Code
P1387
HONDA
P — Powertrain
CYP Sensor B No Signal
AI status
Completed
Completed
100%
Causes
- Open or shorted wiring between sensor and ECM
- Loose, corroded, or damaged sensor connector
- Faulty CYP Sensor B (internal failure)
- Poor or missing sensor power or ground
- Damaged reluctor/target wheel or timing component preventing signal generation
- ECM input circuit fault or internal ECM failure
Symptoms
- Check Engine MIL illuminated (P1387 stored)
- Engine may crank but not start, or hard starting
- Rough idle, misfire, or reduced power
- Stalling or intermittent loss of drive
- Poor fuel economy or abnormal idle behavior
What to check
- Read DTC(s) and freeze-frame data with a scan tool; record operating conditions when code set
- Visually inspect sensor connector and wiring for corrosion, pins pushed out, heat damage or chafe
- Check related fuses and engine/ECM grounds
- Backprobe sensor connector to verify reference voltage and ground with key ON and engine cranking
- Monitor sensor signal with a lab scope or scan-tool live data while cranking and running
- Check continuity and resistance from sensor connector to ECM connector
Signal parameters
- Hall-type (electronic) sensor: 0–5 V square wave; low ≈ 0 V, high ≈ 4–5 V; frequency increases with RPM
- Inductive (magnetic) sensor: AC waveform amplitude increases with RPM (typical 0.1–4 Vpp depending on speed)
- Reference supply: typically 5 V or switched 12 V depending on sensor type — verify in service manual
- Sensor coil resistance (if applicable) varies by design — consult OEM specs before replacing based on resistance
Diagnostic algorithm
- Verify the code: clear codes, attempt to reproduce; note conditions when code returns.
- Visual inspection: disconnect battery negative if required, inspect sensor connector and wiring for damage, corrosion or oil intrusion.
- Power/ground check: backprobe the sensor connector with key ON to confirm reference voltage and ground presence per service data.
- Signal check: with a lab scope or digital multimeter (AC for inductive or DC for hall) monitor the signal while cranking and at idle; look for proper square/AC waveform and continuity to ECM.
- Continuity and resistance: unplug harness ends and check continuity from sensor pins to ECM pins and measure sensor resistance if specified.
- Wiggle test: with connector connected, wiggle harness and connector while monitoring the signal to detect intermittent opens/shorts.
- Inspect timing/target wheel: verify reluctor/target wheel and timing components are intact and aligned if physical damage suspected.
- Substitute or bench-test sensor (if available) or replace sensor if bench/signal tests fail. Re-check for code clearance and perform road test.
- If wiring and sensor test good but code remains, inspect ECM connector and grounds; consider ECM input circuit diagnosis or replacement per manufacturer procedures.
Likely causes
- Connector corrosion or pushed pin at sensor/ECM
- Broken or shorted wire in the harness near the engine or firewall
- Failed sensor from heat or oil contamination
- Missing reference voltage or ground at sensor due to blown fuse or bad chassis/ECM ground
Fault status
Status
P1387 — CYP Sensor B: No signal received by ECM. Circuit fault, open/short, or sensor failure suspected.
Repair difficulty: Medium
Diagnostic time: 1.0-2.0 hours
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