Code
P1656
INFINITI
P — Powertrain
Automatic Transmission System Malfunction
Views:
UK: 14
EN: 55
RU: 24
AI status
Completed
Completed
100%
Causes
- Low or unstable battery/charging system voltage
- Blown fuse or faulty transmission relay
- Poor power or ground to the TCM
- Damaged wiring harness or poor connector at the TCM or transmission
- Faulty Transmission Control Module (TCM)
- Faulty transmission speed/input sensors (turbine/input, output)
Symptoms
- Transmission may go into limp/limitation mode (reduced gears)
- Harsh, delayed, or erratic shifting
- Warning lights illuminated (Check Engine, AT/Trans light)
- Loss of drive or inability to engage gears
- Unusual transmission noises
- Intermittent faults that may clear then return
What to check
- Read and record all stored/active/ freeze-frame DTCs with a capable scanner; note related transmission codes
- Check battery voltage and charging system under load (12.4–14.8V range while running)
- Inspect transmission-related fuses and relays
- Visually inspect TCM and transmission connectors for corrosion, water ingress, bent pins, or loose terminals
- Check continuity and resistance of power and ground circuits to the TCM
- Scan live data: vehicle speed, input/turbine speed, output speed, gear command, shift solenoid statuses, line pressure if available
Signal parameters
- Battery voltage at TCM (V)
- Power (12V) and ground continuity to TCM
- CAN high/low voltage and message activity
- Input (turbine) speed sensor (RPM)
- Output speed sensor (vehicle speed) (RPM or km/h)
- Gear selection/command from TCM
Diagnostic algorithm
- Connect a factory-level or full-feature scan tool; read and record all codes and freeze-frame data. Note any other transmission or communication codes.
- Verify battery state of charge and charging system. Recharge or replace battery if below specification; retest.
- Inspect and test fuses/relays for the TCM and transmission circuits. Replace if faulty.
- Visually inspect the TCM and transmission harness/connectors. Repair corroded pins, damaged wires, or poor connections.
- Check power and ground continuity to the TCM using a multimeter; backprobe connectors with ignition ON and engine OFF as required by service manual.
- Use live-data to compare input vs output speed sensors, gear command vs actual gear, and shift solenoid commands. Look for mismatches or missing sensor signals.
- Perform actuator/solenoid tests via the scan tool (if supported) to confirm solenoids respond and to observe current/duty values.
- Check transmission fluid level and condition; if low or burnt, correct level and consider fluid/filter service. If contamination or metal debris present, suspect internal damage and proceed accordingly.
- If electrical and fluid checks are normal, perform further hydraulic/mechanical tests per service manual: line pressure test, stall test, or valve body inspection as required.
- If wiring and hydraulic systems check good and fault remains, consider TCM reprogramming or replacement only after confirming no external cause. Always clear codes and verify repair by road testing.
- Document findings. If unsure or repairs are extensive, consult factory service information or a transmission specialist.
Likely causes
- Weak battery or poor charging causing intermittent TCM operation
- Corroded/loose TCM connector or pin damage
- Related transmission electrical component (sensor or solenoid) short/open
- Internal transmission hydraulic problem (valve body, line pressure)
- TCM hardware or software fault
Fault status
Status
Automatic Transmission System Malfunction — the Transmission Control Module has detected a fault in the transmission control system. Vehicle may enter limp mode, show transmission-related warning, and exhibit shifting problems. Further diagnosis required.
Repair difficulty: Hard
Diagnostic time: 2-6 hours
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