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Mercedes Citan W415 / Renault Kangoo K9K 872 — DF839 / DTC16BC13 “Coolant thermostat: open circuit” — faulty auxiliary thermostat location and diagnosis

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I am posting this because diagnosing DF839 / DTC16BC13 on the later K9K engine turned out to be confusing. There are two different thermostats/coolant-control areas, and it is very easy to spend time looking at the wrong one.

Vehicle

Fault

DF839 — COOLANT THERMOSTAT: open circuit — State: Active

The corresponding Renault/OBD fault is commonly shown as:

DTC16BC13 / P16BC — electrically controlled coolant thermostat / open circuit

The important discovery is that this fault does not refer to the ordinary main engine thermostat in the large plastic coolant manifold on the cylinder head.

The main thermostat housing is NOT the DF839 component

The main plastic coolant manifold/thermostat housing on this engine is in the cylinder-head cooling circuit.

The electrical component fitted into that housing is a coolant-temperature sensor, Renault:

7700110664 / 77 00 110 664

I removed this sensor and measured approximately:

2.5 kΩ at roughly room temperature

which is entirely plausible for an NTC coolant-temperature sensor.

Therefore, do not confuse this sensor with the electrically controlled thermostat mentioned by DF839.

The large main thermostat housing is commonly referenced as:

Renault 110606973R
Mercedes A6082000015

but replacing this assembly is not the obvious cure for an electrical-open-circuit DF839.

The DF839 component is a SECOND thermostat

The component that DF839/DTC16BC is actually concerned with on this K9K 872 installation is an electrically controlled auxiliary / low-temperature-circuit thermostat.

The original part fitted to this vehicle is:

Renault 212009132R

Cross-referenced/replacement numbers include:

212002554R
212009526R
212002893R

and the Mercedes-Benz cross-reference is:

A4152030100 / A415 203 01 00

Current aftermarket catalogues group 212009132R, 212002554R, 212009526R and 212002893R as equivalents for the same thermostat family.

There is also independent Renault technical/community material specifically describing 212009132R as the auxiliary thermostat used with K9K 872/873 and DTC16BC13, with 212002554R described as a replacement number.

Where the thermostat actually is on the Citan W415

This was probably the most useful discovery.

On this particular Mercedes Citan W415 installation, 212009132R is NOT readily visible from the engine compartment.

It is located:

At the front-left of the vehicle, in front of the left/front wheel, behind the wheel-arch liner.

  1. Go to the left front wheel.
  2. Gain access behind/remove the appropriate section of the wheel-arch liner.
  3. The auxiliary coolant circuit and electrically controlled thermostat are hidden behind it.
  4. Look for a fairly large black plastic thermostat body incorporated into the coolant hoses.
  5. It has a 2-pin electrical connector.

This explains why searching around the cylinder head and main thermostat housing does not reveal the component referred to by DF839.

Electrical test

Disconnect the 2-pin connector from 212009132R and measure resistance directly across the two terminals of the thermostat itself.

On this vehicle the measured resistance was approximately:

2 MΩ — 2,000,000 ohms

The ECU simultaneously reported:

DF839 — COOLANT THERMOSTAT: open circuit — ACTIVE

That is essentially an open electrical circuit and very strongly indicates failure of the electrical element inside the auxiliary thermostat.

Reports from technicians/owners dealing with this same electrically controlled thermostat family put a healthy heater in approximately the low tens of ohms, around 15–17 Ω. I have not found a Renault workshop specification that I can independently verify for that exact resistance, so treat 15–17 Ω as field information rather than an official Renault specification.

However, there is no realistic ambiguity between approximately 15 Ω and 2,000,000 Ω. A reading in the megohm range is effectively an open heater circuit and agrees exactly with an active “open circuit” fault.

Wiring principle

The electrically controlled thermostat has two wires.

Available K9K 872 wiring information identifies the thermostat heater as an ECU-controlled load, with one side supplied from the engine-management/main-relay 12 V circuit and the other side controlled by the ECU.

Component test:

Measure directly across the disconnected thermostat.

Harness test:

With the thermostat disconnected and engine-management relay active/engine running, check each vehicle-side terminal against battery negative.

One side should have approximately system voltage, generally:

~12 V engine off / ~13.5–14.5 V with engine running

The other wire is the ECU-controlled side.

Do not expect to necessarily measure 12 V across the two disconnected connector terminals, because the ECU switches the control side according to its thermal-management strategy.

Important distinction between the two electrical components

For anyone finding this post through Google or an AI search:

7700110664 = coolant-temperature sensor in the main engine thermostat/coolant manifold.

212009132R = electrically controlled auxiliary/low-temperature thermostat associated with DF839/DTC16BC on this K9K 872 installation.

They are completely different components.

The temperature sensor measured approximately 2.5 kΩ and appeared normal.

The auxiliary thermostat measured approximately 2 MΩ and was effectively electrically open.

Parts information

Original component found on the vehicle:

Renault 212009132R

Useful search numbers:

Aftermarket catalogue information cross-references the Renault numbers above to the same thermostat family.

Listings also specifically associate 212009132R with K9K 872 70 kW / 95 hp applications.

Additional symptom: slow coolant loss

This vehicle was also slowly losing coolant.

At the time of diagnosis, the source of that coolant loss had not yet been positively identified, so I would not automatically assume the electrically failed 212009132R is responsible.

Anyone with the same combination of symptoms should inspect:

and preferably pressure-test the cooling system cold.

Short version / search summary

Mercedes Citan W415 2021 109 CDI / K9K 872 70 kW 95 hp — DF839 ACTIVE “COOLANT THERMOSTAT: open circuit”, Renault DTC16BC13 / P16BC. The faulty component is the separate electrically controlled auxiliary/low-temperature coolant thermostat Renault 212009132R, not the main thermostat housing or coolant-temperature sensor. On the Citan W415 it is hidden behind the LEFT FRONT wheel-arch liner, in front of the left front wheel. The thermostat has a 2-pin connector. Faulty unit measured approximately 2 MΩ across its terminals, effectively open circuit. Replacement/cross-reference numbers include 212002554R, 212009526R, 212002893R and Mercedes A4152030100. Coolant-temperature sensor 7700110664 in the main plastic thermostat manifold measured approximately 2.5 kΩ and was not the DF839 component.