P1718 – Automatic Transmission CAN Communication Circuit (Subaru)

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By Reinier (Contact Me)
Last Updated 2023-11-21
Automobile Repair Shop Owner
CodeFault LocationProbable Cause
P1718 Automatic Transmission CAN Communication Circuit
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Table of Contents

  1. What Does Code P1718 Mean?
  2. Where is the P1718 sensor located?
  3. What are the common causes of code P1718?
  4. What are the symptoms of code P1718?
  5. Get Help with P1718

What Does Code P1718 Mean?

OBD II fault code P1718 is a manufacturer-specific trouble code that is defined by carmaker SUBARU as, “P1718 – “Automatic Transmission CAN Communication Circuit” and is set when the PCM (Powertrain Control Module) detects a failure, malfunction, or abnormal voltage in the CAN (Controller Area Network) serial communications system that connects the automatic transmission with other control modules.

All modern vehicles use at least two serial communications systems that connect different categories of control modules to form a communications network in which all connected control modules can transmit and receive information that is shared by all the control modules in the network.

In practice, though, some information, such as messages that control critical functions such as ignition, ignition timing, fuel delivery, and others that control the operation of the engine is typically transmitted over a high-speed communications system that forms part of the larger CAN (Controller Area Network) system. In addition to the above, information that both monitors and controls the operation of the automatic transmission is also transmitted over this system, since the TCM (Transmission Control Module) and the PCM (Powertrain Control Module) aka the Engine Control Module are deeply integrated into one another.

As a practical matter, and despite the high level of integration between the TCM and the PCM, each module has the primary function of controlling the engine and transmission, respectively. However, gearshift points change according to the engine speed and load, so to ensure that gearshifts always occur at the optimal points regardless of the engine speed or the load the engine is under at any given point, the TCM and PCM communicate critical operational data over the CAN network.

For example, before the TCM initiates a gearshift, it gathers information about the engine speed and load from the PCM and it will only initiate the gearshift if both the engine speed and the load on the engine meet certain pre-programmed thresholds. Put differently, this means that the TCM will not initiate a gearshift from say, second gear to third gear if the load on the engine is very high, even though the engine speed may meet the minimum allowable threshold.

Similarly, if all parameters meet certain minimum thresholds, the TCM will alert the PCM over the CAN communication system that a gearshift is imminent. This information prompts the PCM to retard the ignition timing by a few degrees and reduce the fuel injectors’ pulse width at the same time, to “smooth out” the transition from one gear to the next higher (or lower) gear. In a fully functional CAN system, this reciprocal communication occurs in considerably less than a millisecond, which has the effect of making gearshifts almost undetectable to an average driver.

Moreover, the communications between the TCM and PM are also shared by other control modules that control systems like the ABS brakes, cruise control, or adaptive cruise control, if fitted, stability control, traction control, and others over the same communication system. Therefore, if a communication failure occurs between the TCM and the PCM, some of the above systems are directly affected, meaning that some of these systems may be deactivated by the PCM because the control modules that control these systems may be deprived of critical operational data as a result of the communication failure.

Nonetheless, the primary effect of a communication failure between the TCM and the PCM is the loss of coordination of critical operational information between the engine and the transmission. In translation, this means that gearshifts – if gearshifts still occur at all after the communication failure- may be harsh, erratic, or unpredictable. In addition, and depending on the nature of the communication failure, the transmission fluid may overheat severely as a result of malfunctions in the transmission’s valve body, which malfunctions can occur as a direct result of the communication failure between the TCM and the PCM.

It is worth pointing out that severe overheating of the transmission fluid can degrade the fluid to the point where its lubrication properties are destroyed, which can, and often does, cause fatal mechanical damage to the transmission.

Thus, if the PCM detects a communication failure between itself and the transmission control module, it will recognize that it cannot control and/or manage gearshifts effectively, and as a result, the PCM will set fault code P1718 and illuminate one or more warning lights.

Where is the P1718 sensor located?

