P0738: TCM Engine Speed Output Circuit Low

What does P0738 mean?

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.

The code tells you which monitor failed, not which part is broken. Test before replacing parts.

Symptoms

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

Common causes

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

Likelihood is a general estimate across vehicles; your make and model may differ.