Network Faults

9-05 Break down communication network faults.

Networks an input that is sent to only a single module to be shared with other modules. A failed input may not be isolated to only one module. In a network, a failed input may affect the operation of other associated modules requiring that sensor’s input. For example, the crankshaft position (CKP) sensor outputs engine rpm to the PCM. Other modules that need engine rpm include the ABS, supplemental restraint system (airbag), BCM, instrument cluster (IC), and audio control module. Modules not receiving this message will set a DTC. Depending on the system, a U-code, indicating a network fault, may set in any module that failed to receive the input that it is waiting for the PCM to send.

Before beginning any network or communication diagnosis, always verify that the battery is fully charged. Low battery voltage, typically 10 volts or under, may prevent communication with the modules and fail to power up the scan tool. When doing extended key on, engine off (KOEO) testing, install a battery charger to maintain battery voltage.

Communication Fault Types

The following lists the types of communication faults:

No Communication with All Modules/Network Inoperative

An internal module short or wiring faults can cause an inoperative network and no communication on the bus or with a scan tool. Since none of modules can be communicated with, there will not be any associated DTCs. Before beginning an in-depth diagnosis, check the scan tool and its DLC connector for damage (FIGURE 9-11). Verify the tool’s operation by installing it on another vehicle and testing it. Furthermore, some vehicles will shut down communication with a short to ground on the sensor reference voltage supply, also known as the voltage reference (Vref) circuit. Some vehicles use multiple 5-volt reference circuits, so refer to the vehicle’s service information. Typically, a 5-volt reference signal runs to multiple three-wire sensors, so look for the one that’s the easiest to access. Disconnect the sensor and check the reference wire for 5-volts. If the voltage on the reference wire is 0 volt, look for a short in the wiring or any of the three-wire sensors on the circuit.

FIGURE 9-11 As the diagnosis of the vehicle continues, the technician must verify that the scan tool they are using is operable, which should be verified on another vehicle.

Causes of an Inoperative Network

A loss of communication with all modules can result from any of the following:

  • shorted battery or low battery voltage (most modules will shut down when voltage starts to drop below 10 volts)
  • CAN HI shorted to CAN LO
  • short to ground on CAN HI or CAN LO
  • short to power, battery, or system voltage—on CAN HI or CAN LO
  • shorted sensor reference voltage supply (Vref) on some networks/modules, which varies by manufacturer
  • internal module failure—shorted to battery positive or ground
  • open in both CAN circuits
    • Communication and operation may be present if only one CAN line is open.
Network Inoperative Diagnosis
  • Verify battery/system voltage.
  • Begin all diagnoses by verifying that the DLC connector and pins are not damaged.
    • If the vehicle is performing normally other than a lack of communication with the scan tool, suspect a possible concern at the DLC connector.
    • Faulty ground pins at the DLC can prevent a scan tool from communicating properly.
    • Spread terminals on the communication pins (HS-CAN Pins 6 and 14) or the power supply Pin 16, and ground pins, Pins 4 and 5, can result in a failed network test.
  • When testing an HS-CAN, check the terminating resistance across Pins 6 and 14 by using the appropriate terminal ends or a DLC breakout box. Do not front probe the DLC with anything other than the properly sized terminals. Damage to the pins can create a diagnostic headache.
    • Less than 60 Ω reveals a short to ground in the network’s wiring or internal to a module; it will not allow voltage activity or communication on the bus.
    • 120 Ω indicates one open terminating resistor or circuitry leading to that terminating resistor.
    • High resistance, above 60 Ω but not 120 Ω, is normally a result of a high-resistance fault in the network wiring or a connector issue due to corrosion, pin spread/terminal tension, or a module failing internally.
    • Out of limits (OL) indicates a break somewhere in the communication network. Refer to a wiring schematic and a network topology chart to determine which modules are on the network.
  • If the resistance is near 60 Ω, check for shorts-to-power (12 volts) or shorts to ground KOEO on HS-CAN HI, and check for short to power on HS-CAN LO using a digital multimeter (DMM) or graphing multimeter (GMM).
  • If there are no faults found at this point, remove one module at a time, and perform the network test after removing each module.
  • Removing modules one at a time and testing the network checks each module for an internal short.
  • Once communication has been established, the shorted module that has just been removed is most likely internally shorted.
  • Verify proper power and ground circuits to the module.
  • Verify that the communication lines to the module are not shorted to ground and were not moved when the module was disconnected.
  • If communication is not established after removing all but one of the network’s modules, remove the final module and reinstall one of the previously removed modules. If there is still no communication, suspect a wiring concern and retest.

Loss of Communication with One or More Modules

Communication network diagnosis can be overwhelming without having and following a thorough, repetitive process. Develop and stick to a plan on all network faults. Repetition prevents missing or skipping steps.

Networks come in a wide variety of configurations and protocols. Success with any network concern requires a basic understanding of its operation, how it is laid out, and what protocol it uses to communicate. Refer to the manufacturer’s service information, and gather as much diagnostic information as possible before starting a diagnosis.

