out of Calibration is a common issue on Caterpillar C9 engines and is usually related to the accelerator pedal position sensor circuit or calibration.
This short guide explains what the code means, why it occurs, and how to fix it efficiently.
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What Triggers Code 28 / PID 91-13?
The Engine Control Module (ECM) will log this fault when all of the following conditions are met:
- The ECM 8-volt sensor supply drops below 7.5 VDC for more than 1 second
- The ECM has been powered for more than 3 seconds
- The accelerator pedal position signal is detected outside the calibrated range
Accelerator Pedal Position Sensor Overview
The accelerator pedal position sensor provides the throttle input signal to the ECM.
- Signal type: PWM (Pulse Width Modulated) / Duty Cycle
- Output range: 3% – 100% duty cycle
- Low idle: 10% – 22%
- Full throttle: 75% – 90%
- ECM converts this signal to 3% – 100% throttle position
The sensor is mounted directly on the pedal assembly and does not require adjustment.
⚠️ Important:
If the pedal assembly is out of calibration, it cannot be adjusted.
👉 The complete accelerator pedal assembly must be replaced.
Sensor Power Supply Information
- Sensor supply voltage: +8 VDC from the ECM
- Power source: ECM connector P1-4 → terminal “A” of the pedal sensor
- The sensor itself can be replaced separately, but calibration faults usually indicate a pedal assembly issue
PTO Configuration Note (Often Overlooked)
If the engine is operating in PTO mode, the accelerator pedal signal may be ignored when PTO is configured as:
- Remote switches
- Remote throttle
Cab Switches PTO Mode
- The cab accelerator pedal can control engine RPM
- Or it can be ignored, depending on customer parameter settings
- Incorrect PTO programming can mimic a throttle calibration fault
👉 Always verify PTO configuration before replacing components.
Probable Causes (Check in This Order)
- ECM electrical connections
- Accelerator pedal position sensor or harness
- Wiring between pedal sensor and ECM
- Accelerator pedal assembly (out of calibration)
- ECM failure (least common)
Recommended Diagnostic Steps
- Verify ECM 8-volt sensor supply
- Inspect pedal connector and wiring
- Check live throttle duty-cycle data
- Confirm PTO parameter configuration
- Replace the accelerator pedal assembly if calibration is invalid
Using a Caterpillar-compatible diagnostic tool is strongly recommended for accurate live data monitoring.
Summary
- Code 28 / PID 91-13 is usually not an ECM failure
- Accelerator pedals cannot be recalibrated
- PTO configuration errors can cause false symptoms
- Proper diagnosis prevents unnecessary parts replacement
More repair case for Caterpillar machine,please refer to:Caterpillar Machine Repair
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