P0846 indicates an issue in the transmission fluid pressure sensor switch 'B' circuit. Diagnosis requires systematic checks:
Common Causes
- Wiring Damage: Shorted/open wires or corroded connectors in the sensor circuit.
- Faulty Transmission Fluid Pressure (TFP) Sensor: Internal sensor malfunction.
- Low/Poor Fluid: Contaminated or insufficient transmission fluid.
- Internal Transmission Faults: Clogged passages, worn pump, or valve body issues.
- ECM/PCM Failure: Faulty control module processing sensor signals.
Diagnostic Steps
1. Check Transmission Fluid:
Verify fluid level and condition. Brown/burnt fluid or low level indicates internal problems.
2. Inspect Wiring & Connectors:
Locate the TFP sensor (typically on valve body). Examine the harness for:
- Chafing or exposed wires near transmission case.
- Corrosion/bent pins in connectors (disconnect battery first).
- Continuity/resistance checks between sensor and PCM using a multimeter.
Refer to wiring diagrams for specific pinouts.
3. Test Sensor Voltage/Resistance:
With ignition on (engine off), probe sensor signal wire voltage (compare specs). Use a scan tool to monitor live data—unexpected pressure readings indicate sensor failure.
4. Rule Out Mechanical Issues:
If wiring/sensor test normal, inspect valve body for debris or valve sticking. Perform hydraulic pressure tests if accessible.
Repairs
- Repair or replace damaged wiring harnesses/connectors.
- Replace TFP sensor if voltage/resistance values deviate from specifications.
- Flush fluid and replace filter; address internal transmission repairs if mechanical faults are found.
- Update or replace PCM if other tests confirm no circuit/sensor issues.
Pro Tip: Clear DTC after repairs and test-drive to confirm resolution.