What Does P0738 Mean?
The TCM has detected a low voltage condition on the engine speed output circuit. The TCM outputs an engine RPM signal to other control modules, and this output is reading lower than the expected threshold. The driver may notice a tachometer reading lower than actual RPM, cruise control disengaging, or other engine-speed-dependent systems not functioning correctly.
Common Causes
35%
Short to ground in the engine speed output wiring between the TCM and receiving module(s)
25%
TCM output driver weakened or partially failed, unable to drive the signal to proper voltage
20%
Corroded connector or high-resistance connection in the output circuit pulling voltage down
10%
Receiving module input circuit fault loading down the signal line
10%
Poor ground at the TCM affecting output signal reference voltage
Diagnostic Steps
1
With the engine running, measure the voltage on the TCM engine speed output pin — compare to the expected signal voltage (typically a 5V or 12V square wave). A consistently low voltage confirms the circuit fault.
2
Disconnect the receiving module(s) from the circuit and retest the voltage at the TCM output. If voltage returns to normal, the receiving module is loading down the circuit.
3
Check for shorts to ground in the output wiring — disconnect both ends and measure resistance from the signal wire to chassis ground. Any reading below several megaohms indicates a short.
4
Inspect the TCM connector and receiving module connector for corrosion, moisture, or damaged pins that could cause high resistance or ground paths.
5
Verify the TCM ground circuits — measure voltage drop across TCM grounds with the engine running. More than 0.1V drop indicates a poor ground that can affect output driver performance.
Estimated Repair Cost
$80 - $700
Parts + labor, varies by vehicle and location
The TCM has detected a low voltage condition on the engine speed output circuit. The TCM outputs an engine RPM signal to other control modules, and this output is reading lower than the expected threshold. The driver may notice a tachometer reading lower than actual RPM, cruise control disengaging, ...
The most common cause of P0738 (TCM Engine Speed Output Circuit Low) is: Short to ground in the engine speed output wiring between the TCM and receiving module(s)
Typical repair costs for P0738 range from $80 to $700, depending on the vehicle, location, and whether you do it yourself or go to a shop.
This code typically does not affect actual transmission shifting since the TCM still processes engine speed internally. Systems receiving the output signal (tachometer, cruise control) may malfunction. The vehicle is generally safe to drive to a repair facility.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to P0738 to identify the root cause.
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Quick Info
Category
Powertrain
System
Automatic Transmission / TCM Communication
Difficulty
Type
Generic (SAE)
Recommended Tools
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