For 1990-2009 cars

Class II serial data is a 0 to 7 volt variable pulse width message scheme (logic 0's and logic 1's) used to communicate information between modules and for communication to an off-board Scan tool. Contained in each module memory is a list of serial data messages and default actions taken by the module if one of the important messages is not received. One of the most important messages is the state of health (SOH) message used to inform each module that other modules on the Class II link are functioning correctly.

Each module on the Class II link must first "learn" all modules connected to the Class II link every time the ignition is turned On. Each message that a module sends out on the link contains a source indentifier used to identify where the information came from. Once a module receives information, it will store the attached controller ID. If this same information is not received later in the ignition cycle, lost communications with a specific controller DTC will set. DTC U1016 sets when a controller does not receive specific information from the powertrain control module (PCM) after it had already learned its source ID in that ignition cycle.

### DTC Parameters

DTC U1016 will set if the BCM or EBTCM lost Class II communications with the PCM when:

• | Condition exists for longer than 3 seconds for BCM, 5 seconds for EBTCM

• | BCM or EBTCM already learned the PCM source identifier in the ignition cycle. The PCM had initially communicated with both the BCM and EBTCM when the ignition was first turned On.

• | Ignition is On

DTC U1016 diagnostic runs continuously with ignition On.

DTC U1016 is a type C DTC.

### Diagnostic Aids

To locate an intermittent problem, use the scan tool to monitor PCM located under DIAGNOSTIC CIRCUIT CHECK - - - CLASS II MESSAGE MONITOR with ignition On, engine Off. INACTIVE will be displayed if the PCM is not communicating on the Class II link.

If an intermittent loss of battery voltage to the PCM via circuit 1140 exists or an intermittent loss of ignition voltage to the PCM via circuit 439 exists, scan tool communication with the PCM will be lost and the engine will stall and not restart.

An independent open in any of the 551 circuits between the PCM and chassis ground will NOT cause a communication problem with the PCM since all 551 circuits are tied together in the PCM.

Check PCM ground tightness at the cylinder head.