What does it mean?
P1602 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Power Supply (B+) Terminal 15 Low Voltage .
Symptoms
- MIL (check engine lamp) on, often with several other modules storing their own low voltage codes
- Long cranking, hard starting or no start when the battery is weak
- Engine cutting out during cranking or stalling at idle
- Dash lights flickering, slow wipers and window motors
- Learned adaptations lost, so the idle is rough after every start
- Battery warning lamp on when the charging system is the root cause
Likely causes
- Weak, discharged or sulphated battery with high internal resistance
- Loose, corroded or undersized battery terminals and main ground straps
- Charging system fault: worn brushes, failed regulator, slipping or loose drive belt
- High resistance in the ignition switch, main relay contacts or the fuse feeding terminal 15
- Corroded or backed-out terminals in the ECM power connector
- Poor engine-to-body ground causing a voltage drop under load
- Extra accessories tapped into the ignition feed or a parasitic drain flattening the battery
How to fix it
- Load test the battery and check its state of charge before touching anything else
- Measure charging voltage at idle and at raised rpm with the electrical loads switched on
- Clean and torque the battery terminals, main ground straps and the engine-to-body ground
- Back-probe the terminal 15 feed at the ECM and compare it with battery voltage during cranking; more than a few tenths of a volt difference points to resistance upstream
- Voltage-drop test the ignition feed through the fuse holder, the main relay contacts and each connector
- Check for a parasitic draw if the battery keeps going flat overnight
- Repair or replace the faulty relay, fuse holder or corroded connector, then clear the code and verify the supply voltage in live data