P0599 on Jeep
Thermostat Heater Control Circuit High
P0599 on Jeep vehicles indicates thermostat heater control circuit high. Au7o has documented this code across 3 Jeep models — most commonly on Commander, Compass, Patriot. 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. Typical repair costs on Jeep range from $60 to $1,000, depending on the specific model and root cause.
Common Causes of P0599
- •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)
P0599 on Jeep by Model
Jeep Commander(1 issue)
- Engine Overheating / Cooling System Failures2006-2010
The Commander is susceptible to engine overheating caused by failing water pumps, cracked radiator tanks, leaking heater core connections, and thermostat failures. The radiator end tanks (plastic) are prone to cracking at the seam where they meet the aluminum core. Power steering cooler line leaks can also compound cooling issues. The 4.7L V8 is especially vulnerable.
Jeep Compass(1 issue)
- Thermostat Housing Coolant Leak2007-2017
The plastic thermostat housing on the 2.0L and 2.4L World engines is prone to cracking and leaking coolant. The housing warps from heat cycling, causing the O-ring seal to fail. This leads to external coolant leaks, overheating, and potential engine damage if not addressed. The same housing is used across Compass, Patriot, and Dodge Caliber platforms.
Jeep Patriot(2 issues)
- Thermostat Housing Coolant Leak / Cracking2007-2017
The plastic thermostat housing on the 2.0L and 2.4L World engines cracks from heat cycling, causing coolant leaks and potential engine overheating. This is the same design flaw shared with the Jeep Compass and Dodge Caliber that use the same engine family. The plastic becomes brittle over time and cracks at the O-ring seal area.
- Engine Overheating / Water Pump and Radiator Fan Failure2007-2017
Beyond the thermostat housing issue, the Patriot suffers from additional cooling system failures including water pump failure, radiator fan motor burnout, and radiator cap failure. The water pump can develop an internal leak that mixes coolant into the engine oil, causing severe engine damage. The radiator fan may stop working intermittently, leading to overheating in slow traffic or idle.
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View P0599 across all makes →Frequently Asked Questions
What does P0599 mean on Jeep?▼
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 Jeep specifically, this code is documented across 3 models.
What causes P0599 on Jeep vehicles?▼
Common causes on Jeep: 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. Specific causes vary by model and year — see the per-model sections below.
How much does it cost to fix P0599 on a Jeep?▼
Repair costs on Jeep range from $60 to $1,000, depending on the specific model and root cause.
Which Jeep models have P0599 documented?▼
Au7o has documented P0599 on 3 Jeep models: Commander, Compass, Patriot.