Powertrain, generic · Transmission

P0738 TCM Engine Speed Output Circuit Low

Check engine lightUsually off
EmissionsNot relevant
Drive cycleNot required
Limp modeNo

Some transmission modules pass an engine speed signal on to other devices such as a tachometer over a wire of their own. CARB requires output circuits to be checked. No manufacturer document available to us uses P0737–P0739 – engine speed is shared over CAN today; the description follows CARB and the engine module's own speed output (P0654), which sends, for example, two pulses per revolution. P0738 means this output is too low, typically shorted to ground, so it can no longer switch high. The opposite is P0739, the general fault P0737. A tachometer or dependent display then reads zero or jumps, while driving is usually unaffected.

Likely causes

Output wire shorted to ground
Receiver shorted internally
Connector corroded
Driver stage in the transmission module faulty

Symptoms

  • Tachometer at zero or jumping
  • Warning lamp depending on manufacturer
  • Usually no effect on driving

Repair estimate

DifficultyMediumDIY possible
Parts and labour€50–300
Time0.5–2 h

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

Parts involved

engine speed output circuittachometertransmission control module

Related codes

P0737P0739P0654P0725

References: law.cornell.edu, saabwisonline.com.