Faulty map-controlled thermostat (heater element open circuit or internal failure).; Open or high-resistance wiring between the DME and the thermostat connector.; Corroded or damaged connector pins at the thermostat or DME.; DME internal driver circuit failure (rare but possible).; Aftermarket or incorrect thermostat installed that does not match BMW specifications.; Coolant contamination or debris causing thermostat malfunction.

DTC codes / BMW codes / P1620

P1620 · BMWMAP Cooling Control Circuit Signal High

Q: What does code P1620 mean?

MAP Cooling Control Circuit Signal High. P1620 on BMW vehicles indicates that the engine control module (DME) has detected an abnormally high signal voltage on the map-controlled thermostat circuit, which is used to regulate coolant temperature for improved efficiency. The DME expects a duty-cycled ground signal to control the thermostat heater, but it sees a voltage that is too high, suggesting an open circuit, a failed thermostat, or a wiring fault.

P1620 on BMW vehicles indicates that the engine control module (DME) has detected an abnormally high signal voltage on the map-controlled thermostat circuit, which is used to regulate coolant temperature for improved efficiency. The DME expects a duty-cycled ground signal to control the thermostat heater, but it sees a voltage that is too high, suggesting an open circuit, a failed thermostat, or a wiring fault.

SystemPowertrain

Code typeBMW specific

SeverityMedium

Fix difficultyIntermediate

AreaComputer and output circuits

Symptoms you may notice

Illumination of the check engine light, often with no other noticeable symptoms.
Engine may run cooler than normal or take longer to reach operating temperature.
Poor fuel economy or increased emissions due to incorrect coolant temperature regulation.
In some cases, the engine may enter a failsafe mode with reduced power.
Possible overheating if the thermostat remains stuck open or closed, though this is less common.

Common causes, most likely first

Faulty map-controlled thermostat (heater element open circuit or internal failure).
Open or high-resistance wiring between the DME and the thermostat connector.
Corroded or damaged connector pins at the thermostat or DME.
DME internal driver circuit failure (rare but possible).
Aftermarket or incorrect thermostat installed that does not match BMW specifications.
Coolant contamination or debris causing thermostat malfunction.

How to diagnose and fix it

Scan all modules for fault codes and verify P1620 is current; clear and road test to see if it returns.
Visually inspect the thermostat connector and wiring for corrosion, chafing, or loose pins.
Measure resistance of the thermostat heater element and compare to BMW specifications; replace thermostat if out of range.
Check for continuity and voltage drop on the control circuit from DME to thermostat; repair wiring as needed.
If wiring and thermostat test good, test the DME output signal with an oscilloscope or bidirectional scan tool; replace DME only if confirmed faulty.
After repair, clear codes and perform a coolant temperature regulation test drive to confirm the fix.

Map Thermostat Circuit Diagnostics

A common misdiagnosis is replacing the thermostat without checking the wiring harness, as the connector at the thermostat is exposed to heat and coolant and often corrodes internally.
The map-controlled thermostat receives a pulse-width modulated ground signal from the DME; a high signal voltage can be set by a simple open circuit in the ground-side wire, not just a failed thermostat.
Some aftermarket thermostats have incorrect heater resistance, causing P1620 to set even though the part is new; always use a genuine BMW or OEM-quality thermostat.
This code can be stored intermittently due to a loose connector that only loses contact when the engine is hot or vibrating; wiggle-test the harness while monitoring live data.
P1620 does not always mean the thermostat is stuck; it specifically refers to the electrical control circuit, so cooling system performance may still be normal.

Can I keep driving with P1620?

You can generally continue driving with P1620, but avoid prolonged high-speed or heavy-load driving because the engine may not maintain optimal temperature, which can affect fuel economy and emissions. If the engine temperature gauge shows overheating or the vehicle enters limp mode, stop and seek professional service.

Tools that help

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Guidance is general and written for BMW vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.