DF is Volvo slang for Do First. Something is sending too much voltage on the high-speed CAN.
Do not replace any parts at this time. The car transmission is in limp mode because it cannot communicate with the CEM. The 1A codes are no communication with the CEM.
Did the CEM just go stupid, I would see what is causing the CEM-DF17 code and go from there.
Replacing the CEM may not help with anything. The dealer should not have released the car.
The dealer can contact Volvo support, open a help ticket for this if they cannot figure out what is going on. Not to cover for the dealership, there is a good chance that the tech has no idea about fault tracing and want to throw $$$$ parts at it, that may not help. Yes, the CEM could have failed at the wrong time.
The CAN signal too high needs to be addressed first.
On P2 cars I have seen soft drink spill from the cup holder and get into the SRS brain connector.
Dried soft drink sugar gunk conducts electricity and can short across the SRS pins, messing with CAN voltages. I am not saying to look here, but do not rule it out.
KOEO, you can check CAN voltage at OBD pins 6 and 14 for high speed, pin 3 and 11 for low speed
Wires 3, 6, 11, 14 will have 1.5 to 3.5 volts, if 5-12 volts, something is feeding voltage into the CAN system.
The following is info about CEM-DF17 CAN-H
CEM-DF17 CAN-H, high speed network. Faulty signal
Diagnostic trouble code (DTC) information CEM-DF17
Condition
The central electronic module (CEM) checks the voltage in CAN L and CAN H on the high speed network. The diagnostic trouble code (DTC) is stored if the central electronic module (CEM) registers a short-circuit between CAN L and CAN H in the high speed network.
Note! If DFxx diagnostic trouble codes (DTCs) are stored the central electronic module (CEM) must not be replaced as the diagnostic trouble code (DTC) indicates a short-circuit in one or more conductors in the Control area network. The central electronic module (CEM) is the only control module that monitors the voltage level in the Control area network wiring. The central electronic module (CEM) is therefore the only control module that can store a diagnostic trouble code (DTC) for a short-circuit in one of the cables to ground or supply voltage. Do not replace the central electronic module (CEM) in the event of a CAN fault unless VIDA fault-tracing has indicated that the central electronic module (CEM) has an internal fault.
The diagnostic trouble code (DTC) can be diagnosed when the ignition has been on for more than 10 seconds.
Substitute value
None.
Possible source
Short-circuit between CAN L and CAN H in the high speed network.
Fault symptom
The engine does not start.
The engine cuts out when driving.
Malfunction indicator lamp (MIL) lit.
Message in the display.
CEM-DF17 CAN-H, high speed network. Faulty signal
Signal missing
General information
Note! Figures shown are only examples.
Hint: For general information about CAN-troubleshooting, see:
INFORMATION/PRODUCTSPECIFICATIONS/Design and function/37/CAN.
General about CAN-related diagnostic trouble codes.
CEM-DF01 – DF17 can only be stored in Central electronic module (CEM). These diagnostic trouble codes are stored when an electric fault is detected in CAN-lines. Start with these diagnostic trouble codes first.
CEM-1AXX only stored in Central electronic module (CEM). One diagnostic trouble code for every control module on the CAN-net. These diagnostic trouble codes indicate that Central electronic module (CEM) does not receive any reply from other control modules.
XXX-E000 (HS-CAN)/E001 (LS-CAN) can be stored in all CAN-connected control modules. These diagnostic trouble codes indicate that the control module has detected interference on the CAN-net.
XXX-E003 can be stored in all CAN-connected control modules except in Central electronic module (CEM). The control modules on the CAN-net listen for a message (identity number) that is sent out at regular intervals from Central electronic module (CEM). This diagnostic trouble code indicates that the control module has not received any identity number, or that the expected number is not correct. Interference on the CAN-net can also store this diagnostic trouble code.
Note! In case of CAN-related faults, it is likely that a combination of above diagnostic trouble codes occur.
Note! After fault with control modules that interfere with communication, other control modules may remain in Limp-Home (that is, they work with reduced functionality) even when the ignition has been turned off and on a few times. For certain faults in the CAN-net where control modules have stopped communicating, it may help to disconnect/reconnect the battery to “restart” the control module/CAN-net.
If a communication diagnostic trouble code can be read from the control module, the malfunction is probably intermittent.
Intermittent fault in the voltage supply and ground to a control module on the CAN network results in the Central electronic module (CEM) storing a diagnostic trouble code (DTC) for an open-circuit in communication with the control module where the voltage supply does not work. There will probably not be a diagnostic trouble code (DTC) in the control module where the open-circuit in the voltage supply occurred as the control module cannot store a diagnostic trouble code (DTC) when it is not supplied with power.
Note! Central electronic module (CEM) should not be replaced in case of CAN-fault, unless troubleshooting reveals that Central electronic module (CEM) has an internal fault.
Caution! During this fault-tracing use the wiring diagram to provide support when taking readings.