P0738 — TCM Engine Speed Output Circuit Low

powertrain (engine/transmission)

Is it safe to drive?

Short-term driving is OK In most cases the car remains drivable, possibly with reduced performance or a disabled auxiliary system. If symptoms include stalling, no-start or brake/steering warnings, treat it as urgent.

How to diagnose with an OBD scanner

Read the code and freeze frame Connect an OBD2 scanner, confirm P0738 is stored, and save the freeze frame data — it shows engine conditions when the fault was detected.
Check for companion codes and TSBs Related codes often reveal the real root cause. Also check manufacturer technical service bulletins (TSB) for your make and model — many P0xxx issues have factory fixes or software updates. Search by VIN on the manufacturer site or nhtsa.gov.
Inspect wiring and connectors Visually inspect the wiring harness and connectors related to the TCM engine speed output for damage, corrosion, chafing or poor contact.
Subsystem check Check for supply voltage at the TCM engine speed output connector first; then measure resistance from the signal wire to ground — near-zero ohms confirms a short to ground in the harness.
Subsystem check Measure reference voltage, ground and signal continuity at the component connector.
Subsystem check Wiggle-test the harness while watching the signal in live data — dropouts reveal broken wires.
Verify the repair Clear the code, complete a full drive cycle including the conditions from the freeze frame, and confirm P0738 does not return.

Approximate repair cost

Repair cost depends on which component actually failed. Typical scenarios for P0738 (parts and labor, USD, independent shop rates): Replace the faulty sensor or actuator: $150–$500; Repair wiring or connectors: $80–$250; Control module repair: $300–$1,000. Overall, fixing P0738 usually costs between $80 and $1,000. An accurate diagnosis before buying parts saves money — the most expensive scenario is rarely the first one to check.

Common questions

What does OBD2 code P0738 mean?

TCM Engine Speed Output Circuit Low (P0738): a powertrain (engine/transmission) fault affecting the sensor and actuator circuits. The car is usually drivable while you plan the repair. Most often caused by poor ground connection, corroded, loose or damaged connector, blown fuse feeding the circuit.

How much does it cost to fix P0738?

Replace the faulty sensor or actuator — $150–$500; Repair wiring or connectors — $80–$250; Control module repair — $300–$1,000. Exact cost depends on the root cause, your vehicle and local labor rates — a proper diagnosis is the cheapest first step.

What is the most common cause of P0738?

The most common causes are: poor ground connection, corroded, loose or damaged connector, blown fuse feeding the circuit.

What are the symptoms of P0738?

Drivers most often notice: possible limp mode depending on the affected component, Check Engine Light on, the affected system operates intermittently or not at all, increased fuel consumption in some cases.

Will P0738 clear itself?

Sometimes P0738 clears after a few drive cycles if the fault was intermittent — for example a loose connector or a tank of bad fuel. The underlying cause usually remains, so the code tends to return. Clear it once; if it comes back, diagnose rather than erase it repeatedly.

When your engine control unit detects a problem with the signal voltage leaving the transmission control module, it logs trouble code P0738. This particular code points to an abnormal condition in the TCM engine speed output circuit — specifically, the voltage on that circuit has dropped below the acceptable minimum threshold and is sitting dangerously close to ground potential. The PCM does not set this code after a single bad reading; it confirms the fault across multiple drive cycles before committing it to memory, which is why many owners first notice the Check Engine Light appearing after a particular driving pattern or maneuver.

The TCM engine speed output circuit carries a critical signal between the transmission control unit and other vehicle systems. When everything is working correctly, this circuit carries a specific reference voltage. When P0738 is active, that voltage has collapsed toward zero, typically because something is pulling it down — either a direct short to chassis ground, a wire with damaged insulation rubbing against metal, a power supply failure to the circuit, or a failing component that can no longer maintain its proper voltage level. The underlying cause determines the repair cost and complexity, which is why jumping straight to the most expensive fix is almost never the right approach.

