P1921

Symptoms

• Engine may not crank / no-crank condition
• difficulty shifting gears
• erratic or harsh shifting
• gear slippage
• transmission not engaging properly or at all
• often accompanied by related codes P0706
• P0707
• P0708 and/or P1702.

Probable causes

• 1) Faulty transmission range sensor/switch. 2) Transmission range sensor harness open or shorted. 3) Poor electrical connection in the transmission range sensor circuit. 4) On 2011-2018 Ford vehicles with the 6R80 transmission
• the TR sensor is integrated into the molded lead frame
• where heat and vibration degrade the internal electrical connections and send incorrect gear position signals to the PCM. 5) Weak battery causing communication errors between modules
• generating P1921 as a secondary symptom. 6) Issues with the transmission control module.

Diagnostic steps

1. Scan all modules and record P1921 plus companion codes (P0706, P0707, P0708, P1702) with freeze frame data.
2. Test battery and charging system first - a weak battery can generate this code as a secondary fault.
3. Inspect the transmission range sensor wiring harness and connector for opens, shorts, corrosion, and poor connections.
4. Verify the transmission range signal on a scan tool matches the actual selector position through all ranges.
5. Test the transmission range sensor/switch; on Ford 6R80 units suspect the molded lead frame with the integrated TR sensor and replace it if the signal is implausible.
6. If sensor and wiring are good, evaluate PCM/TCM communication on the CAN network and the control modules themselves.
7. Clear the DTC and re-test through all gear ranges to confirm the repair.