P1622 – Map Cooling Thermostat Control Circuit Electrical (BMW)

By Reinier (Contact Me)
Last Updated 2021-03-25
Automobile Repair Shop Owner

Manufacturer Specific Definitions for P1622

Table of Contents

1. What Does Code P1622 Mean?
2. Where is the P1622 sensor located?
3. What are the common causes of code P1622?
4. What are the symptoms of code P1622?
5. Get Help with P1622

What Does Code P1622 Mean?

Special note on trouble code and BMW vehicles: While DTC P1622 is a manufacturer-specific code that may affect several OBD II compliant vehicles, the causes and symptoms of this code typically vary between most vehicle makes. This article will therefore deal with code P1622 as it applies specifically to BMW vehicles.

OBD II fault code P1622 is a manufacturer-specific trouble code that is defined by carmaker BMW as “Map Cooling Thermostat Control Circuit Electrical”, and is set when the PCM (Powertrain Control Module) detects a malfunction or defect in the control, signal, or monitoring circuit(s) of the electronically controlled thermostat in the engine cooling system.

NOTE: The word “Map” in this definition can be somewhat confusing. A better term would be “Mapped”, which refers to the fact that the thermostat’s operating cycle is controlled by the DME (Digital Motor Electronics), which is BMW-speak for PCM (Powertrain Control Module), ECU (Engine/Electronic Control Unit), or ECM (Electronic Control Module).

Before the advent of mapped thermostats, an engines’ temperature was regulated by the temperature of the engine coolant only. In practice, the thermostat reacted to the rising temperature of the coolant: as the coolant’s temperature rose, the thermostat reacted to the increased temperature by opening at a predictable rate to allow progressively more coolant to circulate through the engine. Thermostats of this type were calibrated to be fully open at a certain coolant temperature, and while these devices worked reasonably well, they had some serious drawbacks.

These included the facts that they could only be fully closed or fully open, and since some thermostats took a long time to open fully, they contributed to the excessive exhaust emissions because they kept engines running at low temperatures for longer than was strictly necessary. This became a major sticking point with the introduction of Euro-5 emissions regulations, which prompted car manufacturers to develop strategies that could control/manage engine temperatures more effectively than was possible to do up to that point.

The solution was the development of thermostats that could be controlled independently of the engine coolant’s temperature, and while there are many different mapped thermostat designs today, all work in much the same way, and all also fail in much the same ways.

Mapped thermostats have all of the components/parts of a conventional thermostat, but with the difference that mapped thermostats have integrated electrical heater elements that are controlled by the DME. As a practical matter, the heater element replaces the coolant’s temperature as a control mechanism, in the sense that the thermostat’s valve can be opened by activating the heater element regardless of the engine coolant’s low temperature.

The practical advantage of this is that by heating the element, the opening of the thermostat’s valve can be “mapped” to suit operating conditions and environmental factors, such as low or freezing ambient temperatures. For instance, depending on inputs such as the engine speed, engine load, engine coolant temperature, ambient temperature, and vehicle speed, the DME can activate the heating element at lower temperatures to decrease engine warm-up times.

The practical advantage of this includes the fact that stable combustion is achieved sooner, which produces a significant reduction of the total volume of exhaust emissions an engine generates until it reaches its optimal operating temperature. One other advantage of mapped thermostats is that the DME can exercise precise control over the thermostat throughout the engine’s operating range. For example, at low engine loads, when combustion temperatures are relatively low, the DME can raise the engine’s temperature (to improve combustion) by partially restricting the coolant flow, while under high load conditions, the DME can command the thermostat fully open to maximize the coolant’s flow rate to improve management of the engine’s temperature.

Therefore, since the engine’s temperature has a direct bearing on combustion efficiency, and by extension, the creation of harmful exhaust emissions, any failure, fault, defect, or malfunction in a mapped thermostat, or its control system, will cause the DME to recognize that it cannot control the engine’s temperature effectively, if at all. When this happens, the DME will set code P1622, and illuminate a warning light that may or may not flash, depending on the nature of the problem.

Where is the P1622 sensor located?

This image shows the location (arrowed) of the mapped thermostat on a BMW 325i application. Note though that while the thermostat is relatively easy to access in this example, it may be necessary to remove and/or disassemble unrelated engine parts and/or components to access the thermostat on most other BMW applications.

Typical items that need to be removed or disassembled include, but are not limited to radiators, coolant hoses, viscous fan clutches, the drive belt, and (sometimes) various pulleys and drive belts, as well as power steering hoses and A/C hoses in some cases. Be aware that working space is severely limited on most BMW applications, so if you are not comfortable with the idea of disassembling engine parts in a cramped engine compartment, the better option would be to seek professional assistance with diagnosing and repairing thermostats.

What are the common causes of code P1622?

Common causes of code P1622 are largely similar across all BMW applications, and could include one or more of the following-

• Damaged, burnt, shorted, disconnected, or corroded wiring and/or connectors in the thermostat’s control and monitoring circuits
• Abnormally high or low system voltages
• Defective or malfunctioning engine coolant temperature sensor(s), or defects in any other implicated sensor
• Defective or malfunctioning thermostat
• Low coolant levels, but note that this condition will typically be indicated by a dedicated warning light on the dashboard
• Failed or failing DME, but note that since this is a rare event, the fault must be sought elsewhere before any control module is reprogrammed or replaced

What are the symptoms of code P1622?

Typical symptoms of code P1622 could include one or more of the following-

• Stored trouble code and an illuminated warning light
• In some cases, one or more additional codes may be present along with P1622
• The engine may overheat, or may not reach operating temperature, depending on the nature of the failure
• In cases where the engine overheats, the DME may initiate a forced shutdown of the engine
• Severe, and often fatal engine damage could result from the engine overheating caused by a defective thermostat

WARNING: Any abnormal engine temperature reading (as shown on the gauge on the dashboard) MUST be investigated immediately. While BMW engine temperature gauges can and do fail, never assume that the problem is just a faulty gauge, because there is no easy or reliable way for the average driver/car owner to make a distinction between a faulty temperature gauge and a faulty thermostat. The cause(s) of abnormal engine temperature readings can only be established by performing specific tests so do NOT continue to use the vehicle if you notice abnormal engine temperature readings at any point during a trip or drive cycle. 

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