Start the on-vehicle diagnosis by gaining as much information as possible as easily as possible. This normally means installing the scan tool onto the DLC to determine whether a module can communicate with any modules. The simplest and most efficient way to do this is to check the scan tool by performing a network test of all available modules. A network test will ping each module for a response and report any codes stored in each module that responds to the scanner. Pay particular attention to the modules that reside on the same network as the failed module. If the network test is not available, individually check each module listed on the scan tool for codes.

Any network that has a fault will show one of three network test results:

  1. Certain modules will not respond, or unresponsive modules may or may not actually be present on the vehicle.
    • Not all vehicles have all available modules.
    • Depending on trim and option levels, module content will vary.
  2. U-codes are present in one more modules.
  3. There is no communication with any modules.

Running the all-modules network test will determine which modules are present and can communicate. After completing the test, the scan tool will list the modules it can talk to and any stored codes in those modules. Communication with most or all but one of the modules on the bus reveals that the network cannot be dead or shorted. Refer to the wiring schematic and, if available, the network topology chart to determine which modules are on the network. A module may be wired into one network but may communicate on another.

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To begin a diagnosis, be sure to scan all modules for codes. Any module that is present on the vehicle has the potential to store DTCs that may include valuable diagnostic information. Note: Not all vehicles contain all modules that may be listed on a scan tool. Some modules may be part of an option that the vehicle did not come equipped with but that are included in a different trim level of the same vehicle model. Before looking for a cause of no communication with a module, verify that the vehicle is indeed equipped with the module in question.

With only a single module not communicating, DTCs should be stored in other modules on the network. When diagnosing no communication with one or more modules, check the module(s) and circuitry that did not set the DTC(s). Modules reporting U-codes are pointing to another module, not themselves. The module (if available on the vehicle) that does not set DTCs is generally where the fault lies. For example, if the PCM stores a U-code for a communication fault with the BCM—e.g., a U0140—the fault is generally not with the module that reported the error, in this case the PCM, but rather with the module (or its circuitry) that the code is highlighting. Also, modules that fail to communicate will typically be identified by U-codes in more than one module.

Additionally, depending on which module is not communicating, the vehicle may not start. For example, a faulty PCM, BCM, or antitheft module may cause a no-start condition. Corrupt software inside the module is another fault that can prevent it from communicating with other modules on the bus or the scan tool. Although this is rare, it does occur.

Often when diagnosing module or bus concerns, the process of elimination is used. All modules require three things to communicate and operate, power and ground circuits, and communication lines. To verify the power and ground to the module, use a headlight bulb to load the circuit and perform a voltage drop test across the bulb. A good circuit should show no more than 0.3 to 0.5 volts, depending on the bulb in use (a higher-wattage bulb will show more voltage drop).

After verifying the module’s power and ground circuits, check the integrity of the communication bus between the suspect module(s) and the reporting module(s). To verify circuit continuity, use either a scope or an ohmmeter, along with a system wiring diagram. Using an OBD II breakout box (BOB) makes this process easier and prevents damage to the DLC pins, which can create additional/future issues. To begin, install the scope’s leads into the pins for the communication network in question at the DLC BOB, or back probe the DLC. Turn the key on and monitor the scope, watching for activity. Communication signals should be visible in the form of different voltages that match the specifications of the network’s protocol. While the content of these packets is unknown, the trace (voltage) identifies that communication is occurring, eliminating the possibility of an entire network fault. If activity is present, move the leads to the module that is not communicating. Back probe the communication wires at the module’s connector and watch the scope. If activity is verified and the voltages and pattern match the protocol, the communication wiring is good, so the probable cause is module failure. If there is no communication signal or if the voltage is abnormally high or low for the protocol being tested, disconnect the module and retest. If the voltage range is correct for the protocol being tested and the waveform of each information packet is clean, suspect that the network transceiver in the module has failed. A failed transceiver will affect the voltage readings and the consistency of the waveform. If there is still no signal, trace the wire(s) from the module to the network for a fault.

To check the communication lines, use an ohmmeter from the module in question and connect it to the DLC. Disconnect the module in question. Match the correct pin of the module to the corresponding pin on the DLC. Check the circuit for continuity. A good circuit will show under 5 Ω. On a two-wire network, check both circuits. If a communication line fails the test, find and repair the fault. If the power, ground, and serial data circuits are good, U-codes are present. If the module still fails to communicate, the fault likely lies with the module in question. Don’t load test communication network lines even after de-pinning the circuit between the module under test and the DLC. Without disconnecting all the modules on the network, the excessive amperage draw from the bulb (load device) can damage any module still connected.

Some modules may require a wake-up signal. A wake-up signal is a voltage pulse sent from a master module to all the modules on a network. Additionally, some modules will not communicate if a 5-volt sensor supply circuit (Vref) shorts to ground. A shorted Vref will not stop communication in all vehicles. Some vehicles will not communicate or start with a short in the 5-volt reference supply circuit. Other vehicles will start and run but will illuminate a warning indicator.

