OBD-II Diagnostic Trouble Code
P2122 — Throttle/Pedal Position Sensor/Switch D Low Input
P2122 means the signal from accelerator pedal position sensor D is below the minimum voltage the ECM expects, typically under about 0.2-0.3 V. This normally points to an open signal wire, a short to ground, or a lost 5V reference to the sensor.
Severity 4/5
Truck-relevant
Stop within 50 mi
Severity
Stop within 50 mi
Throttle/Pedal Position Sensor/Switch D Low Input
Code
P2122
Category
Powertrain
Subsystem
Throttle
Truck-relevant
Yes
What this means for truckers
P2122 (TPS D Low) — pedal sensor shorted to ground.
Before you replace parts — try this first
P2122 on the Throttle system is most often open or shorted-to-ground signal wire or damaged connector at the pedal — 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 truck typically goes into idle-only or a heavy derate with the check-engine light on, and the pedal may have no effect at all. If the fault is intermittent the engine may suddenly drop to idle while driving and then recover.
Common causes
- CommonOpen or shorted-to-ground signal wire or damaged connector at the pedal ($80–$350)
- CommonLoss of 5V reference to sensor D (open circuit or reference pulled down by another shorted sensor) ($100–$450)
- OccasionalInternally failed pedal sensor ($180–$550)
- RareECM internal reference circuit fault ($800–$2500)
How to fix
Measure 5V reference and ground at the pedal connector, key on
When to use: First, to see whether the sensor is being powered
Unplug other sensors on the same 5V reference one at a time and watch whether the reference recovers
When to use: When the 5V reference is low or zero
Check the signal wire for continuity to the ECM and for a short to ground
When to use: When supply and ground are good but signal is near zero
Replace the pedal sensor assembly
When to use: When wiring and supply 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 P2122?
Drive with caution. P2122 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 P2122 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 or shorted-to-ground signal wire or damaged connector at the pedal — 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 P2122 fail a DOT inspection?
P2122 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 P2122 and related codes?
P2122 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 P2122 is set without the companions, the issue is more isolated.
How do I prevent P2122 from coming back after a repair?
For P2122 (Throttle/Pedal Position Sensor/Switch D Low Input), prevention centers on the throttle system. Clean throttle body at 100K mile intervals. Don't let carbon build up. Avoid extended idle time.