OBD-II trouble code
P0738: TCM Engine Speed Output Circuit Low
The voltage on the circuit carrying engine speed to the transmission control module is lower than it should be — usually a short to ground or a signal being dragged down. The transmission loses its RPM reference and shift quality suffers.
Quick facts
- System
- Powertrain
- Category
- Transmission / Speed Sensor
- Severity
- Medium severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $100 – $1,100
- DIY difficulty
- Shop recommended
What does P0738 mean?
P0738 is the low-voltage member of the engine speed output family. The engine control module (ECM) works out engine RPM from the crankshaft position sensor and outputs that value to the transmission control module (TCM) so the transmission knows when to shift, how much line pressure to command, and when to lock the torque converter. P0738 sets when the TCM measures a voltage on that circuit that is below the expected range for the conditions.
A "circuit low" fault has a narrower list of causes than the general P0737, and that is what makes it useful. Voltage on a signal wire gets pulled down by three things: a short to ground somewhere in the harness, excessive resistance in the circuit forming a voltage divider, or a transmitting driver inside the ECM that can no longer pull the line to its full high level. Corrosion is the everyday version of the second one — a green, high-resistance terminal in a connector will drop enough voltage across itself to put the signal out of range without ever opening the circuit completely.
It is worth being clear about what this code is not. It is not the transmission's output shaft speed sensor (P0720 through P0723), and it is not a hydraulic or fluid-related fault. Fluid level and condition have nothing to do with whether a voltage reaches a module. The one indirect connection worth checking is the transmission case pass-through connector: on vehicles where the TCM is mounted on or in the transmission, fluid can wick up the wire strands from a leaking connector and corrode terminals from the inside, which is exactly the kind of resistance that produces a persistent circuit-low code.
With no valid engine speed, the TCM reverts to a fallback shift schedule that is intentionally conservative — high line pressure, firm engagement — so drivers usually describe abrupt or slamming shifts rather than slipping. Some vehicles will also refuse to lock the torque converter, which shows up as a noticeable drop in highway fuel economy.
Common causes
- Short to ground in the engine speed signal wire between the ECM and TCM
- Corroded, green, or high-resistance terminals in either module connector
- Chafed harness rubbing on a bracket or the engine block
- Transmission fluid wicking up the harness from a leaking case pass-through connector
- Weak or intermittent crankshaft position sensor producing a low-amplitude signal upstream
- Poor ECM or TCM ground raising circuit resistance
- Water intrusion into an underbody or firewall connector
- Failed ECM output driver or TCM input circuit (verify wiring first)
Symptoms
- Check engine light on, frequently with a transmission warning light
- Harsh, abrupt, or slamming upshifts and downshifts
- Shift points occurring at the wrong engine speeds
- Torque converter clutch not locking up, hurting highway mileage
- Transmission dropping into limp mode and holding one gear
- Erratic or dropping tachometer reading
- Companion P0700 stored by the TCM
Diagnostic steps
- 1.Record freeze frame data before clearing anything — note engine load, coolant temperature, and vehicle speed, since heat-related faults show a clear pattern.
- 2.Compare the ECM's reported RPM against the RPM the TCM reports in live data. A valid ECM value with a low or zero TCM value isolates the fault to the circuit between them.
- 3.Check for crankshaft position sensor codes (P0335, P0336, P0339) — a weak signal upstream produces a low signal downstream.
- 4.With the module connectors unplugged, measure resistance from the signal circuit to ground. A low reading confirms a short to ground; an infinite reading rules it out.
- 5.Measure end-to-end resistance on the signal wire. Anything more than a fraction of an ohm on a short run points to corrosion or a partially broken conductor.
- 6.Inspect both connectors under good light for green corrosion, spread terminals, and pins pushed back into the housing.
- 7.Check the transmission case pass-through connector for fluid contamination and clean or replace it if fluid is present.
- 8.Perform voltage-drop tests on the TCM's power and ground circuits rather than relying on continuity checks alone.
- 9.If the wiring passes every test, scope the ECM output. A signal that never reaches its full high level with good wiring points to the ECM driver.
Repair cost
$100 – $1,100
Cleaning or repairing a corroded connector is often $100-$250 including diagnosis. A crankshaft position sensor runs $150-$400 installed. A chafed harness section repair typically lands at $200-$500. Replacing and programming a TCM or ECM is the top of the range at $600-$1,100 and should only follow a complete wiring check.
Estimate your repair
Run the numbers for your vehicle
Open the Repair Cost Estimator with crankshaft position sensor replacement preselected. Adjust labor rate and vehicle category to fit your situation.
DIY vs shop
Leave this one to a qualified shop. It typically involves emissions-critical components, refrigerant handling, or other work that requires manufacturer-grade tooling, training, or certification. DIY attempts often produce a more expensive problem than the original code.