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DTC codes / P0Cxx · Hybrid propulsion / P0C99

P0C99Hybrid/EV Battery Temperature Sensor L Circuit Low

P0C99 indicates the hybrid or electric vehicle battery pack's temperature sensor 'L' circuit is reporting a voltage below the normal range, which the battery energy control module interprets as an excessively low temperature reading or a short to ground. The computer sets this code when the signal voltage stays under the calibrated minimum for a set time, often during key-on or self-test routines.

SystemPowertrain
Code typeGeneric (all makes)
SeverityMedium
Fix difficultyProfessional
AreaHybrid propulsion

Symptoms you may notice

  • Illumination of the hybrid system warning light or service vehicle soon message
  • Often no symptoms besides the warning light, especially in mild weather
  • Reduced or disabled hybrid battery charging and discharging capability
  • Powertrain power limitation or limp-home mode in some vehicles
  • Inaccurate or missing battery temperature display on the dash or energy monitor
  • Possible failure to enter ready mode or start the hybrid system in extreme cases

Common causes, most likely first

  1. Faulty battery temperature sensor 'L' with an internal short or drifted resistance
  2. Damaged or chafed wiring between the sensor and the battery energy control module, shorting to ground
  3. Corroded, loose, or water-intruded connector pins at the sensor or module harness
  4. Battery energy control module internal fault on the sensor input circuit
  5. Moisture or contamination inside the battery pack causing signal leakage to ground
  6. Aftermarket or improper battery pack service work that pinched or misrouted the sensor harness

How to diagnose and fix it

  1. Scan all modules and record freeze-frame data for P0C99, then verify the battery temperature sensor 'L' reading against ambient and other sensor readings
  2. Inspect the sensor harness and connectors for chafing, corrosion, moisture, and loose pins, repairing any damage found
  3. Measure resistance of the 'L' sensor at the connector and compare to the manufacturer's temperature-resistance chart
  4. Backprobe the signal circuit at the battery energy control module to check for a short to ground with the sensor disconnected
  5. Substitute a known-good sensor or simulate the correct resistance at the harness to confirm whether the sensor or the module is at fault
  6. Replace the faulty temperature sensor or sensor harness as required, following high-voltage safety procedures
  7. Clear codes, perform the battery pack learn or calibration routine if specified, and verify the code does not return after a road test

Before condemning the battery temperature sensor

  • A shorted sensor harness is far more common than a failed sensor itself, so always ohm the wiring before replacing parts inside the high-voltage pack.
  • This code can set intermittently from water intrusion after a car wash or heavy rain, and the sensor may test perfectly once the connector dries out.
  • Some vehicles set P0C99 when the 12-volt auxiliary battery is weak because module voltage references shift, so load-test the low-voltage battery first.
  • The battery energy control module may command reduced fast-charge or EV range to protect the pack when it cannot trust a temperature reading, even if the car drives normally.
  • Do not confuse this with a high-temperature code; a low signal voltage means the module sees cold, not heat, and the repair path is completely different.

Can I keep driving with P0C99?

The vehicle is usually safe to drive to a workshop, but avoid aggressive acceleration, fast charging, and long trips until the fault is repaired because the module may limit power or charging. If the hybrid system warning is accompanied by a ready-mode failure or severe power loss, stop driving and arrange a tow.

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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