Error Code P0738 is defined as TCM Engine Speed Output Circuit Low. This is a generic but uncommon trouble code, meaning it applies to all vehicles equipped with the OBD-II system, especially those made since 1996 up to present. It commonly applies among, but not limited to Chevrolet, Dodge, Honda, Hyundai, Jaguar, Toyota, etc. Specifications on the definition, troubleshooting, and repairs, of course, vary from one make and model to another.
The TCM (Transmission Control Module) works the same as the PCM (powertrain control module, also known as ECM or engine control module in other vehicle makes) – it monitors, controls, and regulates the functions of the transmission, where the PCM does it for the engine.
The TCM comes equipped with various sensors and switches to monitor, control, and regulate the transmission’s drivability. It uses the ESS (Engine Speed Sensor) to determine the shift points among other functions. Just like as its name implies, the ESS monitors the speed of the engine. ESS is sometimes called CKP (Crankshaft Position Sensor) or OSS (Output Speed Sensor).
Many times this measures the speed of the crankshaft using a fall effect type sensor. This sensor comes with notches in the flywheel that passes through the sensor to monitor the position of the crankshaft. It’s important that the PCM and TCM both have the accurate readings here, as it influences among other things, such as speed, drivability, and shift points.
The PCM stores the Error Code P0738 (and other associated error codes) when it sees one or multiple conditions outside a specific electrical range within the ESS, or its circuit. Given the nature of the code and the location of the sensor, this error code is usually an electrical issue. There are specific mechanical issues that may cause this code, but they are rare.
This code is registered when the PCM determines a low electrical value within the ESS circuit(s).
- Hard shifting
- Low speed
- No start
- Increase in fuel consumption
- Inaccurate speedometer
- Less throttle response
There are lots of factors that could lead to this problem. Common causes for this code include:
- Defective ESS
- Defective PCM
- Defective TCM
- Dirty pick up on ESS
- Faulty wiring
- Connector problem
How to Check
If the engine cranks fine but doesn’t start, this may indicate a problem with the ESS. For this, check with the manufacturer’s manual and get the exact electrical values, then test the sensor. Repair or replace accordingly. Usually, the sensors read the speed of the crankshaft, so it would most likely be mounted somewhere on the engine block.
Check the transmission fluid. Make sure the automatic transmission fluid is clean and full. Add as needed. Low fluid can cause automatic transmission to fail. Dirty transmission fluid (black or extremely dark) must be replaced to ensure the efficient function of the transmission. Now only can this low and dirty transmission fluid can cause this code to be activated but can cause further damage to the internal transmission of the components. This could be expensive, thus, it’s extremely important to ensure the fluid is in good condition.
Note: Make sure you use the transmission fluid as recommended by the manufacturer, for the make and model of the vehicle. A simple mistake could lead to expensive repair, so research thoroughly.
Check the wiring. Since the ESS wires are exposed to the elements and road debris, it could be vulnerable to damages. Check for any signs of wears and damages to the harness. Chaffing may occur if the harness is routed close to the engine and other sharp components. Repair as necessary.
Note: Make sure you refer with the service manual for the proper diagnosis of the specific wiring harness.
Clean the sensor. The steps may be similar to removing the ESS. In this case, remove the sensor from the engine block. Check the sensor’s pickup, these are magnetic and are susceptible to picking debris/shaving, which can potentially alter electrical readings. Reinstall the sensor after cleaning.
Check the TCM. In many cases, these are mounted to the transmission of the vehicle. This means the electrical module can be exposed to harsh conditions. All that being said, make sure to check for any signs of damages, defects, or corrosion.
How to Fix
Depending on the diagnosis, common repairs for this code include:
- Change of transmission fluid and filter
- Fixing transmission fluid leaks
- Replacement of engine speed output sensor (ESS)
- Replacement of transmission output speed sensor
- Repair or replacement of damaged connectors and wirings
- Replacement of solenoids
Depending on the situation, this code is not considered as a severe problem. Prolonged use of the vehicle with this code, however, can cause expensive repairs, in the long run, so make sure to keep an eye on it.
When diagnosing this code, make sure you let the engine cool down first and wear proper PPE (personal protective equipment).