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1997 Chevrolet C1500 failure to communicate and failure to pass emissions testing

I have a 1997 Chevrolet C1500 with a 305 that is failing emissions. Only one mechanic can get a reading off the computer, everybody else’s diagnostic tools come up with a failure to communicate code. The one mechanic that can get readings told me it was the O2 sensors, thermostat, it needed a tune up, and it may or may not need cats. I also discovered that the water temperature sensor valve was bad and replaced that. So far I have invested a little over $350 by installing new O2 sensors, thermostat, doing a tune up, and replacing the water temperature sensor. The fail sheet from emissions says the computer was not able to communicate at that time too. The check engine light is on sometimes and off others. How can I get the truck up to code so I can pass emissions and get my license plates renewed?

While the check engine light is on, have the exact codes read and post them here. That’s the fastest way to get to he bottom of it.

If the codes can’t be read b/c of the inability of your computer to chat with the shop’s diagnostic scanner, that might be b/c the software versions of the engine computer and scanner are not compatible, very possible given the vintage of your drive. You may need to take your truck to the local Chevy dealership service department. They’ll likely be able to read the codes for you and tell you what needs to be done so the emissions computer can communicate with your truck’s engine computer. While there ask them to check the customer interest, recalls, and tsbs for problems already documented about communications problem with the engine computer.

I have to disagree about the failure to communicate. This is standard OBD II for emissions testing, nothing esoteric about it.

For starters, make sure there is no crud in the port. Spraying some electrical contact cleaner into it might help. Also be sure all the contacts are securely in place and not pushed out the back of the connector. I believe that in some vehicles, a fuse is involved. Check 'em all.

The OBD-2 failure to communicate issue is a fairly common issue due to the lack of DC power on the OBD-2 connector. A number of manufacturers use Pin 16 for power and some use Pin 1. Pin 4 is for chassis ground. First check the fuses for a blown fuse. If they are good and you don’t have power then you need to check the wires to the connector for a problem.

This is my kids post under my name, he is the owner of the Silverado and is having troubles. Anyway, today we are planning to replace the cats on it with the splice-into-the-original pipe parts, because it is cheaper than the full set of pipes.

This brings up the question: Are the flange bolts in the manifolds threaded in or pressed in? If We go the full Y pipe route, I am curious if we go that route instead, how to replace broken manifold bolts.

Then we will tackle the ODB port problems.

Well, we got most of the exhaust on today, from the manifolds to the H pipe in back of the catalytics. Turns out the whole exhaust system was cobbled up with non-OEM conforming parts welded together!!

Next steps are to put on a new muffler and tail pipe then work on the ODB port and drive it a bit to see if the fault code clears and engine lite goes off.

Best of luck, thanks for giving us a progress report. You comments about the state of the prior exhaust system aren’t surprising. Exhaust shops tend to use the weld-it-'till it works method because welding is an inexpensive way to accomplish exhaust pipe work once you have the proper welding equipment on hand.

We have now got the “Check Engine” lite to stay off!! Now we need to figure out how to get the OBD port working. We checked with a voltmeter and there is no power on pin 16. We checked all of the fuses and they are good, including the fuses under the hood.

Any ideas? In the meantime, we will start checking schematics…once we can find them!

I have info for the 1998 C/K trucks that is probably the same as the 1997.

For '98, power to pin 16 of the DLC (Data Link Connector) comes from Fuse 7 in the Instrument Panel fuse block. This fuse is labeled “AUX PWR” and also feeds the Auxiliary Power Outlet (and/or Lighter) in the dash. Check to see if there is power at this outlet. If there is, then there may be a problem with a splice within the harness where the Fuse 7 output, DLC pin 16, and the power outlet wires connect together. This splice is supposedly located about 1.5" from where the power outlet wire exits the main harness. If there’s no power at the power outlet, there may be a problem with the Fuse 7 connections in the block or with the Fuse 7 output wire exiting the block. The Fuse 7 and power outlet positive wires are 16 gauge orange.

Bugmenot, that did the trick. We checked Aux Power Fuse 7 and it was indeed toast. We replaced it and found 12VDC on PIN 16. Once I get back home I will see if the truck passed.

Thank you!!!

It still may not pass the emissions.The code will still be in the history of the ECM.

Normally it takes x number of drive cycles to clear the memory. A drive cycle may consist of the engine reaching operating temp and traveling x number of miles and other factors.
I’d give it a few days before taking it in for testing.


Great diagnostic work there Part Throttle, and kudos for the informative assist from @Bugmenot . @Yosemite is correct the emissions problem may not be entirely solved, but you are a long ways down the road to the solution now. Best of luck OP.

Well, it passed today. Hoorah!!! My kid has been driving it for a few days and must have got enough cycles on it. Thanks again to everyone!