P0838 moderate

Transmission Control Module Battery Voltage High

The P0838 code means the vehicle's onboard diagnostic system has detected: Transmission Control Module Battery Voltage High. This is a moderate severity code.

My Garage →
Keep driving?
Short distances only
DIY difficulty
moderate
Estimated cost
$50 - $300 (parts and labor)
Check Your Fluid First
ANCEL AD310 OBD-II Scanner

Read and clear transmission codes. Many trans codes are caused by low or burnt fluid -- check level and color before assuming the worst.

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Read Transmission Codes
LAUNCH CRP123X (Reads All Systems)

Basic scanners only read engine codes. This reads transmission, ABS, and airbag too.

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Common Symptoms

  • Check Engine Light on
  • TCM detecting high battery voltage
  • Transmission shifting issues
  • Erratic transmission behavior

Probable Causes (Ranked by Likelihood)

  • Overcharging condition affecting TCM Very High
  • Wiring damage or corrosion in the circuit High
  • Weak or failing battery Moderate
  • ECM/TCM input circuit failure Low

Step-by-Step Diagnostic Procedure

  1. Use a scan tool to monitor TCM battery voltage data

  2. Test battery voltage with a multimeter; should be 12.4-12.7V with engine off

  3. Check alternator output voltage; should be 13.5-14.7V with engine running

  4. Test for parasitic drain that could cause low voltage conditions

Frequently Asked Questions

What does P0838 mean?

P0838 indicates a problem with the transmission control module battery voltage high. The TCM monitors its own battery voltage supply and has detected an abnormal condition that can affect shift quality and module operation.

Is P0838 a safety concern?

Voltage issues affecting the TCM can cause erratic shifting, which is a safety concern, especially during highway driving or towing. Have the electrical system tested.

Can I diagnose this myself?

Battery and charging system testing requires a multimeter. Most auto parts stores will test your battery and alternator for free.

Sources

This page is built from documented references. Verify against your own service info before repair work.

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