OBD-II Diagnostic Trouble Code
P2123 — Throttle/Pedal Position Sensor/Switch D High Input
P2123 indicates the accelerator pedal position sensor D signal is above the maximum voltage the ECM accepts, typically near the 5V reference. Common reasons are an open ground at the sensor or the signal wire shorted to the reference or battery voltage.
Severity 4/5
Truck-relevant
Stop within 50 mi
Severity
Stop within 50 mi
Throttle/Pedal Position Sensor/Switch D High Input
Code
P2123
Category
Powertrain
Subsystem
Throttle
Truck-relevant
Yes
What this means for truckers
P2123 (TPS D High) — pedal sensor shorted to power.
Before you replace parts — try this first
P2123 on the Throttle system is most often open sensor ground circuit or corroded ground terminal — 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 ECM will usually ignore the pedal and command idle or a limp mode, with the check-engine light on. Drivers report no throttle response or an engine stuck at a fixed RPM.
Common causes
- CommonOpen sensor ground circuit or corroded ground terminal ($80–$300)
- CommonFailed pedal sensor ($180–$550)
- OccasionalSignal wire shorted to 5V reference or battery voltage in the harness ($100–$400)
- RareECM fault ($800–$2500)
How to fix
Check the sensor ground at the pedal connector for continuity and voltage drop
When to use: First, since an open ground drives the signal high
Unplug the sensor and check whether the signal stays high; if it does, look for a harness short to voltage
When to use: To separate a wiring short from a sensor fault
Replace the pedal assembly and verify readings at idle and full travel
When to use: When ground and harness test good
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 P2123?
Drive with caution. P2123 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 P2123 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 — Open sensor ground circuit or corroded ground terminal — 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 P2123 fail a DOT inspection?
P2123 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 P2123 and related codes?
P2123 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 P2123 is set without the companions, the issue is more isolated.
How do I prevent P2123 from coming back after a repair?
For P2123 (Throttle/Pedal Position Sensor/Switch D High Input), prevention centers on the throttle system. Clean throttle body at 100K mile intervals. Don't let carbon build up. Avoid extended idle time.