Powertrain, generic · Computer and auxiliary outputs

P0634 Control Module Internal Temperature A Too High

Check engine lightUsually off
EmissionsNot relevant
Drive cycleNot required
Limp modePossible

The module measures its own circuit board temperature with an internal sensor and sets P0634 when it overheats. In practice this concerns mainly transmission modules mounted in or on the gearbox, such as GM's 6T70/6T75 and 6L units with the module in the valve body, surrounded by transmission fluid. GM sets it at 142 °C or more for 5 s (Saab 9-4X: 146 °C), or at 50 °C or more with system voltage of 18 V or higher for 2 s, because overvoltage heats the output stages. The Saab 9-4X then goes into limp mode with only reverse and 5th gear, manual shifting disabled and maximum line pressure; GM's table rates it without the check engine light. Check transmission overtemperature (P0218) first, then fluid level and cooler, engine cooling and charging voltage. Faults of the internal temperature sensor circuit are P0666–P0669.

Likely causes

Transmission fluid too hot: cooler clogged, lines kinked, air flow blocked, wrong level
Engine overheating
Charging system overvoltage (18 V or more)
Faulty internal temperature sensor or valve body module
High sustained load (towing, mountain driving, heat)

Symptoms

  • Transmission limp mode (fixed gear), harsh shifts
  • Manual shifting disabled
  • Transmission warning, check engine light depending on the maker
  • Often together with transmission overtemperature (P0218)

Repair estimate

DifficultyHardWorkshop job
Parts and labour€100–2,000
Time1–6 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

transmission control moduletransmission fluid coolercharging system

Related codes

P0218P0711P0562P0563P0666

References: law.cornell.edu, gsi.ext.gm.com, saabwisonline.com.