P0597 on Cadillac: How to Diagnose & Fix
Thermostat Heater Control Circuit Open
1 Cadillac model · 1 documented issue · $250-$650 repair
P0597 on Cadillac by model: symptoms, diagnosis and fix
The engine thermostat on the 3.6L LGX V6 commonly sticks in the open position, so the engine never reaches its target operating temperature. The ECM sets P0128, and because the steering module needs a valid coolant-temperature signal to compensate for cold, thick steering-gear grease, it stores U0401 and the driver gets a 'Steering Assist is Reduced - Drive With Care' message, sometimes with noticeably heavier steering effort. Owners report this in NHTSA filings, and GM bulletin 17-NA-158 documents it for the 2017 XT5 with the LGX. It is a cooling-system fault masquerading as a steering fault, which is why many owners are first quoted power-steering diagnostics.
Common Symptoms
- 'Steering Assist is Reduced, Drive With Care' message on the DIC
- Check engine light with P0128
- Air conditioning stops blowing cold / compressor disabled
- Engine temperature gauge reads low or takes a very long time to warm up
- Heater blows lukewarm in cold weather
- Steering effort feels heavier than normal
How to Fix
Replace the thermostat/water outlet housing assembly (the thermostat is integral to the plastic housing on the LGX - it is not serviced separately) and refill/bleed the cooling system with DEX-COOL. Clear P0128, then verify coolant temperature reaches roughly 195-215F on a scan tool before condemning the steering rack or A/C compressor. Do not replace the power steering rack for this message until engine coolant temperature has been verified.
What you need to fix it
Only diagnosis- and fitment-reviewed repair parts are linked here.
- DEX-COOL 50/50 pre-mix engine coolant (for the refill and bleed)OEMGM 12378390 / ACDelco 10-5027 (1 gallon)Cross-refPrestone Dex-Cool 50/50 (orange, ELC/OAT), Peak OET Dex-Cool 50/50$18–$35
The LGX cooling system is DEX-COOL only - do not mix in green IAT coolant.
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Sources for this issue (4)
P0597 code reference
General diagnostic-code context. The code alone does not confirm a cause for your vehicle; use the applicable issue card and its sources.
Frequently asked questions
What does P0597 mean on a Cadillac?
P0597 stands for "Thermostat Heater Control Circuit Open." P0597 is a cooling-system code indicating an open electrical circuit in the control for the heated (electronically controlled) engine thermostat. Many modern engines use a thermostat with a built-in heater element that the computer can energize to open the thermostat sooner for better temperature management and emissions. When the PCM commands that heater but detects no electrical continuity (an open circuit), it sets this code. It generally means the thermostat heater isn't being controlled, which can affect coolant temperature regulation, fuel economy, and emissions. On Cadillac it is documented across 1 model: XT5.
What are the most common causes of P0597?
The most common causes of P0597 are: Failed thermostat heater element or integrated control thermostat, Open or broken wiring to the thermostat heater circuit, Corroded, loose, or disconnected thermostat connector, Blown fuse or open in the heater power feed, Poor ground or control-circuit fault from the PCM, Damaged PCM driver for the heater (rare). The specific cause varies by vehicle.
How much does it cost to fix P0597 on a Cadillac?
Published repair estimates on Cadillac range from $250 to $650 depending on the model and root cause. If an issue mentions a recall or campaign, check your VIN with the manufacturer or NHTSA and confirm eligibility and coverage with a dealer.
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Content compiled with AI assistance using NHTSA complaints, TSBs, and owner reports. May contain errors. Always verify with your vehicle's service manual.