P1115

What the Code Means

Code P1115 is set by the engine cooling system and points to a fault in the thermostat control circuit. The source data describes P1115 as “Coolant Thermostat, Activation” — meaning the DME can't properly activate the thermostat or read its data. The circuit is damaged, so reading data isn't possible.

The same source mentions a related code, P1102 (low heater resistance, oxygen sensor), which also relates to the oxygen sensor. For P1115, though, the core problem sits in the thermostat circuit: when the thermostat sticks open, the table lists P1115 together with P0128. That means the engine never reaches operating temperature and the system flags a temperature-regulation mismatch.

Symptoms

• Long warm-up, or the engine never reaches operating temperature.
• Check Engine light with P1115 (“Coolant Thermostat, Activation”).
• Engine overheating.
• The table lists P1115 together with P0128 when the thermostat sticks open.

Diagnosis and Repair

1. Check the thermostat.
2. Use ISTA or Launch X431 for diagnosis.
3. If faulty, replace the thermostat. Original part — 11 53 7 578 300; an aftermarket equivalent may be sourced.

Secondary Problems

• Engine overheating.
• Coolant boiling in the coolant reservoir.
• The engine may drop into limp mode.
• Risk of thermal shock if an overheated engine is shut off.