Technical Description of OBD-II Trouble Code
Low Output Circuit for TCM Engine Speed
What does that signify?
This is a generic diagnostic trouble code (DTC) for powertrain and is applicable to OBD-II vehicles. This includes vehicles from various manufacturers such as Dodge, Chevrolet, Honda, Toyota, Hyundai, Jaguar, etc. Although it is a generic code, the specific repair steps may vary depending on the year, make, model, and powertrain configuration.
The TCM (Transmission Control Module) functions similarly to the ECM (Engine Control Module) as it monitors, controls, and regulates the transmission’s functions, while the ECM does the same for the engine.
The TCM, along with the ECM, uses various sensors, switches, and an Engine Speed Sensor (ESS) to monitor, control, and regulate the drivability of the transmission. The ESS, also known as the CKP (Crankshaft Position Sensor) or OSS (Output Speed Sensor), measures the speed of the engine using a hall effect type sensor that detects notches in the flywheel.
When the ECM detects one or multiple conditions outside the specified electrical range within the Engine Speed Sensor or its circuit, it activates the P0738 code and associated codes. While this code is usually caused by an electrical issue, mechanical problems can also be a rare cause.
The severity of this DTC is generally low, as it mainly causes drivability issues. However, prolonged use with this fault may lead to further problems, so it is important to monitor and address it.
Some symptoms of the P0738 code may include hard shifting, a drop in fuel mileage, no start, low speed, engine stalling or bucking, an inaccurate speedometer, and reduced throttle response.
Common causes of the P0738 code include a defective Engine Speed Sensor (ESS), defective ECM (Engine Control Module), defective TCM (Transmission Control Module), wiring issues, a dirty pickup on the ESS, or connector problems.
When troubleshooting the P0738 code, it is recommended to check for technical service bulletins (TSBs) specific to your vehicle, as they may provide known fixes that can save time and money during diagnosis.
Some tools that may be needed for diagnosing or repairing engine speed output circuits and systems include an OBD code reader, multimeter, basic socket set, basic ratchet and wrench sets, basic screwdriver set, battery terminal cleaner, and a service manual.
Before further troubleshooting, it is important to verify and record the integrity of the battery and charging system.
Basic troubleshooting steps for the P0738 code include testing the Engine Speed Sensor (ESS) using desired electrical values from the manufacturer and repairing or replacing it if necessary, checking the automatic transmission fluid and topping up as necessary, inspecting the wiring for any damage or chafing and repairing any damaged wiring, cleaning the sensor to remove debris that may affect electrical readings, and inspecting the TCM for signs of corrosion, damage, or defects.
