Peugeot Partner 2023 C1387 Rear Speed Sensor Reads High – ABS Coherence Fault Diagnosis

User question

Hello Peugeot partner 2023 code C1387
front speed sensor okay but rear speed sensor red more? Please

Artificial Intelligence Response

Direct answer to the question

• DTC C1387 on a 2023 Peugeot Partner usually flags a rear wheel speed signal “coherence” (plausibility) fault. If your scan tool shows the rear sensor in red and “reading more” (higher speed than the others), the system is seeing an implausibly high or noisy signal from one rear wheel.
• Most likely causes (in order): debris or damage on the magnetic encoder ring in the rear hub, incorrect tire size/pressure causing coherence error, mis-seated rear sensor/air gap issue, wiring/connector damage on the rear harness, or a mismatched hub/bearing encoder pole count. Replace parts only after confirming which side is bad.

Detailed problem analysis

• How the system works
  • Your Partner (K9) uses active wheel speed sensors that read a multi‑pole magnetic encoder integrated into the wheel bearing/hub. The ABS/ESP ECU compares all four wheel speeds for coherence. If one rear sensor reports a speed consistently higher than the others (especially during straight‑line driving), it sets C1387.
• Why a rear sensor can “read more”
  • Magnetic contamination: fine ferrous particles on the encoder ring distort the magnetic pattern, creating extra transitions (spurious pulses) and apparent speed spikes.
  • Tire/rolling radius mismatch: a smaller effective radius (wrong size or low pressure) spins faster and can cross the plausibility threshold.
  • Sensor mounting/air gap: corrosion or debris under the sensor flange tilts it, increasing noise and double‑triggering.
  • Bearing/encoder issues: damaged, de‑magnetized, or wrong pole‑count encoder (after bearing replacement) produces a fixed ratio error versus the other side.
  • Wrong sensor variant: some aftermarket sensors with different internal conditioning can scale or distort pulse timing.
  • Wiring faults: intermittents in the 2‑wire active sensor supply/signal can be interpreted as rapid edges, seen by the tool as “higher speed.”
• Tests that separate causes (do these in order)
  1. Identify the side: read freeze‑frame and live data. At a steady 30–60 km/h straight, rear left and rear right should match front speeds within ~1–2 km/h. The side that spikes or reads consistently higher is suspect.
  2. Basic coherence checks:
     • Verify tire sizes on all four corners match the door‑label spec; correct pressures cold; check tread depths (gross mismatch can trigger coherence faults).
  3. Visual/mechanical inspection (suspect side first):
     • Remove the sensor; inspect tip for metallic fuzz. Clean with lint‑free cloth.
     • Inspect the encoder face on the hub/bearing: remove metallic debris; look for scuffs, tears, or missing magnetic sectors. Do not scrape with sharp tools and do not use a magnet.
     • Ensure the sensor seats fully and squarely; remove any rust lift on the mounting pad; torque the retainer properly.
     • Check wheel bearing play/runout; excessive play varies the air gap and creates noise.
  4. Harness/connector inspection:
     • Follow the rear sensor harness along the trailing arm/axle to the body connector; look for chafing, tension, crushed points, or water ingress. Gently tug‑test pins.
  5. Electrical tests (active sensors; do NOT judge by resistance):
     • Key ON, back‑probe: expect a supply from the ABS ECU (commonly 7–12 V on 2‑wire systems with current modulation, or 5 V on 3‑wire types).
     • Spin the wheel by hand while monitoring with an oscilloscope:
     • 2‑wire active: observe a current‑modulated or low‑amplitude voltage square‑wave; stable frequency proportional to speed; no double edges or dropouts.
     • 3‑wire active: observe a clean digital signal toggling roughly 0.5–4.5 V.
     • A DMM in mA (series) can sometimes show the 2‑wire current toggling, but a scope is far more reliable.
  6. Side‑swap/part‑swap logic:
     • If your rear sensors are separate bolt‑in units and identical, swap left/right sensors. If the “red/high” reading moves with the sensor, replace that sensor.
     • If it does not move, suspect the hub/bearing encoder or wiring on the original side.
  7. Encoder pole‑count sanity check (after any bearing work):
     • If one rear wheel always reads a fixed percentage higher (e.g., ~5–10%) with no spikes, you likely have a hub with a different pole count. Fit the correct bearing/hub for your VIN.
  8. Finalization:
     • After any corrective action, clear DTCs and perform a straight‑line road test; log live data to confirm stable, matched rear speeds.

