P0599 on Cadillac: How to Diagnose & Fix
Thermostat Heater Control Circuit High
1 Cadillac model · 1 documented issue · 1 critical · $3,000-$6,000 repair
P0599 on Cadillac by model: symptoms, diagnosis and fix
The Northstar V8 in the STS is infamous for head gasket failure caused by the head bolts pulling out of the aluminum block. GM used torque-to-yield head bolts threaded directly into the aluminum block without steel inserts, and over time the bolt threads strip from the soft aluminum. Coolant enters the combustion chambers, causing overheating and white exhaust smoke. This issue affects virtually every high-mileage Northstar engine and is often terminal for the vehicle due to repair costs exceeding vehicle value.
Common Symptoms
- Coolant loss with no visible external leak
- White smoke from exhaust (sweet smell)
- Engine overheating, especially under load
- Coolant in oil (milky appearance on dipstick)
- Bubbles in coolant reservoir
How to Fix
The permanent fix is the Northstar Performance "Norm's Inserts" or Time-Sert procedure — the engine is removed, heads pulled, and steel thread inserts are installed in all head bolt locations in the aluminum block. This is a 30+ hour job. Alternatively, a used/reman engine with inserts already installed can be swapped in. Bar's Leaks Head Gasket Fix (1111) is a temporary measure that can buy time.
Sources for this issue (1)
P0599 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 P0599 mean on a Cadillac?
P0599 stands for "Thermostat Heater Control Circuit High." This code relates to the engine cooling system's thermostat heater control circuit, used on engines with an electrically heated (map-controlled) thermostat that the computer can open early to fine-tune coolant temperature. The 'high' designation means the computer detected the control circuit voltage was higher than expected, indicating an electrical fault such as a short to voltage or an open circuit. It is generally an electrical/wiring problem rather than a mechanical cooling failure. It can affect how precisely the engine manages its operating temperature and fuel economy. On Cadillac it is documented across 1 model: STS.
What are the most common causes of P0599?
The most common causes of P0599 are: Faulty electronically heated thermostat element/heater, Short to voltage in the heater control circuit, Open or damaged wiring/connector, Corroded or loose connector at the thermostat, Poor electrical ground, Faulty PCM/ECM driver (less common). The specific cause varies by vehicle.
How much does it cost to fix P0599 on a Cadillac?
Published repair estimates on Cadillac range from $3,000 to $6,000 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.
Looking for P0599 on a different make?
View P0599 across all makes →Content compiled with AI assistance using NHTSA complaints, TSBs, and owner reports. May contain errors. Always verify with your vehicle's service manual.