CAT Engine 7748 Fault Code: Secondary Throttle Position Sensor: Abnormal Frequency, Pulse Width, or Period
Also called Accelerator Pedal Position #2 : Abnormal Frequency, Pulse Width, or Period, Secondary Throttle Position Sensor - Abnormal Frequency, Pulse Width, or Period, Secondary Throttle Position Sensor : Abnormal Frequency, Pulse Width, or Period, Secondary Throttle Position Sensor abnormal frequency, pulse width, or period, Secondary Throttle Position Sensor:Abnormal Frequency, Pulse Width, or Period
Secondary Throttle Position Sensor: Abnormal Frequency, Pulse Width, or Period · ai-assisted, editor-reviewed · Last updated 2026-07-13
TL;DR
CAT code 7748 (SPN 29 / FMI 8) means the ECM sees an abnormal PWM signal frequency, pulse width, or period from the secondary throttle position sensor (accelerator pedal position #2) on C13, C15, and C18 engines. The ECM will fall back to the primary throttle, and if that fails too, the engine drops to a limp home speed until the fault is fixed and the key is cycled.
High severity. The engine keeps running but loses normal throttle response and can be locked at a limp home speed until the fault is repaired and the keyswitch is cycled, which is a real operational and safety concern in traffic or work cycles.
What does CAT Engine error code 7748 mean?
This code points to the secondary throttle position sensor, also called accelerator pedal position sensor #2, which is mounted on the pedal assembly. It sends the ECM a PWM (pulse width modulated) signal that changes duty cycle with pedal travel: minimum duty cycle at released pedal, maximum at full throttle. The ECM translates that signal into a 0 to 100 percent throttle position.
Code 7748 sets when that PWM signal's frequency, pulse width, or period falls outside the range the ECM expects. In practical terms, the ECM is no longer confident the secondary sensor is telling the truth about pedal position, so it stops trusting that input.
Because these engines run two throttle position signals for redundancy, the ECM will switch to whichever throttle is still healthy. If the primary throttle also has a fault, the engine has no valid throttle command left and defaults to a limp home speed until the problem is corrected.
What triggers a CAT Engine 7748 code?
The ECM has been powered for 3 seconds and sets the code when the digital (PWM) secondary throttle signal frequency is less than 150 Hz for more than 1 second, or greater than 1050 Hz for more than 1 second. Related logic also flags the signal frequency sitting at 0% or 100% duty cycle for more than 2 seconds. These conditions only apply when codes 0168-01, 0262 (analog sensors), 0041 (digital/PWM sensors), and 0774-03/0774-04 are not already active.
Common causes of 7748
- Damaged, corroded, or abraded connectors and wiring at the throttle pedal assembly or harness
- General harness problems, including pinch points along the routing to the ECM
- A faulty secondary throttle position sensor itself
- An intermittent connection or wiring fault that only shows up under vibration or movement
- A faulty ECM (less common, but listed as a possible cause)
- A faulty throttle assembly, since the sensor is mounted to the pedal geometry and cannot be adjusted separately
How to troubleshoot CAT Engine 7748: first checks
- Turn the key off and back on after any repair, since the ECM will keep ignoring the repaired throttle input until the keyswitch is cycled
- Inspect the throttle pedal connector and harness for corrosion, damaged pins, chafed insulation, or pinch points along the routing
- Wiggle-test the harness near the pedal assembly and along its run while watching for the code or an active fault to reappear, since several causes are intermittent
- Check for other active codes first (0168-01, 0262, 0041, 0774-03, 0774-04, and codes 29-3, 29-4, 774-3, 774-4, 678, or 41), since these must be inactive for 7748 logic to apply and may point to the real root cause
- Confirm the sensor is producing a proper PWM signal from minimum duty cycle at released pedal to maximum duty cycle at full throttle, since the sensor cannot be adjusted and a bad reading usually means a bad sensor or harness, not a calibration issue
- If wiring and sensor both check out, consider the ECM itself, though this is listed as a less likely cause
How the code clears
There is no separate reset procedure listed beyond repairing the fault. Once the underlying wiring, connector, sensor, or throttle assembly problem is fixed, the ECM will still ignore the repaired throttle's input until the keyswitch is turned OFF and then back ON. Cycling the key is required for the ECM to trust that input again.
Affected models and serial ranges
7748 appears in our records across 3 CAT Engine models. Match your machine by model and serial number.
| Model | Serial ranges |
|---|---|
| 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 7748 mean?
It means the ECM detected an abnormal frequency, pulse width, or period on the secondary throttle position sensor's PWM signal. This is SPN 29, FMI 8, and it affects C13, C15, and C18 engines.
Will the engine still run with code 7748 active?
Yes, but with reduced function. The ECM switches to the primary throttle while the secondary is faulted. If both throttles are unavailable, the engine drops to and stays at a limp home speed until the code is fixed.
Why won't my throttle respond normally after I fixed the wiring?
The ECM ignores input from a repaired throttle sensor until the keyswitch is cycled off and back on. Fixing the wiring alone will not restore normal throttle response until you cycle the key.
Can I adjust the throttle position sensor to fix this code?
No. The sensor is mounted to the pedal assembly, and the pedal geometry controls its travel and output range. The sensor itself cannot be adjusted, so a bad signal points to wiring, connectors, or a faulty sensor or throttle assembly.
Is this a wiring problem or a sensor problem?
It can be either. Possible causes include damaged or corroded connectors, harness problems such as abrasion or pinch points, an intermittent connection, a faulty sensor, a faulty throttle assembly, or in rare cases a faulty ECM.
Does code 7748 log every time it happens?
The diagnostic code logs if the engine is running when the fault condition occurs. It will not log if the engine is cranking at the time.
What other codes should I check alongside 7748?
Check for 0168-01, 0262, 0041, 0774-03, 0774-04, 29-3, 29-4, 774-3, 774-4, 678, and 41. These must be inactive for the 7748 logic to apply, and any of them being active may explain the real underlying problem.