DTC P0122 – Throttle/Pedal Position Sensor A Circuit Low Input.

P0122 – TPS/APP Sensor A Circuit Low Input

Meaning:
The ECU is detecting a lower-than-expected voltage from Throttle Position Sensor A (TPS A) or Accelerator Pedal Position Sensor A (APP A).
Typical expected sensor signal range ≈ 0.5V – 4.5V. Low input usually means <0.2–0.3V.

Common Symptoms

Low power / limp mode

Poor acceleration

High idle or unstable idle

Throttle response delay

MIL (Check Engine Light) ON

Common Causes

1. TPS/APP Sensor Fault

Internal sensor failure

Worn or dead spot inside potentiometer

2. Wiring Issues (Most common)

Signal wire shorted to ground

Open circuit in signal wire

Loose connector pins

Corroded terminals (especially in humid areas)

3. Faulty 5V Reference

Shared 5V circuit shorted by another sensor

Low reference voltage from ECU

4. Bad Ground

Ground wire intermittent or high resistance

5. Throttle Body Issues

Electronic throttle actuator fault

Throttle body stuck/dirty (less common for P0122)

6. ECU Fault (Rare)

Step-by-Step Diagnostic Procedure

Step 1: Scan Tool Data Check

Check TPS A / APP A signal voltage with key ON:

Normal: 0.5V–1.0V at idle

If reading is 0.1–0.2V, it confirms low input

Compare TPS A vs TPS B (if available) — should be proportional

Step 2: Visual Inspection

Inspect connectors at throttle body or accelerator pedal

Look for:

Bent pins

Loose fit

Water intrusion

Oil contamination

Damaged wires near throttle body (very common)

Fix if needed → clear codes → recheck.

Step 3: Check Reference Voltage (5V)

Using multimeter:

1. Backprobe the 5V reference wire

2. Should read about 5V

If <4.5V:

Another sensor sharing 5V may be shorting (MAP, MAF, ECT, etc.)

Disconnect sensors one-by-one to see when 5V returns

Step 4: Check Sensor Ground

Check ground terminal continuity

Voltage drop test: should be <0.1V

Poor ground = low output signal.

Step 5: Check Signal Voltage

Backprobe TPS/APP signal:

With the pedal released → around 0.5V

Slowly press pedal → voltage must increase smoothly to 4.0–4.5V

If voltage stays very low or fluctuates:

Sensor bad

Signal wire short to ground

Step 6: Check Wiring Continuity

Signal wire: sensor → ECU

5V wire: ECU → sensor

Ground: sensor → ECU/body ground

Look for broken wires inside insulation.

Step 7: Replace the Faulty Component

Based on findings, replace:

Throttle Position Sensor (for external TPS type)

Accelerator Pedal Position Sensor

Complete electronic throttle body (if integrated TPS)

After replacement → perform throttle body / APP relearn, depending on vehicle.

Quick Fix Summary

Cause Fix

5V missing Find shorted sensor / wiring
Signal wire shorted Repair harness
Loose/corroded connector Clean/replace
Faulty TPS/APP sensor Replace
Throttle body fault Replace throttle body
ECU fault Very rare – confirm carefully.

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