P1799 Transmission Control Module Battery Voltage Circuit Malfunction
The P1799 code indicates a malfunction in the battery voltage circuit as detected by the transmission control module (TCM). This code is specific to Honda and Acura vehicles and is triggered when the TCM reads battery voltage outside the expected parameters, either too low or too high, affecting the operation of transmission solenoids and associated sensors. The TCM continuously monitors battery voltage to ensure proper function of hydraulic solenoids and other electro-hydraulic components in the automatic transmission.
01. Most Common Causes
- Weak, sulfated, or faulty battery
- Alternator with insufficient or excessive charging output
- Corroded, loose, or sulfated battery terminals
- Wiring in the battery voltage circuit to the TCM with excessive resistance, short, or open circuit
- Blown fuses or defective relays in the TCM power supply circuit
- Internal TCM failure (less common)
02. Symptoms You Will Notice
- Check Engine Light illuminated
- Transmission shifting issues, such as harsh shifts, slipping, or stuck in gear
03. Step-by-Step Diagnosis
Scan for codes and verify P1799 is present; check for other related codes.
Inspect battery condition, terminals, and connections for corrosion or looseness.
Test battery voltage at rest (should be ~12.6V) and with engine running (should be ~13.5-14.5V).
Check alternator output and charging system performance.
Inspect fuses and relays related to the TCM power supply.
Test wiring and connectors between the battery and TCM for resistance, shorts, or opens.
If all else passes, suspect internal TCM failure and consider professional diagnosis.
1. Frequently Asked Questions
Is it safe to drive?▼
You can drive short distances, but the vehicle may experience transmission issues such as harsh shifting or slipping. It's best to diagnose and repair promptly to avoid further damage.
How much does it cost to fix?▼
Repair costs typically range from $100 to $500, depending on the cause. Battery or alternator replacement is on the lower end, while TCM replacement can be higher.