P0738: TCM Engine Speed Output Circuit Low

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.

Medium severity. Can I keep driving?It depends on the symptoms. If the car is running normally you can go to a repair shop; if it goes into limp mode, drive as little as possible.

Symptoms

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

Most likely causes

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

What to check, in order

Start with the simplest. You can tick off each step.

Check the battery and grounds
With the connector unplugged, check whether the signal wire is making contact with ground.
Clear the code and see if it returns
If it is a reference voltage code, disconnect sensors one at a time and see if the voltage recovers
Check whether a software update is available
Do not replace the control module without a full diagnosis

Good to know

Repair difficulty: medium

What each character of P0738 means

P: Powertrain: engine and transmission
0: Generic code: same on every make
7: Transmission
38: Specific fault in that system

Frequently asked questions

Can I keep driving with code P0738?

It depends on the symptoms. If the car is running normally you can go to a repair shop; if it goes into limp mode, drive as little as possible.

What causes code P0738?

The most common causes are: output wire shorted to ground; receiver shorted internally; connector corroded.

Related codes

Sources

Values and procedures vary by manufacturer. If it affects brakes, steering, fuel or high voltage, see a professional. Updated: 2026-11-04. How we build these pages.