P0597 on 2017-2018 Genesis G90: Thermostat Heater Control Circuit Fix

What's Unique About the 2017-2018 Genesis G90

This is a well-documented issue on Genesis vehicles and their platform mates. The manufacturer released a Technical Service Bulletin (TSB #21-EM-006HG-1) specifically for the P0597 code on G80 and G90 models. This bulletin identifies a faulty electric thermostat as the cause and instructs technicians to replace it. This makes the diagnosis highly certain, pointing directly to a known component weakness rather than a more complex wiring or computer problem.

Diagnostic Flowchart

Tap your situation to follow the diagnostic path that matches what you're seeing on this vehicle.

Symptoms You May Notice

• Check Engine Light is illuminated
• Engine temperature gauge may read slightly lower than normal or take longer to reach operating temperature
• Cooling fans may run continuously or at unusual times
• Reduced fuel efficiency
• Poor cabin heater performance

Most Likely Causes

1. Failed Electric Thermostat Assembly 🔴 High Probability → Shop Integrated Thermostat Housing Assembly Genesis issued a TSB (21-EM-006HG-1) acknowledging this failure and instructing dealers to replace the thermostat with an improved part. The internal heater element within the thermostat housing is the point of failure, creating an open circuit. This is a common failure on other Hyundai/Kia vehicles with the 3.3T engine, like the Kia Stinger.
   How to confirm: Perform a resistance test on the thermostat's electrical connector pins. A new, functional thermostat should have a resistance of around 11 ohms, with a general acceptable range being 5-30 Ω. An infinite resistance reading (Open Line on a multimeter) confirms the internal heater has failed. A technician in a YouTube video demonstrated this, measuring 31 kilo-ohms on the failed part and 11 ohms on the new OEM replacement. 🎬 Watch: A technician demonstrates the resistance test and replacement process.
   Typical fix: Replace the entire electric thermostat assembly. The thermostat and housing are sold as one integrated unit. After replacement, the cooling system must be properly bled of air, which can be a lengthy process 🎬 Watch: A step-by-step guide to replacing the thermostat on a platform mate. on these engines.
   Est. part cost: $180-$260
2. Wiring or Connector Issue ⚪ Low Probability
   How to confirm: Visually inspect the wiring harness and connector leading to the thermostat for breaks, corrosion, or loose pins. Use a multimeter to check for continuity and proper voltage from the ECM to the connector. One pin should have battery voltage (approx. 12V) with the key on, and the other is a pulse-width modulated ground signal from the computer.
   Typical fix: Repair the damaged section of wiring or clean/replace the connector.
   Est. part cost: $5-$50

Rare But Worth Checking

• Powertrain Control Module (PCM) Fault: → Shop Engine Control Module (ECM) This is extremely rare. Only consider this after definitively ruling out the thermostat and wiring. A faulty driver circuit within the PCM can mimic the symptoms of a failed part.

Diagnosis Steps

1. Confirm the presence of the P0597 code using an OBD-II scanner.
2. Visually inspect the electrical connector and wiring harness attached to the thermostat for any signs of damage, corrosion, or loose connections. The thermostat is located at the back of the engine, near the firewall, making access tight. Pay close attention to signs of pink coolant residue around the connector, which indicates a leak through the thermostat body itself.
3. Disconnect the thermostat connector. Use a multimeter set to Ohms (Ω) to measure the resistance between the two pins on the thermostat itself. A good thermostat should read approximately 5-30 Ω; specifically for this platform, a new part reads around 11 Ω. An open circuit (infinite resistance or 'OL') confirms the thermostat's internal heater has failed.
4. If the thermostat resistance is within spec, check the wiring harness. Check for battery voltage (approx. 12V) on one pin of the harness-side connector with the ignition on. The other pin is the control from the PCM.
5. If wiring and the thermostat are confirmed to be good, the issue may be with the PCM, though this is highly unlikely.

