Faulty voltage regulator, either built into the alternator or controlled by the module; Poor or corroded connections at the battery terminals or earth straps causing wrong voltage sensing; Damaged wiring or loose connector in the alternator control or sense circuit; Failing or damaged battery that is not accepting charge normally; Fault in the powertrain control module or its voltage control output; Incorrect or faulty replacement alternator that does not suit the model.

P1594

Charging System Voltage Too High.

Pontiac
Powertrain
Manufacturer code
Medium severity

How this code is built

P: SystemPowertrain
1: Defined byThe manufacturer
5: AreaVehicle speed, idle control and auxiliary inputs
94: FaultThe specific fault in that area

What it means

P1594 on a Pontiac means the powertrain control module has seen the charging system voltage rise above its allowed upper limit. The module monitors system voltage and, if the reading stays too high for long enough, it sets the code. This points to an overcharging fault, a regulation problem or a wiring or sensing fault, and the exact strategy can vary by model and engine.

Symptoms

Battery or charging warning light on the dashboard
Check engine light on
Headlights or interior lights unusually bright or flickering
Battery water loss, a hot battery or a rotten egg smell from it
Erratic gauges, or electrical accessories behaving oddly
Voltmeter reading high, if the car has one

Possible causes

Faulty voltage regulator, either built into the alternator or controlled by the modulelikely
Poor or corroded connections at the battery terminals or earth straps causing wrong voltage sensinglikely
Damaged wiring or loose connector in the alternator control or sense circuitpossible
Failing or damaged battery that is not accepting charge normallypossible
Fault in the powertrain control module or its voltage control outputrare
Incorrect or faulty replacement alternator that does not suit the modelrare

How to diagnose

Read all stored codes and freeze frame data, noting engine speed, load and system voltage when the fault set
Visually inspect the battery, terminals, earth straps and alternator wiring and connectors for corrosion, looseness or damage
Measure battery voltage with the engine off, then at idle and at about 2,000 rpm with loads on and off
Compare the module's reported system voltage with a voltmeter reading at the battery to spot a sensing error
Check voltage drop on the battery positive and earth paths while the engine is running
Test the alternator output and regulator operation, following the procedure for that model and engine
Inspect the alternator control or sense circuit for continuity, shorts and high resistance
Load test the battery and check its condition

How to fix

Replace the alternator or voltage regulator if output is above specification
Clean and tighten battery terminals and earth connections, and replace any corroded or damaged leads
Repair or replace damaged wiring or connectors in the charging control or sense circuit
Replace the battery if it is faulty or has been damaged by overcharging
Repair or replace the powertrain control module only after wiring and charging components are proven good, then clear the code and retest

Can I keep driving?

Medium severity

Short, careful journeys are possible, but sustained overvoltage can damage the battery and electronic components, so get it checked soon. If the voltage is very high, the battery is hot or smells, or lights are flaring, stop driving and arrange recovery.

Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.