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DTC codes / P07xx · Transmission / P07B9

P07B9Transmission Park Position Sensor/Switch B Circuit Low

P07B9 indicates that the transmission control module has detected a low voltage condition on the circuit for the transmission park position sensor or switch B. This sensor tells the computer whether the transmission is mechanically locked in Park, and a low signal means the circuit is grounded, shorted, or the sensor has failed.

SystemPowertrain
Code typeGeneric (all makes)
SeverityMedium
Fix difficultyIntermediate
AreaTransmission

Symptoms you may notice

  • The check engine light or transmission warning light illuminates on the dashboard.
  • The vehicle may fail to start because the computer does not recognize the Park position.
  • The gear selector may feel loose or fail to engage Park fully.
  • A message such as 'Shift to Park' may appear when the lever is already in Park.
  • Often no symptoms besides the warning light, especially if the fault is intermittent.

Common causes, most likely first

  1. A faulty transmission park position sensor or switch B internal short to ground.
  2. Damaged or chafed wiring in the sensor circuit, often near the transmission case or shifter assembly.
  3. Corroded or loose electrical connector at the sensor or transmission control module.
  4. Water or fluid intrusion into the sensor connector causing a short.
  5. A failed transmission control module driver for the sensor circuit (less common).
  6. Aftermarket or improperly installed remote start or alarm wiring tapping into the park signal circuit.

How to diagnose and fix it

  1. Connect a scan tool and record all stored and pending codes, then clear codes and test-drive to see if P07B9 returns.
  2. Visually inspect the park position sensor connector and wiring for corrosion, chafing, or fluid contamination, repairing as needed.
  3. Measure resistance and voltage on the sensor B signal circuit at the sensor connector and compare to manufacturer specifications.
  4. Check the sensor ground circuit for excessive resistance or an open circuit back to the transmission control module.
  5. If the circuit tests good, remove and bench-test the park position sensor or switch, replacing it if it fails the specification.
  6. If the sensor and wiring are intact, test the transmission control module output for a shorted driver and replace the module only if confirmed faulty.
  7. After repair, clear codes and verify the park position status on the scan tool data stream in all gear selector positions.

Park sensor circuit checks most shops miss

  • A low signal code often comes from a chafed wire harness rubbing against the transmission case or a sharp bracket, not from a failed sensor.
  • Aftermarket remote start systems frequently tap into the park position wire, and a poor splice can set P07B9 even when the factory sensor is perfect.
  • The sensor may test fine with a multimeter but fail only when the transmission is hot or vibrating, so a wiggle test while watching live data is essential.
  • Do not replace the transmission control module until you have verified the signal circuit is not shorted to ground with the sensor disconnected.
  • A weak or failing battery can cause erratic park position signals on some vehicles, so check charging system health before condemning the sensor.

Can I keep driving with P07B9?

You can usually drive the vehicle, but do not rely on the Park position to hold the car or allow starting, and avoid parking on any incline until the fault is repaired. If the vehicle fails to start or rolls in Park, stop driving and have it towed.

Tools that help

Tools we recommend are in the Auto Veteran shop on Amazon. As an Amazon Associate we earn from qualifying purchases.

Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.

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