OBD-II Diagnostic Trouble Code
P2120 — Throttle/Pedal Position Sensor/Switch D Circuit
P2120 means the ECM has detected a general circuit fault on accelerator pedal position (APP) sensor D, usually the first of two or three tracks in the pedal assembly. The signal is missing, erratic or outside what the circuit should produce, so the ECM cannot trust driver demand from that track.
Severity 4/5
Truck-relevant
Stop within 50 mi
Severity
Stop within 50 mi
Throttle/Pedal Position Sensor/Switch D Circuit
Code
P2120
Category
Powertrain
Subsystem
Throttle
Truck-relevant
Yes
What this means for truckers
P2120 (Throttle/Pedal Position Sensor D Circuit) — 4th TPS variant on drive-by-wire pedals (some HD trucks have multiple redundant sensors).
Before you replace parts — try this first
P2120 on the Throttle system is most often chafed, corroded or loose wiring or connector at the pedal assembly (floor-mounted pedal harness is prone to water and wear) — 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
The check-engine light comes on and most trucks go into a reduced-power or idle-only mode, often with the engine responding slowly or holding a fixed elevated idle. Throttle response may cut in and out over bumps if the fault is intermittent, and cruise control is usually disabled.
Common causes
- CommonChafed, corroded or loose wiring or connector at the pedal assembly (floor-mounted pedal harness is prone to water and wear) ($80–$350)
- CommonFailed accelerator pedal position sensor assembly ($180–$550)
- OccasionalShorted or loaded 5V reference shared with other sensors ($100–$450)
- RareECM connector pin fit or internal ECM fault ($250–$2500)
How to fix
Inspect the pedal connector and harness under the cab floor for corrosion, pushed-back pins and chafing; repair and retest
When to use: First step, especially if the code is intermittent or appears after washing or rain
With a scan tool, watch APP D voltage/percent while slowly depressing the pedal and look for dropouts or flat spots
When to use: To confirm whether the sensor track itself is faulty
Check 5V reference, signal and ground at the pedal connector with a multimeter, key on
When to use: When the signal is missing or stuck regardless of pedal position
Replace the pedal assembly and clear codes, then verify idle validation and full travel readings
When to use: When wiring and supply are good but the sensor reading is erratic
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
- → P2100 — Throttle Actuator Control Motor Circuit/Open
- → P2101 — Throttle Actuator Control Motor Circuit Range/Performance
- → P2127 — Throttle/Pedal Position Sensor/Switch E Circuit Low Input
- → P2128 — Throttle/Pedal Position Sensor/Switch E Circuit High Input
- → P2138 — Throttle/Pedal Position Sensor/Switch D/E Voltage Correlation
Frequently Asked Questions
Can I drive with code P2120?
Drive with caution. P2120 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 P2120 on a commercial truck?
Total repair cost ranges $80–$2500 depending on the root cause. The cheapest fix is usually the first one listed below — Chafed, corroded or loose wiring or connector at the pedal assembly (floor-mounted pedal harness is prone to water and wear) — 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 P2120 fail a DOT inspection?
P2120 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 P2120 and related codes?
P2120 is closely related to: P2100 (Throttle Actuator Control Motor Circuit/Open), P2101 (Throttle Actuator Control Motor Circuit Range/Performance), P2127 (Throttle/Pedal Position Sensor/Switch E Circuit Low Input), P2128 (Throttle/Pedal Position Sensor/Switch E 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 P2120 is set without the companions, the issue is more isolated.
How do I prevent P2120 from coming back after a repair?
For P2120 (Throttle/Pedal Position Sensor/Switch D Circuit), prevention centers on the throttle system. Clean throttle body at 100K mile intervals. Don't let carbon build up. Avoid extended idle time.