One of the most valuable pieces of information stored alongside P0738 is the freeze frame data. Think of it as a photograph of your engine's operating conditions at the exact moment the fault was detected — engine RPM, calculated load, coolant temperature, and vehicle speed are all captured. Technicians use this snapshot to reproduce the failure condition during testing, which dramatically narrows the diagnostic window and prevents chasing phantom problems.

Common Symptoms of P0738

The Check Engine Light illuminates on the dashboard
The vehicle may enter limp mode, restricting gear selection and performance
The affected system (typically related to transmission operation) functions intermittently or stops working entirely
Fuel consumption increases noticeably in some cases
Reduced driveability depending on which circuit or sensor is involved
In rare instances, secondary systems that depend on TCM data may behave erratically

What Causes P0738?

The signal wire is shorted to ground, often from chafed or melted insulation
The TCM engine speed output circuit itself has failed and is dragging the signal low
Supply voltage to the TCM output circuit is missing or insufficient
An internal driver fault exists within the control module
A related sensor or actuator on the affected circuit has malfunctioned
Open circuit conditions or additional shorts exist in the wiring harness
Poor ground connections at the component or module level

How to Diagnose P0738

Connect an OBD2 scanner and confirm that P0738 is stored in the transmission or powertrain control module. Before clearing anything, save the freeze frame data — it tells you exactly what your engine was doing when the failure occurred.
Scan for other related trouble codes. P0738 rarely appears alone, and accompanying codes often point directly to the failing component. Also search for manufacturer technical service bulletins (TSB) for your specific make, model, and model year — many transmission-related codes have published fixes, software updates, or known issues documented by the factory.
Perform a thorough visual inspection of the wiring harness and connectors associated with the TCM engine speed output circuit. Look for damaged insulation, corroded pins, backed-out terminals, or any sign that the wiring has been rubbing against metal or heat sources.
Check for proper supply voltage at the TCM engine speed output connector. Then, with the connector unplugged, measure resistance from the signal wire to ground — a reading near zero ohms confirms a short-to-ground condition somewhere in the harness.
At the component connector (if accessible), verify that reference voltage, ground, and signal continuity are all within specifications. Compare your readings to the manufacturer's published values for that circuit.
Perform a wiggle test on the harness while monitoring live data on your scanner. Any signal dropout or fluctuation during physical manipulation of the wiring indicates a broken or intermittent wire inside the insulation.
After repairs, clear the code and complete a full drive cycle that recreates the conditions recorded in the freeze frame. Confirm that P0738 does not return — a code that comes back immediately tells you the root cause has not been addressed.

Repair Options and Costs

Not every P0738 repair requires the same investment. Work through cheaper possibilities before committing to component replacement.

Replace the faulty sensor or actuator on the affected circuit: $150–$500 for parts and labor
Repair damaged wiring or connectors: $80–$250 for parts and labor
Control module diagnosis and repair or replacement: $300–$1,000 depending on whether the module can be repaired or must be replaced outright

How Much Does It Cost to Fix P0738?

The total bill for addressing P0738 varies significantly depending on which part of the circuit has failed. Based on typical independent shop rates in the United States, expect one of the following scenarios. Sensor or actuator replacement runs $150 to $500. Wiring harness repair or connector replacement costs between $80 and $250. Control module-level diagnosis and repair, which may involve reflashing, refurbishment, or full replacement, ranges from $300 to $1,000.

Across all possible repairs, you are looking at an overall range of $80 to $1,000. The most important takeaway is that an accurate diagnosis before purchasing parts will save you money in the long run — the most expensive fix is rarely the first one to try.

Can You Drive with P0738?

In most situations, the vehicle remains driveable after P0738 sets. You may notice reduced performance, limited gear selection, or an illuminated limp-mode indicator, but the car will typically still move. However, if the failure causes stalling, prevents the engine from starting, or triggers warnings related to braking or steering assist, treat the situation as urgent and have the vehicle transported to a repair facility rather than driving it.