PACCAR MX P0121 is the fault code page for “Dual PWM accelerator pedal Sticking pedal” in the 2013 PACCAR MX Diagnostic Service Manual. PACCAR publishes it for engines in the EPA 2013 emissions family, and it is worked with the DAVIE diagnostic software.
What this fault code means
The accelerator pedal uses a dual pulse width modulation sensor and switch, with two independent signal, supply and ground circuits into the ECU. The code is a plausibility check on driver input, not a circuit failure.
Set and reset conditions
The diagnostic runs continuously while the engine is on and sets after more than 10 seconds of concurrent application of both the brake and accelerator pedals.
PACCAR MX P0121 wiring and technical data
Six ECU pins carry the two pedal channels; the manual lists each so a suspect channel can be isolated at connector D420.B. The fault code information block records it as MIL trips not applicable, 3 drive cycle recovery, readiness group none, freeze frame type Crankcase.
| D420 pin | Signal | Function |
|---|---|---|
| B38 / B50 | Pedal 1 and 2 supply | 5 V supply |
| B41 / B45 | Pedal 1 and 2 signal | Pedal position |
| B42 / B46 | Pedal 1 and 2 ground | Pedal position |
Possible causes and working notes
- Accelerator pedal and service brake applied together
- Brake pedal or hand brake not fully released
- Faulty wiring
- Faulty connector
- Accelerator pedal mechanical fault
The manual is explicit that this DTC is mainly informational: logged, but lighting no dashboard lamp. If it does not self-clear during driving it produces a reduced torque or derate condition, and the page gives the PACCAR Engine Support Call Center number, 1-800-477-0251.
Document details and format
| Field | Value |
|---|---|
| Brand | PACCAR |
| System group | DTCs, MX Engines (EPA2013) |
| Source publication | 2013 PACCAR MX Diagnostic Service Manual |
| Manual page | 141 |
| Pages | 4 |
| File size | 0.4 MB |
| Format | PDF, English |
Applies to PACCAR MX P0121 repairs
The page sits in the DTCs section for MX engines built to the EPA 2013 emissions standard, so it covers trucks running that engine family with the PCI engine control unit. Coverage and dealer support are published on the PACCAR corporate site.
Who needs this PACCAR MX P0121 page
Shops facing a derate with no warning lamp on a PACCAR MX truck, where a dragging brake or sticking pedal is the real story.
The full PDF page is in the DiagTechPro manuals library; a free account downloads it. Keep the PACCAR MX P0121 sheet open next to DAVIE.
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