What P0599 means
A map-controlled thermostat is a wax thermostat with a built-in heating resistor (Valeo: about 10 Ω).
How the engine computer detects it
Unheated it opens late, for hot running at part load; the engine module heats it by map under full load or towing so it opens earlier. CARB requires the thermostat to be monitored and the heater circuit counts as an output component. GM switches it to ground with a low-side driver and checks it every second, fault after 15 of 30 samples, type B; Hyundai/Kia/Genesis and VW/Audi use the codes as well. P0599 means a short to positive or a shorted heating resistor – GM detects it with the driver on, when it cannot pull the line to ground. The heater has no effect, coolant runs hotter under load; Audi's 4.2 FSI reports it with P2181, shows the coolant warning and limits engine speed.
Written from public technical sources and not yet checked by a mechanic. How we write these pages
Symptoms
- Check engine light on
- Higher coolant temperature under load
- Coolant warning and engine speed limit (Audi)
Common causes
- Heating resistor in the thermostat shorted Most likely
- Control line shorted to positive Possible
- Coolant or corrosion in the connector Possible
- Engine module driver Less likely
How to diagnose P0599
- Read every stored code with an OBD-II scanner. If misfire or fuel trim codes are there too, fix those first: they often cause this one.
- Inspect the parts involved: map controlled thermostat, thermostat heater, heater wiring, engine control module.
- Work through the causes above from most to least likely, cheapest checks first.
- Clear the code and drive a full drive cycle to confirm the repair held.
Reading the code
- P
- Powertrain
- 0
- Generic (SAE) code
- 5
- Vehicle speed, idle control and auxiliary inputs
- 99
- Specific fault
Questions drivers ask
Can I drive with code P0599?
Drive gently and only as far as you need to. The engine computer may limit power to protect the engine, so avoid highways and heavy traffic until it's fixed. If the check engine light starts flashing, pull over.
Will my car pass an emissions inspection with P0599?
Probably not in states that test vehicles with an OBD-II scan: an emissions-related code that keeps the check engine light on is usually an automatic fail. Rules differ by state and county, so check yours.
Will P0599 clear by itself?
The light can go off after several drive cycles without the fault, but the code returns if the cause is still there. After a repair, the readiness monitors need a full drive cycle before an inspection.
Can I fix P0599 myself?
Often yes, with an OBD-II scanner and basic tools. Difficulty: medium. Start with the cheapest checks before replacing expensive parts.
This page is general information, not a substitute for a professional diagnosis. Repair costs are rough estimates for parts and labor and vary by vehicle and region.