Before replacing a module, one more step should be performed. Modules are computers. Like any computer, be it a laptop or cell phone, they sometimes fail to respond. Simply turning the device off, waiting a few minutes, and then turning it back on may fix this. This is known as logic lock. While there is a detailed explanation, the explanation here will be simple. When a computer detects something that it does not like, such as data confusion, a temporary glitch, or power or ground surge, it locks up, requiring a reset to restore operation. Automotive computers are no different. If a module detects something odd or bizarre, it responds the same way. Resetting the suspect module may cause the module to start communicating. To perform a reset, remove power by disconnecting the module or removing both battery cables and connecting them together with a jumper wire to deplete any internal capacitors that may be storing power (FIGURE 9-12). Wait about 5 to 10 minutes, then reconnect and retest. If the fault repeats, a module may be failing and need to be replaced. Check technical service bulletins (TSBs) for any programming updates before replacing a module for repeated logic lock.

FIGURE 9-12 Sometimes a module on the vehicle gets into a situation that is electronically stuck, and the only way to reset it is to disconnect power and discharge any reserve power in that module. This should only be done when a module is stuck, and the cables should be disconnected for 5 to 10 minutes after they have been touched together.

Summary of Diagnosing No Communication with One or More Modules

Intermittent Network Operation and Diagnosis

The following two subsections describe intermittent network operation and how to diagnose intermittent network communication problems.

Intermittent Network Operation

Intermittent concerns are always the most difficult to find and repair. Intermittent faults are typically the result of an electrical fault on one or both communication circuits. The following lists some of the common faults:

  • intermittent internal module failure
  • CAN HI short-to-voltage
  • CAN LO short to ground
  • CAN HI open circuit
  • CAN LO open circuit.
Diagnosing Intermittent Network Communication

There are no “silver bullets” for diagnosing intermittent network communication. Be as thorough as possible. Operate the vehicle under as many different driving and ambient conditions as possible. Do not hesitate to use water, heat, wiggle, tug, and pull testing. Technicians sometimes use heat guns to increase the temperature of a suspected module, or they remove it and stick in the freezer for several hours and then retest its operation when it’s cold.

  • Check the CAN circuits individually, for shorts to ground, power, or each other.
  • Before moving/touching anything, perform a thorough visual inspection.
    • It is easy to move a harness or connector or to tap a module, unknowingly repairing the fault and thus preventing an accurate diagnosis.
  • To help locate or isolate a fault, perform a wiggle test while monitoring the network’s communication circuits by using a scope or a DMM while monitoring Peak Min/Max voltage or resistance. A GMM is beneficial to use here due to its extended time-based recording abilities.
    • In small sections, wiggle and pull on the harness while looking for changes in resistance, paying close attention to areas where the harnesses run close to bare metal, are tightly routed, and are exposed to the elements.
    • Spraying salt water or baking soda water on the wiring harnesses may help find a wiring fault. Both are conductors and help find wiring faults.
  • Check the CAN HI circuit individually for the following:
    • short-to-voltage
    • short to ground.
  • Check CAN LO for
    • short to ground
    • open circuit.
  • Check for a short between CAN HI and CAN LO.

Unexplained/False U-codes

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Run a network DTC scan and clear all codes after programming any module. Leaving DTCs behind can create a difficult diagnostic scenario later for an unsuspecting technician.

Having U-codes in a module’s memory makes diagnosis difficult. When facing a memory code, it could be a result of a legitimate intermittent fault or a false U-code. False DTCs can also be stored by updating a module during programming (reflashing), because of low battery voltage (most modules fail to communicate below 10.0 volts), or inadvertently by another technician working on a vehicle that had a module(s) disconnected with the key on. When reprogramming a module, the other modules on the network need to be shut down to prevent interference and programming errors. After programming communication faults, false U-codes may be present on the network. After programming, the module will usually recommend clearing all codes before releasing the vehicle. Failure to remove the DTCs may result in a diagnostic scenario where a fault is not present.

Using a Network Topology Chart for Diagnosis

Think of a network topology chart as a road map to aid in the diagnosis. Due to the increase in networks, manufacturers are including network topology charts more often in late-model service information (FIGURE 9-13). During a diagnosis, acquire a copy of the topology chart and wiring diagram for the network being diagnosed (FIGURE 9-14).

FIGURE 9-13 An example of a network topology chart from a 2016 F-150. Network topology charts are beneficial during diagnosis to help find which modules are on the network and the location of the terminating resistors and gateway module. This vehicle has two HS-CANs: HS-CAN1 and HS-CAN2. The gateway in this module is a stand-alone design module. Other networks may include the gateway module as part of an existing module.

image courtesy of Ford Motor Company

FIGURE 9-14 Wiring schematic showing the connections of the network topography at the DLC. HS-CAN1 communicates across Pins 6 and 14. HS-CAN2 is found on Pins 3 and 11.

courtesy of Ford Motor Company

The network topology map can be used in the following applications: