U0166 Lost communication with auxiliary heater control module

OBD-II DTC Datasheet

Lost Communication With Auxiliary Heater Control Module

What does this mean?

This is a generic communications system diagnostic trouble code that applies to most makes and models of OBD-II vehicles.

This code means that the auxiliary heater control module (AHCM) and other control modules on the vehicle are not communicating with each other. The circuitry most commonly used for communication is known as Controller Area Bus communication, or simply the CAN bus.

Without this CAN bus, control modules cannot communicate and your scan tool may not receive information from the vehicle, depending on which circuit is involved.

The auxiliary heater control module is usually located behind the dashboard or in the center console area of the vehicle. It receives input data from various sensors, some of which are directly connected to it, most of which are transmitted via a bus communication system. These inputs allow the module to control the interior temperature.

Troubleshooting steps may vary depending on the manufacturer, the type of communication system, the number of wires, and the colors of the wires in the communication system.

Code severity and symptoms

The severity in this case is never serious, as this is a device for the convenience of customers. The lack of AHCM operation does not in any way affect the operation of the vehicle.

Symptoms of a U0166 code may include:

No temperature changes from the auxiliary heater system
Additional heater system does not work / no response

reasons

Usually the reason for installing this code is:

Open in CAN + bus circuit
Open in the CAN bus - electrical circuit
Short circuit to power in any CAN bus circuit
Short to ground in any CAN bus circuit
No power or ground to AHCM
Rarely - the control module is faulty

Diagnostic and repair procedures

A good starting point is always to check the Technical Service Bulletins (TSB) for your particular vehicle. Your problem may be a known issue with a known manufacturer-released fix and could save you time and money while troubleshooting.

First, look for other DTCs. If any of these are bus communication or battery / ignition related, diagnose them first. Misdiagnosis is known to occur if you diagnose the U0166 code before any of the major codes are thoroughly diagnosed and rejected.

If your scan tool can access trouble codes and the only code you are getting from other modules is U0166, try accessing the Auxiliary Heater Control Module (AHCM). If you can access codes from AHCM then code U0166 is either intermittent or memory code. If AHCM cannot be accessed, then code U0166 set by other modules is active and the problem already exists.

The most common malfunction is a loss of power or grounding of the auxiliary heater control module.

Check all fuses supplying the AHCM on this vehicle. Check out all the reasons for AHCM. Locate ground anchorage points on the vehicle and make sure these connections are clean and secure. If necessary, remove them, take a small wire bristle brush and baking soda / water solution and clean each one, both the connector and the place where it connects.

If any repairs have been made, clear the DTCs from memory and see if U0166 returns or you can contact the AHCM. If no code is returned or communication is restored, the problem is most likely a fuse / connection issue.

If the code returns, look for the CAN bus communication connections on your car, most notably the AHCM connector, which is usually located behind the dashboard or in the area of the car's center console. Disconnect the negative battery cable before disconnecting the connector on the AHCM. Once detected, visually inspect the connectors and wiring. Look for scratches, scuffs, exposed wires, burn marks, or molten plastic.

Disconnect the connectors and carefully inspect the terminals (metal parts) inside the connectors. See if they look burnt or have a green tint indicating corrosion. If you need to clean the terminals, use an electrical contact cleaner and a plastic bristle brush. Allow to dry and apply dielectric silicone grease where the terminals touch.

Perform these few voltage checks before connecting the connectors back to the AHCM. You will need access to a digital volt/ohmmeter (DVOM). Make sure the AHCM has power and ground. Access the wiring diagram and determine where the main power and ground supplies enter the AHCM. Connect the battery before continuing with the AHCM still disconnected. Connect the red lead of your voltmeter to each B+ (battery voltage) supply going into the AHCM connector, and the black lead of your voltmeter to a good ground (if unsure, battery negative always works). You should see the battery voltage reading. Make sure you have a good reason. Connect the red lead of the voltmeter to the battery positive (B+) and the black lead to each ground circuit. Once again, you should see the battery voltage every time you connect. If not, repair the power or ground circuit.

Then check the two communication circuits. Locate CAN B+ (or MSCAN + circuit) and CAN B- (or MSCAN - circuit). With the black wire of the voltmeter connected to a good ground, connect the red wire to CAN B+. With the key on and the engine off, you should see a voltage of about 0.5 volts with little fluctuation. Then connect the red lead of a voltmeter to the CAN B circuit. You should see about 4.4 volts with slight fluctuations.

If all tests pass and communication is still not possible, or you were unable to reset DTC U0166, the only thing that can be done is to seek help from a trained automotive diagnostician, as this will indicate an AHCM failure. Most of these AHCMs must be programmed or calibrated in order to properly install the vehicle.

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