Parts You'll Likely Need

• Electric Thermostat Assembly (OEM #25500-3L200 (for 3.3T) or 25620-3FAA1 (for 5.0L)) — This is the component identified as the cause of the P0597 code in the official Genesis TSB 21-EM-006HG-1. The internal heater element fails, creating the open circuit. Using the genuine OEM part with a lot date newer than July 20, 2017 is required to ensure the updated, more durable part is used.
  Trusted brands: Hyundai/Genesis Genuine Parts
  OEM price range: $180-$255
  Aftermarket price range: $150-$220

Related Codes That Often Appear With This One

• P0598 - Thermostat Heater Control Circuit Low (sometimes seen with P0597 on platform mates like the Kia Stinger)

Technical Service Bulletins (TSBs) & Recalls

• Bulletin #21-EM-006HG-1: Addresses DTC P0597, confirms the cause as a faulty electric thermostat, and provides updated part numbers for the fix for G80 and G90 models.

Platform-Specific Known Issues

• Genesis TSB #21-EM-006HG-1 (superseding 20-EM-003HG) directly addresses DTC P0597 on 2017-2018 G90 (3.3T) and 2018 G80 (3.3T/3.8L) models built before August 30, 2017. It confirms the cause is a faulty electric thermostat and instructs dealers to use a replacement part with a lot date newer than July 20, 2017.

Mechanic-Grade Diagnostic Values

• Thermostat Heater Element Resistance — expected: 5-30 Ω (A new OEM part for this platform measures ~11 Ω). Failure: Infinite resistance or 'OL' (Open Line) on a multimeter.
• Supply Voltage Test at ECM vs. Thermostat — expected: Approx. 12V at the ECM output pin for the thermostat heater circuit while the engine is running.. Failure: If voltage is present at the ECM pin but absent at the thermostat connector, the wiring harness is faulty. If voltage is absent at the ECM pin, the ECM driver circuit is the likely suspect (rare).

Scan Tool Commands That Help

• GDS-M (Genesis Diagnostic System): Thermostat Heater Circuit Activation / Actuation Test — To command the heater circuit on and off to verify if the PCM driver is working and if the circuit can carry a load. This helps differentiate a wiring/component failure from a PCM failure.

Real Owner Repair Stories

• Reddit user on r/kiastinger (Kia Stinger 3.3T AWD (platform mate)) — Check Engine Light with codes P0597 (inactive) and P0598 (active). Radiator fan running at high speed and turning on earlier than expected.
  ✅ What actually fixed it Replacing the OEM thermostat assembly. The user found pink coolant residue and corrosion on the electrical connector pins, indicating coolant had leaked through the thermostat housing into the electrical connector, causing the open/low circuit faults.
• Reddit user on r/GenesisG70 (Genesis G70 3.3T (platform mate)) — Check Engine Light with code P0597.
  ✅ What actually fixed it A responding user confirmed they had the same code and fixed it by replacing the thermostat assembly. They noted the job was difficult due to the tight location at the back right of the engine.
🎬 See this walkthrough of the thermostat replacement on the 3.3T engine.
• Reddit user on r/kiastinger (Kia Stinger GT1, 72,000 miles) — Check Engine Light with code P0597. Vehicle would not drive in sport mode.
  ✅ What actually fixed it The thermostat was replaced under warranty, which resolved all symptoms and codes.

OEM Part Supersession History

• Unknown PNs on early build vehicles → 25500-3L200 (for 3.3T) — The original thermostats were prone to internal heater circuit failure.
  Heads up: Per TSB 21-EM-006HG-1, the replacement part MUST have a lot date newer than July 20, 2017, to be the improved 'countermeasure' part. A blue marking indicates the lot date placement.

Model Year Variations Within This Range

• 2017-2018 (pre-facelift): According to TSB 21-EM-006HG-1, the faulty thermostat issue specifically affects G90 models built before August 30, 2017. Vehicles built after this date likely received the improved thermostat from the factory.