Failed voltage regulator inside the generator, allowing uncontrolled full-field output; Corroded, loose, or damaged voltage sense wire between the generator and the ECM; Poor ground connection at the generator, engine block, or battery negative terminal; Faulty ECM, though this is rare and should be the last suspect; Aftermarket or incorrect generator installed that is not compatible with the ECM's control strategy; Battery with an internal short or high internal resistance that skews voltage regulation

DTC codes / GM codes / P1594

P1594 · GMCharging System Voltage Too High

Q: What does code P1594 mean?

Charging System Voltage Too High. P1594 on GM vehicles means the engine control module (ECM) has detected that the charging system voltage has risen above the safe limit, typically above 15.5 volts, for a sustained period. The ECM monitors the generator's output through the voltage sense circuit and commands the generator to reduce output when voltage climbs too high; this code sets when the commanded reduction fails to bring voltage back into range.

P1594 on GM vehicles means the engine control module (ECM) has detected that the charging system voltage has risen above the safe limit, typically above 15.5 volts, for a sustained period. The ECM monitors the generator's output through the voltage sense circuit and commands the generator to reduce output when voltage climbs too high; this code sets when the commanded reduction fails to bring voltage back into range.

SystemPowertrain

Code typeGM specific

SeverityHigh

Fix difficultyIntermediate

AreaVehicle speed, idle control and auxiliary inputs

Symptoms you may notice

Battery warning light or charge indicator illuminated on the dashboard
Headlights and interior lights noticeably brighter than normal, sometimes flickering
Battery acid smell or visible swelling of the battery case
Electrical accessories such as the radio or climate control behaving erratically
Battery repeatedly overheating or boiling over electrolyte
Often no symptoms besides the warning light, especially on short trips

Common causes, most likely first

Failed voltage regulator inside the generator, allowing uncontrolled full-field output
Corroded, loose, or damaged voltage sense wire between the generator and the ECM
Poor ground connection at the generator, engine block, or battery negative terminal
Faulty ECM, though this is rare and should be the last suspect
Aftermarket or incorrect generator installed that is not compatible with the ECM's control strategy
Battery with an internal short or high internal resistance that skews voltage regulation

How to diagnose and fix it

Perform a visual inspection of the battery, generator, and all related wiring for corrosion, loose connections, or physical damage
Measure battery voltage at rest and with the engine running; a reading above 15.5 volts confirms overcharging
Check the voltage sense circuit and ground circuits at the generator with a multimeter for excessive resistance or voltage drop
Test the generator's voltage regulator output with a scan tool capable of commanding generator output, if available
Inspect the battery for internal faults using a conductance tester and replace it if it fails
Replace the generator if the regulator is internal and proven faulty, then clear the code and verify charging voltage returns to normal
If all external circuits and components test good, diagnose the ECM control circuit and replace the ECM only as a last resort

Overcharging faults that fool good technicians

A loose or corroded ground strap between the engine and chassis can cause the ECM to see a lower voltage than actual, commanding the generator to increase output and triggering P1594.
Some GM vehicles use a battery current sensor on the negative cable; a failed sensor can skew the ECM's voltage regulation strategy and set this code without any generator fault.
Aftermarket generators with incorrect internal regulator calibration are a common cause on older GM trucks and SUVs, even when the unit bench-tests as good.
P1594 can be set by a weak or deeply discharged battery that causes the ECM to command high charging voltage for extended periods, so always load-test the battery before condemning the generator.
On vehicles with remote start or dual batteries, a failing auxiliary battery or isolation relay can create voltage spikes that the ECM interprets as overcharging.

Can I keep driving with P1594?

Do not continue driving with this code active, because sustained overcharging can boil the battery, damage the ECM and other electronics, and potentially cause a fire. If you must move the vehicle, keep the drive under a few minutes and turn off unnecessary electrical loads, then have it diagnosed immediately.

Tools that help

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Guidance is general and written for GM vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.