OBD-II Diagnostic Trouble Code
P0120 — Throttle/Pedal Position Sensor/Switch A Circuit
P0120 is a general circuit fault for throttle or pedal position sensor A. On gas trucks this is usually the throttle body position sensor; on some diesels it refers to the accelerator pedal position sensor, though many diesel ECMs report pedal faults under P2120-P2138 or J1939 codes instead. The ECM saw a signal it could not use, without a specific high or low reading.
Severity 4/5
Truck-relevant
Stop within 50 mi
Severity
Stop within 50 mi
Throttle/Pedal Position Sensor/Switch A Circuit
Code
P0120
Category
Powertrain
Subsystem
Throttle
Truck-relevant
Yes
What this means for truckers
Stop within ~50 miles on a Cummins X15, Detroit DD15, PACCAR MX-13. Driving longer risks damaging adjacent components (turbo housing, head gasket, aftertreatment) and turning a $300 fix into a $3,000 fix.
Before you replace parts — try this first
P0120 on the Throttle system is most often corroded or loose pedal or throttle sensor connector — not a dead part. On commercial diesels especially, work these in order before spending money on parts:
- Confirm the symptoms. Match what your truck is doing to the Symptoms section below. If the code is set but the symptoms are not there, clear the code and re-test after a drive cycle — it may not return.
- Clean and dry the sensor connector. Water, road salt, or oil seeping into a connector is the #1 fake sensor failure. Disconnect, look for green corrosion or moisture, spray with electrical contact cleaner, dry, reseat. Many faults clear here for $0.
- Inspect the wiring harness. Trace the wire from the sensor toward the ECM — look for chafing against the engine block, melted insulation near the turbo or exhaust, or pinch points where a clamp has rubbed through.
- Test the sensor before swapping it. Most Common Causes (below) list the expected voltage or resistance range — a $20 multimeter saves a $300 sensor swap that does not fix the problem.
- Only then, replace the part. Use the OEM or a tier-1 aftermarket. Cheap sensors throw the same code in two weeks.
Symptoms
Check engine light with the engine often stuck at idle or a fixed fast idle, poor or no response to the pedal, or limp mode. Cruise control may not work.
Common causes
- CommonCorroded or loose pedal or throttle sensor connector ($30–$200)
- CommonDamaged wiring, often at the pedal under the cab floor, at the firewall or near the throttle body ($100–$450)
- CommonWorn or failed pedal or throttle position sensor ($150–$600)
- RareECM input fault ($900–$2500)
How to fix
Inspect the pedal or throttle connector and harness for corrosion and damage
When to use: First step on P0120
Check 5V reference, ground and signal while slowly moving the pedal or throttle
When to use: To find dropouts or a dead circuit
Replace the pedal assembly or throttle position sensor
When to use: When the signal does not move smoothly with power and ground good
Run any required pedal or throttle relearn after repair
When to use: After replacing the sensor, if the engine requires it
Affects most
No specific vehicle pattern found in our complaint database. This code can appear on any vehicle that supports OBD-II diagnostics.
Related
- → Browse all DTC trouble codes
- → Decode a VIN to check codes for your truck
- → Owner-operator maintenance guides
- → P0100 — Mass or Volume Air Flow Circuit
- → P0101 — Mass or Volume Air Flow Circuit Range/Performance
- → P0102 — Mass or Volume Air Flow Circuit Low Input
- → P0103 — Mass or Volume Air Flow Circuit High Input
- → P0104 — Mass or Volume Air Flow Circuit Intermittent
Frequently Asked Questions
Can I drive with code P0120?
Drive with caution. P0120 is severity 4 — stop within ~50 miles. Most commercial diesel codes at this severity (turbo, fuel rail, transmission solenoid) can damage adjacent expensive components if left unrepaired. Get to a service location before highway-speed loads.
How much does it cost to fix P0120 on a commercial truck?
Total repair cost ranges $30–$2500 depending on the root cause. The cheapest fix is usually the first one listed below — Corroded or loose pedal or throttle sensor connector — which is also the most common. On commercial trucks the cost is usually higher than passenger vehicles due to part availability and shop labor rates ($120–185/hour at most truck shops, vs $90–130 at general auto shops). DIY-capable fixes (sensors, hoses, filters) are at the low end; major component replacements (turbo actuator, high-pressure fuel pump, transmission solenoid pack) are at the high end. Always diagnose with a scan tool BEFORE replacing parts — guessing wrong on a $1,500 turbo actuator when the actual issue was a $30 boost hose is a common expensive mistake.
Will P0120 fail a DOT inspection?
P0120 is generally not a direct DOT inspection fail item by itself. Inspectors focus on safety-critical systems (brakes, steering, tires, lights, hours-of-service). However, the check engine light alone can trigger additional inspection scrutiny. Best practice: fix the code and clear it before a planned inspection.
What's the difference between P0120 and related codes?
P0120 is closely related to: P0100 (Mass or Volume Air Flow Circuit), P0101 (Mass or Volume Air Flow Circuit Range/Performance), P0102 (Mass or Volume Air Flow Circuit Low Input), P0103 (Mass or Volume Air Flow Circuit High Input). These codes share the same subsystem (throttle) and often appear together. If you see this code, scan for the full code list — fixing the root cause usually clears multiple companion codes at once. If only P0120 is set without the companions, the issue is more isolated.
How do I prevent P0120 from coming back after a repair?
For P0120 (Throttle/Pedal Position Sensor/Switch A Circuit), prevention centers on the throttle system. Clean throttle body at 100K mile intervals. Don't let carbon build up. Avoid extended idle time.