Current information and trends

• Contemporary PSA/Stellantis small vans use active MR/Hall sensors with magnetic encoders integrated into the hub. Many rear sensors are serviceable separately, but any encoder damage requires hub/bearing replacement.
• Common field trend: coherence faults after brake or bearing work due to debris on the encoder, incorrect hub supplied, or sensors not fully seated. Another frequent cause is harness chafe along the rear suspension travel path.

Supporting explanations and details

• Active vs. passive sensors
  • Passive VR sensors output an AC voltage and can be ohmed; modern active sensors contain electronics and are powered—resistance checks are not conclusive. Use voltage/current and waveform observation instead.
• Why debris matters
  • The encoder ring is a rubberized strip with alternating magnetic poles. Ferrous particles bridging poles distort the magnetic field, creating apparent extra edges—your tool interprets these as higher wheel speed.
• Air gap sensitivity
  • Active sensors tolerate a narrow air‑gap window. Corrosion “lift” under the sensor or bearing play modulates the gap and causes jitter/double‑triggering.

Ethical and legal aspects

• ABS/ESP may be disabled while C1387 is present; braking distance and stability can worsen, especially in wet or emergency maneuvers. Drive cautiously and repair promptly.
• If the vehicle is still under manufacturer warranty, avoid cutting/splicing harnesses or fitting non‑approved parts that could jeopardize coverage.

Practical guidelines

• Tools recommended
  • Scan tool with live graphing, tire pressure gauge, torque wrench, lint‑free wipes, non‑residue cleaner, oscilloscope (preferred), or automotive current probe; magnetic viewing film if available.
• Quick triage path you can do today
  1. Confirm tire sizes/pressures.
  2. Read live data; identify which rear wheel reads “more.”
  3. Remove/clean that sensor and the encoder face; reseat sensor.
  4. Inspect harness along the rear arm to the body connector.
  5. Clear codes and road test. If still bad, scope the signal and plan for sensor or hub replacement as indicated.

Possible disclaimers or additional notes

• Sensor “resistance” values listed generically on the internet do not apply reliably to active sensors and can mislead diagnosis.
• On some trims the rear sensor is molded into the hub; on others it’s a separate bolt‑in. Use your VIN to confirm parts architecture before planning a swap test.

Suggestions for further research

• Consult Peugeot Service Box/official workshop manual for your exact VIN to confirm sensor type, hub part numbers, and pinouts.
• Search technical service bulletins (TSBs) for K9 Partner ABS coherence faults after brake/bearing service.
• If available, use Diagbox to access detailed fault descriptors and freeze‑frame (vehicle speed, wheel speeds, steering angle) to pinpoint conditions that set C1387.

Brief summary

• C1387 on a 2023 Partner typically indicates a rear wheel speed coherence fault. A rear wheel showing “red”/higher speed is commonly due to encoder contamination or damage, sensor seating/air‑gap issues, tire/pressure mismatch, wiring faults, or an incorrect hub encoder pole count. Identify the side via live data, clean/inspect sensor and encoder, verify harness and air gap, then confirm signals with a scope. Replace the sensor or hub as indicated and verify with a road test. If you share which rear side shows the error and whether any brake or bearing work was done recently, I can tailor the next steps and pinouts for your exact setup.

Disclaimer: The responses provided by artificial intelligence (language model) may be inaccurate and misleading. Elektroda is not responsible for the accuracy, reliability, or completeness of the presented information. All responses should be verified by the user.