P0738
UniversalTCM Engine Speed Output Circuit Low
Medium SeverityThe Transmission Control Module (TCM) is detecting a signal from the engine speed output circuit that is lower than expected. This indicates a problem with the signal pathway that provides engine speed information to the TCM, which is critical for smooth and timely gear changes.
⚠️ Before You Begin (Safety First)
Always disconnect the battery before working on electrical components. Allow the engine and transmission to cool down completely to avoid burns. Use appropriate personal protective equipment (PPE) such as gloves and eye protection.
Common Symptoms
- Check Engine Light illuminated
- Harsh or erratic shifting
- Transmission slippage
- Transmission stuck in a single gear (limp mode)
- Poor fuel economy
- Engine stalls at low speeds
Common Causes
- Faulty Engine Speed Sensor (crankshaft or camshaft position sensor providing signal to TCM)
- Wiring harness issues (chafed, corroded, or open circuit) between the sensor and TCM
- Poor electrical connection at the sensor or TCM connector
- Faulty TCM (Transmission Control Module) (less common)
- PCM (Powertrain Control Module) internal fault affecting signal processing
Frequently reported on: General/Universal
Diagnostic Steps
- Retrieve all stored OBD-II codes using a scanner to confirm P0738.
- Inspect the wiring harness and connectors leading to the engine speed sensor (usually crankshaft or camshaft position sensor) and the Transmission Control Module (TCM) for visible damage, corrosion, or loose connections.
- Using a multimeter and referring to your vehicle's wiring diagrams, test the continuity and resistance of the circuit to identify any breaks, shorts, or high resistance.
- If no wiring issues are found, test the engine speed sensor itself according to manufacturer specifications (e.g., resistance check, signal output test with an oscilloscope if available).
- Clear the code and perform a test drive to see if the issue reappears. If it does, consider professional diagnosis or replacement of the suspected faulty component (sensor or TCM).
👨🔧 When to Call a Professional
If you are not comfortable performing detailed electrical diagnostics, including using a multimeter to test continuity and voltage, or if the issue persists after basic checks, it is highly recommended to seek assistance from a certified automotive technician. Transmission-related electrical problems can be complex.
Commonly Asked Questions
What is the Engine Speed Output Circuit?
It's the electrical pathway that transmits information about the engine's rotational speed from the engine's sensors (like the crankshaft position sensor) to the Transmission Control Module (TCM). The TCM uses this data for precise gear shifting and torque converter control.
Can I drive with a P0738 code?
Driving with a P0738 code is not recommended. It can lead to erratic or harsh shifting, reduced fuel economy, and potentially cause the transmission to enter a 'limp mode' to prevent further damage, which can be unsafe or unreliable.
Is a P0738 always a bad sensor?
Not necessarily. While a faulty engine speed sensor is a common cause, wiring issues (such as frayed wires or corrosion), poor electrical connections at the sensor or TCM, or a problem with the Transmission Control Module (TCM) itself can also trigger this code.
How do I find the engine speed sensor?
The engine speed information for the TCM is typically derived from the crankshaft position sensor or, less commonly, a camshaft position sensor. Their exact locations vary by vehicle, but they are usually mounted on or near the engine block. Always refer to your vehicle's service manual for precise locations.
What tools do I need to diagnose P0738?
A basic OBD-II scanner to read codes, a digital multimeter for electrical testing (voltage, resistance, continuity), and your vehicle's specific wiring diagrams are essential tools for diagnosing a P0738 code.