Damaged, chafed or shorted wiring in the engine speed signal circuit (short to earth or high resistance); Corroded, loose or water-damaged connector at the TCM, engine control module or crankshaft sensor; Faulty crankshaft position sensor or transmission input/output speed sensor feeding the signal; Poor power or earth supply to the TCM or engine control module; Fault within the TCM itself, or in the engine control module providing the signal; Outdated or faulty control unit software, where the manufacturer has documented it as a cause for the specific model.

P0738

TCM Engine Speed Output Circuit Low.

Generic
Powertrain
Generic code
Medium severity

How this code is built

P: SystemPowertrain
0: Defined bySAE, the same on every car
7: AreaTransmission
38: FaultThe specific fault in that area

What it means

P0738 is the generic code for a low signal in the engine speed output circuit between the transmission control module (TCM) and the rest of the vehicle. The TCM monitors the engine speed signal it sends out, or receives, and has seen a voltage or signal level lower than expected. The exact wiring, signal path and which module sets the code vary by make, model and transmission, so check the manufacturer's data for the vehicle.

Symptoms

Malfunction indicator lamp or transmission warning lamp on
Rev counter erratic, reading low or not working
Harsh, delayed or erratic gear changes
Transmission in limp mode or stuck in one gear
Torque converter lock-up not working, with poor economy
Possible stalling or rough running if the signal is shared with other systems

Possible causes

Damaged, chafed or shorted wiring in the engine speed signal circuit (short to earth or high resistance)likely
Corroded, loose or water-damaged connector at the TCM, engine control module or crankshaft sensorlikely
Faulty crankshaft position sensor or transmission input/output speed sensor feeding the signalpossible
Poor power or earth supply to the TCM or engine control modulepossible
Fault within the TCM itself, or in the engine control module providing the signalrare
Outdated or faulty control unit software, where the manufacturer has documented it as a cause for the specific modelrare

How to diagnose

Read all stored and pending codes and note freeze frame data; look for related engine speed, crankshaft sensor or communication codes
Check whether the rev counter works and whether live engine speed data matches between the engine and transmission control modules
Inspect the wiring and connectors between the engine control module, TCM and relevant sensors for chafing, corrosion, moisture and bent or pushed-back pins
Check battery condition and charging voltage, then test power and earth supplies at the TCM
Using the wiring diagram for the vehicle, test the signal circuit for short to earth, open circuit and resistance
Check the crankshaft sensor and transmission speed sensor signals with a scope or live data against the manufacturer's specification
Check for technical service bulletins and available software updates for the model and transmission
Clear the code and road test to see if it returns, confirming the repair

How to fix

Repair or replace damaged wiring, and secure it away from heat and sharp edges
Clean, dry or replace corroded or damaged connectors and terminals
Replace a faulty crankshaft or transmission speed sensor once testing confirms it
Restore correct power and earth supplies, including fixing a poor earth point or a weak battery
Repair, replace or reprogram the TCM or engine control module only after the wiring and sensors are proven good, following manufacturer procedure

Can I keep driving?

Medium severity

The car can usually be driven carefully for a short time, but limp mode and harsh shifts can stress the transmission, so get it checked soon. Avoid heavy loads or long journeys, and stop if the car stalls or loses drive.

Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.