2008 Freightliner Business Class fault codes

[NH2112]

One of the trucks at work is throwing this code:

MID 128 PID 106 FMI 3, Air inlet pressure sensor voltage high.

All we have for scan tools is a generic Mac scanner, and it doesn't give any info on the fault code. 10 pages in on Google I still haven't found anything that tells me what to check, and the Freightliner dealership says "We can send a tech out" (which I don't blame them for.)

So I guess my question is "what the hell is the problem, and how do I repair it?"

[DmaxMaverick]

What engine/series and power rating? What condition does the error set? Sensor/sender errors are usually not too difficult.

[NH2112]

It has the 4-cyl Mercedes in it, no idea of the power rating or anything else. As far as the condition it sets, it basically runs like crap, it doesn't seem to be running on fewer than 4 cylinders but it does seem to be very jerky. Sometimes it feels like the engine, sometimes it feels like the transmission is abruptly making a hard downshift. I figured it was either the boost sensor or whatever the equivalent of a MAP sensor is in these things and started looking for likely candidates to swap out with one from another truck. The bossman said to just call the Freightliner dealer and have them send a road mechanic, I'd like to have figured it out myself because it's in a deicer and I'd rather not have to rely on Freightliner if something else breaks during a huge snowstorm. Hopefully it's just a matter of a bad sensor returning too high a voltage to the ECU, and not the ECU sending high voltage to the sensor.

Computers simply don't belong in anything you want or need to be reliable, IMO. We spend SO MUCH MONEY on PQ cards for boom functions, joysticks, electronic heater controllers, etc, for our newer deicers, usually resulting in them being down for a couple days because who can afford to have $10K or so tied up in 3 or 4 spare parts? If one of our older trucks breaks it's one of the burners 90% of the time, and troubleshooting basically involves jumpering out the flow switch, temp switch, and flame detector, just like on your home furnace. I can be at the heating/plumbing supply store in 15 minutes and installing the new parts 20 minutes after that. It's SO much simpler! So what if the older trucks have open buckets, as opposed to the enclosed cabins of the newer ones?

/rant :D

[DmaxMaverick]

The code may indicate it's as simple as a bad sensor. Often it's that simple. Having other sensors available (like in other trucks) should make it simple to eliminate that. I'd swap them. The tech manuals, if you have them, should have troubleshooting charts for each sensor, as well.

[NH2112]

Tech manuals? From MY employer? Hahahahaha!

The call was already made to our local dealership, they'll be here Monday morning. I really hate having to let someone else fix our stuff, but I don't blame them for not giving me any help over the phone because they have to make money from their customers just like we do. I'm going to try to get the boss's OK to order service manuals for these chassis.