Since code P1718 is almost invariably caused by wiring issues, as opposed to control module failures, this image shows an example of a poorly executed electrical diagnostic procedure that had progressed into major damage to the wiring.

In this example, the yellow circle indicates sites where wiring had been pierced with a sharp object to obtain voltage readings. The holes in the insulation allowed moisture to enter the wires, and over time, the accumulation of corrosion between the two piercings built up to the point where the two wires shorted out, thus causing a major electrical failure on this vehicle.

Note this is only one example of how damaged insulation can cause major issues such as communication failures between control modules but the bigger issue is that it can be extremely difficult even for professional mechanics to locate burnt or shorted wiring such as the example shown here. Therefore, we strongly recommend that non-professional mechanics do not attempt to locate this kind of damage to wiring due to the very high likelihood that additional damage to wiring and/or sensitive electronic components could result from incorrect or ill-considered testing procedures. Therefore, we recommend that you seek professional assistance with diagnosing and repairing suspected damaged wiring.

What are the common causes of code P1718?

NOTE: It should be noted that code P1718 rarely, if ever, involves mechanical or other failures in, or malfunctions of, the automatic transmission itself but be aware that the most common causes of this code could include one or more of the following-

  • Damaged, burnt, shorted, disconnected, or corroded wiring and/or electrical connectors almost anywhere in the CAN serial communications system that serves the engine and transmission
  • Failed or defective transmission power relay, but note that this will be indicated by an additional, dedicated trouble code
  • Poorly executed repairs to wiring, or the improper installation of some types of aftermarket accessories
  • Poor ground connections almost anywhere in the vehicle’s wiring
  • In some cases, a defective brake light switch could contribute to the setting of code P1718
  • The use of incompatible versions of software in either, or both, the PCM and TCM- or any other implicated control module, for that matter
  • Failures of, or malfunctions in, most of the control modules that share information with either, or both, the engine and transmission control modules
  • Faulty or corrupted programming in any control module that shares information with either, or both, the engine and transmission control modules
  • In some cases, abnormally low or high system voltages caused by flat or discharged batteries or defects in the charging system can corrupt or damage the programming in some control modules that share information with either, or both, the engine and transmission control modules
  • Failed or defective PCM and/or TCM, but note that since these are rare events, the fault must be sought elsewhere before any control module is replaced or reprogrammed

NOTE: While it is possible to reset the transmission control module on most SUBARU vehicles, performing this procedure when code P1718 is present will not resolve the problem. This is because code P1718 is set by a failure in the communication path()s) between the transmission control module and other control modules, as opposed to a malfunction or “glitch” in the transmission control module itself.

What are the symptoms of code P1718?

The most common symptoms of trouble code P1718 are largely similar across all applications and could include one or more of the following-

  • Stored trouble code and one or more illuminated warning lights, depending on the nature of the communication failure, but note that the most common additional warning lights are those that relate to the ABS, cruise control, stability control, and traction control. Be aware that if these warning lights are illuminated ABS braking and one or more other safety systems may not be available
  • Multiple additional codes could be present along with P1718. Note that the most common additional codes are generic transmission and UXXXX( communication) codes
  • Depending on the nature of the failure, some symptoms such as additional codes and warning lights may appear and disappear intermittently
  • Fuel consumption may increase noticeably
  • Gear shifts may be harsh, erratic, or unpredictable
  • Depending on the nature of the failure, the transmission may be locked in one gear; most often, this would be the gear the transmission was in when the failure occurred
  • In some cases, the vehicle may be in a fail-safe or limp mode that will persist until the fault is found and corrected
  • The transmission fluid may overheat severely, but note that severe and even fatal mechanical damage to the transmission may result from overheating of the transmission fluid

NOTE: Note that although the transmission fluid may overheat when code P1718 is present, the fluid will not necessarily overheat when this code is present. In most cases, the TCM will illuminate the AT temperature warning light merely as an indication that there is something wrong with the transmission, or with the control and/or management of the transmission.

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