Code
P0725
MERCEDES-BENZ
P — Powertrain
Engine Speed input Circuit Malfunction
Views:
UK: 11
EN: 20
RU: 16
AI status
Completed
Completed
100%
Causes
- Broken, corroded or disconnected wiring/connectors between ECM and TCM
- Faulty crankshaft position (engine speed) sensor or hall/VR pickup
- Faulty ground or power supply to the sensor or control module
- Intermittent connector contact or pin damage
- Defective ECM or TCM (internal electronics or input circuit failure)
- Software/firmware mismatch or module programming error
Symptoms
- Transmission may not shift properly, harsh or erratic shifts
- No torque converter lock-up or limp-home mode
- Engine RPM reading on dash may be erratic or missing
- Check Engine Light / Transmission warning lamp illuminated
- Loss of some drivability functions (reduced performance)
What to check
- Read/record all stored and pending DTCs from ECM and TCM; note freeze frame
- Compare live engine RPM from scan tool to instrument cluster and to TCM input
- Visually inspect sensor, wiring harness, and connectors for damage or corrosion
- Check fuses and module grounds related to engine speed sensor and TCM/ECM
- Backprobe sensor connector and measure signal with multimeter and oscilloscope
- Perform wiggle test on wiring while watching live RPM/signal data for intermittent faults
Signal parameters
- Hall-effect sensor: square wave 0–5V (or 0–12V on some systems), ~50% duty at steady RPM
- Variable reluctance (VR) sensor: AC sine wave, amplitude increases with RPM (tens to hundreds of mV to volts)
- Expected stable RPM pulse frequency proportional to engine speed (compare to scanner RPM)
- No signal or heavy noise/voltage spikes indicates fault
- Short to power or ground will show fixed voltage or no pulse on waveform
Diagnostic algorithm
- With a capable scan tool, confirm P0725 and capture live data: compare engine RPM from ECM, TCM and dash. Note discrepancies.
- Check related fault codes (ECM/TCM) to identify upstream sensor faults such as crankshaft position codes (e.g., P0335).
- Visually inspect the crank sensor connector, harness and mounting for damage, oil ingress or corrosion. Repair as needed.
- Verify sensor supply and ground at the connector with key on: check reference voltage (if applicable) and ground continuity to chassis.
- Backprobe the signal lead and observe with a digital multimeter and preferably an oscilloscope while cranking and at idle. Look for clean pulses without dropouts or excessive noise.
- If waveform/signal is absent or out of spec, test sensor per manufacturer resistance/voltage procedure; replace sensor if faulty.
- If sensor output is good, trace wiring for continuity and shorts between sensor and ECM/TCM; repair any faults (repair or replace harness/connector).
- Check and clean module connectors and verify module grounds. Re-test after repairs.
- If wiring and sensor are good but the TCM input remains absent, consider module fault—check for updated software and consult technical service bulletins before replacing modules.
- Clear codes and perform road test; verify engine RPM and transmission behavior and confirm code does not return.
Likely causes
- Damaged or chafed engine speed sensor wiring harness
- Failed crankshaft position sensor (common failure item)
- Corroded connector at sensor or at transmission/ECM
- Blown fuse or poor ground feeding sensor or modules
- Interference/noise on the signal (short to power or ground)
Fault status
Status
Engine speed input circuit malfunction reported — the transmission control module is not receiving a valid engine RPM signal (no signal, intermittent signal, or out-of-range signal). This may affect transmission shift control and torque converter lock-up.
Repair difficulty: Medium
Diagnostic time: 1.5-3 hours
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