P0738 — TCM Engine Speed Output Circuit Low #

Please note: The definition and description of P0738 may vary depending on the vehicle manufacturer (OEM). Below are known variations in wording:

• P0738 — TCM Engine Speed Output Circuit Low (Generic/SAE)
• P0738 — Transmission Control Module Engine Speed Output Circuit Low (Honda)
• P0738 — Output Speed Sensor Circuit Low (Nissan)
• P0738 — Output Shaft Speed Sensor Signal Low (Mazda)

Summary of Code P0738 #

P0738 is a diagnostic trouble code (DTC) that signals an abnormal low voltage condition in the Engine Speed Output Circuit monitored by the Transmission Control Module (TCM). The TCM is the computer that manages your vehicle’s transmission operations, while the engine speed output circuit typically involves an output speed sensor (OSS) responsible for relaying the transmission’s output shaft speed to the TCM. When the TCM detects a voltage lower than the expected range in this circuit, it sets the P0738 code. This may result in shifting issues or trigger the transmission to go into “limp mode” to protect itself. Addressing this code promptly helps prevent further transmission or drivability problems.

Common Symptoms for Code P0738 #

When a P0738 code is present, your vehicle might exhibit a range of symptoms, from subtle to unmistakable. The transmission could act like it’s “confused,” shifting at odd times or not shifting at all. Some drivers notice a loss of power, especially when accelerating or climbing hills. In some cases, the check engine or transmission warning light will illuminate. Watch for these telltale signs:

• Illuminated check engine light
• Transmission stuck in a single gear (limp mode)
• Harsh, delayed, or erratic shifting
• Reduced fuel economy
• Speedometer behaving erratically or not working
• Loss of engine power or poor acceleration

Probable Causes for Code P0738 #

The most common causes for a P0738 code are related to the wiring, connectors, or the sensor itself within the engine speed output circuit. Other issues can include internal transmission faults or problems with the TCM. Here’s the likely order of culprits, based on frequency:

1. Faulty output speed sensor (sometimes called OSS or Vehicle Speed Sensor/VSS)
2. Damaged or corroded wiring or connectors in the output speed sensor circuit
3. Poor electrical connection at the TCM or sensor
4. Failed TCM (Transmission Control Module)
5. Internal transmission issues affecting sensor operation

Step-by-Step Diagnosis for Code P0738 #

Diagnosing P0738 requires patience and a methodical approach. Start by addressing the most likely problems, then dig deeper if needed. Here’s a recommended step-by-step process:

• 1. Scan for additional codes: Check for other transmission or sensor-related DTCs that could provide more context.
• 2. Inspect wiring and connectors: Visually examine the output speed sensor wiring and connectors for corrosion, breaks, or loose pins.
• 3. Test the output speed sensor: Use a multimeter to check sensor resistance and confirm it matches OEM specifications.
• 4. Check voltage and ground: With the key on, verify that proper voltage and ground are present at the sensor connector.
• 5. Examine the TCM: Inspect for signs of water intrusion, burnt smells, or damaged pins at the TCM harness.
• 6. Road test and monitor live data: Use a scan tool to observe the output speed sensor readings while driving; look for erratic or flat-line signals.
• 7. If all else fails: Consider possible internal transmission faults or consult OEM diagnostic flowcharts for advanced troubleshooting.

Possible Repairs for Code P0738 #

The right fix depends on what the diagnosis reveals. Most repairs are straightforward, but some may require professional equipment or expertise. Here are the most common solutions, linked to their corresponding causes:

• Replace the output speed sensor if it fails resistance or live data tests
• Repair or replace damaged wiring or connectors in the sensor circuit
• Clean corroded terminals at the sensor or TCM connectors
• Replace the TCM if it’s found to be faulty (rare, but possible)
• Address internal transmission issues if sensor and wiring are verified good

Can You Drive with Code P0738? #

Limited — Not Recommended for Extended Driving. While the vehicle may still run with a P0738 code present, continued operation can strain the transmission and reduce overall drivability. If the transmission enters limp mode, you’ll be limited to one gear, which significantly affects acceleration and highway speeds. It’s best to address the issue promptly to avoid further damage and restore normal performance.

Vehicles Most Commonly Affected by Code P0738 #

P0738 is seen most frequently in vehicles with electronically controlled automatic transmissions, particularly those manufactured by Asian brands in the late 1990s and 2000s. Vehicles that are often reported with this code include:

• Honda Accord, Civic, Odyssey (2000s models)
• Nissan Altima, Maxima, Frontier
• Mazda 3, 6, CX-5
• Hyundai Sonata, Elantra
• Kia Optima, Sportage

Common Mistakes & Best Practices for P0738 #

Technicians and DIYers sometimes jump to conclusions or overlook basics when troubleshooting P0738. Avoiding these pitfalls will save time and money. Here’s what to watch out for:

• Skipping a thorough wiring inspection — Many issues are caused by simple corrosion or broken wires.
• Replacing the sensor before testing it — Always test the output speed sensor before replacement.
• Overlooking TCM connections — Poor contacts at the TCM can mimic sensor faults.
• Neglecting to check for related codes — Co-existing DTCs may indicate a larger issue.
• Ignoring OEM service information — Always consult the manufacturer’s diagnostic flowcharts for model-specific steps.

Express FAQ — P0738 #

Here are quick answers to some of the most common questions about code P0738:

• Q: Can a bad output speed sensor cause transmission problems?
  A: Yes. The TCM relies on the output speed sensor to determine shift timing and operation. A faulty sensor can cause harsh or erratic shifts and may trigger limp mode.
• Q: Will clearing the code fix the problem?
  A: No. Clearing the code will temporarily turn off the warning light, but unless the underlying issue is repaired, the DTC will return after the next drive cycle.
• Q: Is P0738 an expensive repair?
  A: Often, the fix is affordable—replacing a sensor or repairing wiring is usually not costly. However, if the TCM or transmission internals are at fault, repairs can become more expensive.