There has to be a code. A big majority of detected problems are intermittent, and they would be impossible to diagnose without those codes to tell us where to start looking. There's only two ways to not have a code. One is if it got erased before it was read. That will happen if the battery is disconnected or run dead, or in some cases if the problem doesn't act up again for a certain number of engine starts, commonly 50.
The other way is if there's an intermittent connection on one of the power wires or power to the computer is lost intermittently. Most commonly power is lost due to arced and pitted contacts in the ignition switch. Computers for safety systems like anti-lock brakes and air bags have at least two power wires. One is to run the warning light when power is lost on the other one. The systems can't function when they're missing one power feed, so the light is turned on. There won't be a code set because the computer is not interpreting the missing feed as a defect. It's seeing it as the ignition switch was turned off, and that's a normal condition.
Also consider that some aftermarket scanners don't access all air bag computers, and those that do never do as much as the dealer's equipment. GM is one of the top manufacturers at designing in tricks that force you to go back to the dealer. Independent shops are able to buy the same scanners the GM dealers use, but many shops bought the Chrysler DRB3 because it was designed to work on any brand of car for engine and emissions systems. Because all scanners are very expensive, the shop owners don't want to buy other brands of equipment if it won't see a lot of use.
If your mechanic's scanner won't access your air bag computer when the warning light is on, that is proof there's a power wire problem because that computer is turned off. If it does communicate with the scanner, there will be a fault code or there will be something displayed to indicate why the computer isn't responding.
GM also has a real big problem with their generators. Due to their design, they develop huge voltage spikes that can damage the internal diodes and voltage regulator, and interfere with computer sensor signals. If one of the six diodes fails it reduces the generator's output capacity to exactly one third of its rated value. 30 amps from the common 90 amp generator is not enough to run the entire electrical system under all conditions. The battery has to make up the difference, and it can slowly run down to the point the voltage is too low for the air bag computer to reliably trip the bag. The clue here is the air bag light will usually turn of when you raise engine speed. You need a professional load tester to test the generator's maximum current output. If it is low, "ripple" voltage will also be very high. That's the other indication of a failed diode.
Saturday, February 15th, 2014 AT 8:49 PM