P0599
Thermostat Heater Control Circuit High
Same as P0597 with the control line shorted to supply. The sensor heater warms the element to operating temperature (~600 °C); without it the lambda sensor stays inactive in closed loop and the ECM cannot trim mixture.
- Check engine light: Yes
- Emissions related: Yes
- Repair difficulty: medium
Common causes
Ordered by how often each turns out to be the cause.
- Control line shorted to supplyCommon
- Connector pins corroded - high contact resistanceLess likely
- Supply wire pinched against B+ sourceLess likely
Parts involved
- MAP Thermostat Heater
P0599 is a generic OBD-II code defined by SAE J2012. Exact wording and diagnostic procedure vary by manufacturer and model. Code definitions are drawn from the public-domain OBDex dataset; the workshop notes are our own.