FAQ
TL;DR: For Peugeot 307 TU5JP4 1.6 16V with P0121/Antipollution Fault, confirm a stable 5V reference to the throttle/pedal sensor; “one of the external pins should have 5V after ignition.” [Elektroda, sulof1601, post #16645022]
Why it matters: This helps you diagnose high idle, limp mode, and throttle non-response without buying parts blindly. This FAQ is for DIYers and techs asking “how do I fix P0121/high idle on a 307 TU5JP4?”
Quick Facts
- Symptom set: P0121 plus “Antipollution Fault,” intermittent limp, high idle around 1100 rpm, often on first throttle input. [Elektroda, gsmowiec, post #16633304]
- TU5JP4 uses a 6-pin electronic throttle; measured 5V present at pin 5 in fault state. [Elektroda, gsmowiec, post #16645159]
- 1.4 had a cable; 1.6 uses a pedal/sensor setup, with 5V on an outer pin at ignition. [Elektroda, sulof1601, post #16645022]
- Throttle pot wear can appear by 28,000 km, triggering P0121/P0122 and sudden power loss. [Elektroda, tunek, post #18848400]
- Brush shift fix: moving wiper ~0.5 mm and cleaning tracks restored operation short-term. [Elektroda, tunek, post #18848400]
What does OBD-II code P0121 mean on a Peugeot 307 TU5JP4?
P0121 indicates a throttle/pedal position sensor range or performance issue. The ECU sees mismatched or implausible signals. It often triggers the “Antipollution Fault” message and limp mode with poor throttle response. Clear codes, but verify the 5V reference and sensor signals before replacing parts. [Elektroda, gsmowiec, post #16633304]
Why does my idle jump to about 1100 rpm with Antipollution Fault?
The ECU enters a protective strategy when it detects faulty throttle position data. It raises idle and may limit throttle to prevent runaway. Owners reported 1100 rpm idle and intermittent normal behavior after a short drive. Inspect throttle and sensor wiring and confirm reference voltage. [Elektroda, gsmowiec, post #16633304]
Is my car cable-throttle or drive-by-wire?
On 1.6 16V TU5JP4, the system uses an electronic throttle with a pedal sensor, not a cable. The earlier 1.4 had a cable. Expect a multi-pin throttle body and a separate sensor feed with a 5V reference on an outer pin. [Elektroda, sulof1601, post #16645022]
How do I check the 5V reference quickly?
Enter fault mode, unplug the four‑pin sensor connector, and measure between pins 4 and 2. You should see 5V with ignition on. No 5V suggests wiring or ECU supply issues rather than the throttle body itself. “See whether you will have a voltage of 5V.” [Elektroda, sulof1601, post #16635149]
Where is the sensor and what readings should I see?
The pedal/throttle position sensor sits near the bulkhead in the engine bay. Users measured 5V on the sensor supply and also noted 5V at throttle pin 5. Abnormal values, like ~10V between certain pins, point to wiring faults or ECU problems. [Elektroda, gsmowiec, post #16645159]
Can replacing the throttle body alone fix P0121?
Not always. One user replaced the throttle and the fault returned. Investigate the sensor supply, harness, and ECU. A contributor warned it can be a short in the loom or a failed ECU driver. Rule these out before buying more parts. [Elektroda, sulof1601, post #16645216]
What’s a common root cause besides wiring?
Throttle potentiometer wear inside the Bosch unit. The wiper scrapes resistive tracks, causing signal dropout and A/B mismatch (P0121/P0122). One owner saw failure after 28,000 km in two years. This can also cause sudden power loss in traffic. [Elektroda, tunek, post #18848400]
Is there a temporary repair for a worn throttle potentiometer?
Yes. Open the throttle, clean tracks with isopropanol, and shift the brushes about 0.5 mm to unworn paths. The car then “runs like gold,” but lifespan is limited. Replace any wet or loose connectors during reassembly. [Elektroda, tunek, post #18848400]
What’s an edge case I should watch for?
If 5V is missing or abnormally high at the sensor, suspect a loom short or ECU output stage failure. In that case, replacing the throttle will not help. Diagnose wiring continuity and ECU before further parts. [Elektroda, sulof1601, post #16645216]
How do I perform a 3-step voltage test on the throttle circuit?
- With the fault present, key ON, unplug the four‑pin sensor connector.
- Measure 5V between pins 4 and 2 with a multimeter.
- If 5V is absent, inspect wiring to ECU and grounds before replacing hardware. [Elektroda, sulof1601, post #16635149]
Why does the car run fine after a restart yet keep the error?
The ECU may recover sensor tracking after a key cycle, restoring throttle response. However, it retains the stored DTC until cleared and the fault does not reoccur. This matches reports of normal driveability after restarting, with the code still stored. [Elektroda, gsmowiec, post #16644961]
What should I check first if the car won’t respond to the accelerator?
Confirm the 5V reference and grounds at the sensor and throttle. Inspect the throttle connector for moisture and the pedal plug for loose contacts. Clean or repair as needed before calibration. “The plug was wet, and the gas pedal plug had very loose contacts.” [Elektroda, tunek, post #18848400]
Do I need to calibrate the throttle after cleaning or repair?
Yes. After cleaning or internal repair, perform throttle calibration with a capable tool. One user cleaned and calibrated, yet faults persisted until addressing the potentiometer wear and connectors. Calibration confirms learned positions. [Elektroda, marcomp_net, post #17503605]
Could the crankshaft sensor cause these symptoms?
A faulty crank sensor can cause stalling and misfires, but P0121 points at throttle/pedal feedback. One owner replaced the crank sensor during overhaul, yet P0121 behavior remained until addressing throttle electronics. Prioritize the throttle circuit for this code. [Elektroda, marcomp_net, post #17503605]
What is CAN bus in this context?
CAN bus is the car’s high-speed network linking the ECU and modules. P0121 itself is from the throttle/pedal sensor circuit, not CAN faults. However, unstable 5V or bad grounds can mimic communication issues. Fix power and signal integrity first. [Elektroda, sulof1601, post #16645022]
