Air Bag Light Stays on Constantly

[crowe47]

Several months ago a mechanic had to replace the upper intake manifold on my 98 Ford Ranger XLT 4x4 4.0L OHV (97,000 miles and owned since new) that somehow had developed a dime size hole on the underside and messed up the idle badly. After he finished installing the new manifold that fixed the idle problem, for the first time, the air bag light lit up and has stayed on constantly since then when the engine is on - no normal flashing when you first turn the ignition key, light just turns on and stays on. Also, the passenger air bag light does not ever come on and had not been a problem before the aforementioned repairs.
The mechanic recently checked all of the fuses inside and under the hood and all checked out okay. He said there is nothing he did under the hood several months ago that would would have disturbed anything connected with the air bags. He thinks that there is some issue with the wiring to the air bags or the ECM itself, since the fuses are fine and when he hooked up his scanner, it would not communicate with the air bag module in the ECM. The scanner did have normal readouts from the other modules. Do you think my mechanic is on the right path or does anyone have any other ideas about what may be causing my air bag issues? If I have to replace the ECM, how costly is this repair? Is this a relatively easy DIY project?