A communication failure can be caused by a number of different problems, the most common are wiring and a bad PCM - yes it is possible for the PCM to fail to communicate and yet work perfectly for everything else.
The least expensive and easiest thing you can do is check for communication with a Generic OBD code reader. If it communicates ok and you get "no codes" then take it back for the smog check and let them know - ask them to do a "code and monitor status check" prior to running the test.
CAUTION! do NOT clear codes, reset it, or disconnect the battery! if you do you will have to drive it long enough to run the OBD Monitors prior to the smog test.
If your tool does not communicate, look under the dash at the DLC and check for broken or disconnected wires, beyond that without the proper tools, information and training it may be beyond what you can do yourself and more expensive than having a professional do it.
I have seen a few times where the system would not communicate and I had to exit and initiate communications again or cycle the key off, then back on. but If you get no communication after several attempts then it's better to take it in.
Your car's engine computer (PCM/ECM) communicates with the Smog test Machine through the data link (DLC) connector using the Global OBDII protocol (computer language). The PCM also has a factory protocol.
Typically we (professional technicians) would try to communicate with the PCM on the factory protocol, then use a labscope to verify if there is communication at the DLC, if not next we go to the PCM and test the circuit directly at the pcm. If it works at the PCM but not at the DLC then it's a wire, but it is also possible for the PCM to fail to communicate and still work fine.
Robert Grove, ASE World Class Technician Automotive Hall of Fame 1996 www.wego2umobilemechanic.com www.orangecountymobileautorepair.com
Checked following OBDII pins: 1- Pin 4 to 16, battery supply 13.5volts 2- pin 5 to 16, battery supply 13.5 volts. 3- by removing battery -ve terminal, restance between pin4 and pin 5, 0.1 ohm. 4- chasis ground to pin 4, its has 0.1 ohms. 5- chasis ground to pin 5, its has 0.1 ohms. 6- Voltage at pin 5 (pin 4) to pin 6 is 2.5volts. 7- voltages at pin 5( pin 4)to pin 14 is 2.3 volts. 8- Resistance beteen pin 6 and 14 is 60.2 ohm (CAN bus end termination restance, 120+120/2= 60 ohm). 9- ECM connector cleand with electrical wire cleaner CRC, dried with blower, wait for half an hour, install battery +ve connector. 10- All fuse checked, especially, Cigrate Lighter. Finally, i think i have to perform Drive cycle of OBDII Reddiness, two times, inwhich, i ahve to drive my car continuously at speed of 95KM/Hr or 2000 rpm to 3000 rpm for 3 minutes, and keep idle 1 minute, OFF one minute, again start and drive for 3 minutes, same as before, without decreasing speed, breaks if possible to complete the two consective cycles. May be my OBDII can work again.