What Does It Mean?
This is a generic powertrain diagnostic trouble code (DTC) that applies to OBD-II vehicles. This may include, but is not limited to, Dodge, Chevrolet, Honda, Toyota, Hyundai, Jaguar vehicles, etc. Although generic, the exact repair steps may vary depending on the year, make, model, and powertrain configuration.
The TCM (Transmission Control Module) operates similarly to the ECM (Engine Control Module) in that it monitors, controls, and regulates transmission functions while the ECM does the same for the engine.
The TCM uses various sensors and switches along with the ECM to monitor, control, and regulate transmission operation. Specifically, the TCM uses an Engine Speed Sensor (ESS) to determine shift points among other functions. Just as its name implies, the ESS monitors engine speed. Other names for the ESS may be CKP (Crankshaft Position Sensor) or OSS (Output Speed Sensor).
Most of the time, it measures crankshaft speed using a Hall effect type sensor. The sensor uses notches in the flywheel that pass through the sensor to monitor crankshaft position. It is imperative that the ECM and TCM have an accurate reading here as they influence, among other things: shift points, speed, drivability, etc.
The ECM triggers P0738 and related codes when it monitors one or more conditions outside a specific electrical range in the engine speed sensor or its circuit. Given the nature of the code and the sensor’s location, I would say most of the time, it would be an electrical issue. Specific mechanical problems can cause this code but I would say they are somewhat rare.
P0738 TCM Engine Speed Output Circuit Low code is set when the ECM recognizes a low electrical value in the engine speed output circuit(s).
How Severe Is This DTC?
I would say the severity here would be low. Obviously, it depends on the specific situation but generally, this fault will only cause driving issues. That being said, prolonged use with this fault may cost you in the long run, so be sure to monitor it.
What Are Some of the Symptoms of the Code?
Symptoms of a P0738 fault code may include:
Harsh shifting
Decreased fuel consumption
No start
Low speed
Engine stalling/surging
Inaccurate speedometer
Reduced throttle response
What Are Some of the Common Causes of the Code?
Causes
of this P0738 transmission code may include:
Faulty engine speed sensor (ESS)
Faulty ECM (Engine Control Module)
Faulty TCM (Transmission Control Module)
Wiring issue
Dirty pickup on the ESS
Connector problem
What Are the P0738 Troubleshooting Steps?
Be sure to check the Technical Service Bulletins (TSBs) for your vehicle. Accessing a known fix can save you time and money during diagnosis.
Tools
Some of the things you might need when diagnosing or repairing engine speed output circuits and systems:
OBD code reader
Multimeter
Basic socket set
Ratchet and wrench sets
Basic screwdriver set
Battery terminal cleaner
Service manual
Safety Tips
Let the engine cool down
Chock the wheels
Wear PPE (Personal Protective Equipment)
NOTE: ALWAYS check and record the integrity of your battery and charging system before proceeding with troubleshooting.
Basic Step #1
If your car’s engine cranks properly but does not start, it may indicate a problem with the ESS (Engine Speed Sensor) itself. Test the sensor using the manufacturer’s desired electrical values. Repair or replace accordingly. Typically, these sensors read crankshaft speed so it would most likely be mounted somewhere on the engine block.
Basic Step #2
Check your fluids. Be sure to check the automatic transmission fluid. Ensure it is clean and full. Top up if necessary. Low fluid level can cause automatic transmissions to do funny things, so go ahead! Black or extremely dark transmission fluid will need to be replaced to ensure proper transmission function. Not only can low and/or dirty transmission fluid trigger this code, but it can further damage internal transmission components. Which is one of the most expensive repairs in the business, so make sure the correct clean fluid is present.
NOTE: Always use the transmission fluid recommended by the manufacturer specific to your make and model. A mistake here could potentially cost thousands. Do your research!
Basic Step #3
Check the wiring. Given the location of the ESS, the wires are exposed to the elements and road debris. Inspect for any damage to the harness. Chafing can occur if the harness is routed near the engine and other sharp components. Repair any damaged wiring.
NOTE: Refer to your service manual to ensure you correctly identify the desired wiring harness.
Basic Step #4
Clean the sensor. It may be straightforward enough to remove the ESS. In that case, remove the sensor from the engine block. Inspect the sensor pickup; after all, they are magnetic and are extremely likely to pick up debris/shavings that could potentially alter electrical readings. Reinstall the sensor once cleaned and free of debris.
Basic Step #5
Inspect the TCM (Transmission Control Module). I have seen them mounted directly on the transmission many times. This means this electrical module is exposed to harsh conditions. That said, take a look for any signs of corrosion, damage, and/or any kind of fault.