This is especially true for a module specific U code. Let’s say you do a DTC scan of a CAN bus equipped vehicle and find DTC U2111 (Electronic Steering Control Module Not Communicating) in several chassis related modules. Don’t suspect those modules. Suspect the module they are unable to communicate with by the definition of that code. CAN networks have an abundance of very definitive DTCs when the bus has voltage level problems such as intermittent shorts to power or ground, open circuits and accumulated errors in communications that can temporarily cause a module to pull itself offline.
You can accidentally set the stage for a U code by coding a new module with incorrect information.
Another really important thing to remember when interpreting U codes on CAN bus vehicles are non-consequential history codes that set. Sometimes when there has been a service procedure where a module has been w/o power codes will set in history. They mean nothing if not accompanied by a real complaint with the vehicle. Simply record the DTCs for possible referral in the future then clear them. Some scan tools might cause U codes to set in select modules when they are unplugged from the DLC while still in a linked-up communication condition. This isn’t a condemnation of the car or the tool, just a reaction to two parties not wishing to part ways yet. Avoid this anomaly by simply backing out of the communications screens with your tool prior to disconnecting.
Total bus failures like shorts to power or ground by a network circuit or module on the network can occur on a CAN HS network. This can prevent the vehicle from starting, (immobilizer) cause the entire PRNDL to light up all at once or induce multiple IPC telltale lights to illuminate. A short to power might be 12 volts measured at DLC pin 6 or 14, but don’t assume a short to ground is 0 volt or 0.1 ohms between ground and the network pin. I’ve seen 50 ohms of resistance between ground and a network circuit due to a faulty aftermarket accessory wired into one of the bus wires causing communications failures with some modules while others still would communicate. My scope showed a perfect pattern on that vehicle too but a quick measurement between ground and both network circuits revealed several thousand ohms of resistance on one of the two wires but only 50 ohms between the bus and ground on the other wire.
Aftermarket radios quite frequently are the culprit on my problem vehicle. Always consult the service manual for exact specs, but a rule of thumb for a short circuit by definition on a data bus network circuit is anything less than 350 ohms measured between ground and each network circuit.
End of the Line - Terminating the Bus
Some networks including CAN buses require termination resistances at each end of the bus. If the bus is a vehicle wide bus such as a GM or Ford HS dual wire CAN bus, there will be a pair of 120 ohm resistors either in two modules or a single resistor in one module and the other resistor wired in the network harness somewhere. If the vehicle uses one resistor in the harness as opposed to locating them internal to two modules, the resistor might be a simple stick lead resistor soldered into the harness and taped up with electrical tape or it may be in a small plastic housing.
I have seen cases with GM where the terminating resistor was in a black plastic housing about the size of a small relay and located under the vehicle near the fuel tank. The reasoning for this location was for the resistor to be near the end of the line of the HS CAN bus where the fuel pump control module is located. This has led to corrosion in the field which means resistance build-up. The resistors are there for noise immunity (EMI) so if the total resistance isn’t right – reliable bus communications suffer and phantom U codes and strange symptoms can be the result.
Remember, a network failure between an immobilizer module and the ECM can cause a no start complaint.
Because these two terminating resistors are 120 ohms each and they are wired into parallel, the total resistance should be 60 ohms +/- five percent. Because you are using your ohm meter, make sure the vehicle and the bus are asleep meaning no current draw and no bus activity. Measuring this terminating resistance is simple; apply your ohm meter between pins 6 and 14 and look for 60 ohms. If you see 120 ohms, there is a module unplugged or another type of open circuit in the network. If you see some other resistance much higher than 120 ohms, you have a complete open on the bus.
f you are using a DLC break out box (they are invaluable), check to see if a DLC pin has backed out as this has happened before. If you see something between 60 and 120 ohms, look for corrosion in places such as at that undercar resistor location (if used) or even a connector in the network being dripped on courtesy HVAC condensation.
On the subject of bus terminators, a note specific to Chryslers with CAN bus networks involving the use of that Diagnostic CAN C network as the exclusive bus for scan tool communications must be pointed out. These vehicles use a single 60 ohm resistor in the gateway module (usually the TIPM) as opposed to two 120 ohms resistances in two separate locations wired in parallel. The resistance measured at the DLC is the same: 60 ohms. To get to the terminating resistors total resistance on the regular Chrysler CAN C bus, you must back probe both CAN C bus circuits at a module or in the harness somewhere as there are no pins for this bus actually in the DLC.
Other Tips and Tricks
I’ve had good luck with scoping various networks to look for flat lining modules and wiring shorts to ground that were intermittent. Tap test, heat and freeze modules and connections until you see the pattern drop out and stay dropped out. Keep in mind some networks won’t just sit there active with the key on making your scope toggle a square wave. Some vehicles require a scan tool to initiate conversation with another module on the network before activity begins on some buses.
Most J1850 and CAN bus vehicles will show active square waves on the bus when the ignition is on or something is activated which leads me to a couple of parting tech tips. One is regarding the bus that won’t go to sleep. If after several minutes the networks on a vehicle don’t go flat line on a scope after the ignition is switched off and RAP functions have timed out, look for some kind of BCM input (door jamb switch, keyless entry, etc.) that might be keeping the network awake, which as you might imagine can lead to battery drains.
If you have a DLC break out box plugged in, make sure you don’t have a scan tool plugged into the break out box as it can keep the bus active. On the flip side of the bus that stays too active with the key off, I also have found a quick way of diagnosing power locks as far as whether the key fob or the receiver is bad. If my scope pattern on the low speed CAN bus or J1850 bus (body electronics) goes active when I press a button for lock or unlock, the key fob is good and the problem is somewhere upstream from the remote lock receiver.
Keep on increasing your knowledge about communication networks and how to apply your scan tool, meter and scope to troubleshoot those instances where a module is having a Failure to Communicate, and you’ll remove a lot of frustration from your diagnostics on the ever increasing numbers of F.R.E.D.s rolling into your service bay.
Subscribe to Motor Age and receive articles like this every month…absolutely free. Click here