OBD-II Code P0596 — What It Means & How to Fix It

What Does This Error Mean?

P0596 means the cruise control servo control circuit is reading a voltage that is too high. The control signal from the PCM to the cruise control servo has exceeded the maximum expected voltage. This typically happens when a wire in the servo circuit is shorted to battery voltage, or when the circuit has an open connection that lets the voltage float high. Cruise control will not work while this code is active. Normal driving is completely unaffected.

Affected Models

All vehicles 1996+
Common in GM vehicles
Common in Ford trucks and SUVs
Common in Dodge and Ram vehicles
Common in any vehicle where the cruise control servo shares a wiring harness with the fuel system

Common Causes

Short to battery voltage in the servo control wire
Open circuit in the servo control wire allowing the voltage to float high
Failed servo with internal damage causing voltage feedback into the control circuit
PCM output driver failure outputting full battery voltage instead of the commanded signal
Wiring harness damage from heat or abrasion causing the control wire to contact a power wire

How to Fix It

With the key off, unplug the cruise control servo connector. Check the harness-side wiring for any obvious damage — melted insulation, chafing, or wires that appear to contact other wires in the bundle.

A short to power inside a wiring bundle is sometimes caused by heat damage from a nearby exhaust component melting the insulation.
With the key on (engine off) and the servo unplugged, use a multimeter to measure voltage on the control wire pin at the harness connector. It should show low or no voltage when the circuit is not being commanded.

If you see battery voltage (12-14V) on the control wire when it should be low, there is a short to power in the harness.
If harness voltage is normal, plug the servo back in and recheck the voltage at the PCM connector for this circuit. A voltage discrepancy between the PCM side and the servo side helps locate where the short is in the harness.

This step-by-step isolation technique is the most reliable way to find a hard-to-see wiring fault.
If the wiring checks out clean, replace the cruise control servo. An internal servo fault can sometimes push voltage back up the control wire.

Always confirm with a circuit test before replacing the servo — replacing a good servo wastes money and may not fix the problem.
After any repair, clear the code and perform a thorough cruise control function test on an open road. Verify that all speed adjustment buttons work as expected.

If the code returns immediately after clearing, the short is still present and further harness inspection is needed.

When to Call a Professional

A short to power can damage the PCM output driver if left unchecked. If you cannot identify the cause quickly, have a technician trace the circuit with a wiring diagram. Diagnosis typically costs $75 to $125 per hour. Servos are generally affordable to replace ($80 to $200) and wiring repairs vary based on the location and severity of the damage.

Frequently Asked Questions

Can P0596 damage my PCM?

Potentially yes. A sustained short to battery voltage on a PCM output circuit can overheat and damage the output driver inside the PCM. It is worth diagnosing and repairing P0596 promptly rather than driving with it indefinitely.

How does an open circuit cause a high voltage reading?

PCM output circuits often use a pull-up resistor internally. When the wire is broken (open), the resistor holds the signal at a relatively high voltage because there is nothing on the other end to pull it lower. This is why both shorts to power AND open circuits can both trigger a 'high' code.

Is P0596 common after engine bay work?

Yes. Accidentally routing a wire near a hot exhaust component, or pinching a harness during reassembly, are common causes of high-circuit codes after engine repairs. Always inspect recent repair areas first when diagnosing a new code.