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One more thing I forgot, and this should probably be the first thing to check, is blockage in the EGR tube connected to the intake manifold. If it gets grungy with caked oil vapours at the top then that would restrict flow and throw that 2-4-1 code. If the throttle body throat and plate is often found to be dirty then definitely suspect this.
If you disconnect the top of the tube you may not be able to move it back enough for inspection and cleaning without also loosening the EGR. Simplest thing is to remove the throttle body and try to use a flashlight and mirror followed by a stiff piece of wire to ream it out. Failing that, disconnect the EGR tube at the top so you can get into a bit better. The lower end of that EGR tube is often thoroughly frozen onto the EGR valve so you may have to loosen the EGR mounting on the block if you need to push it back a bit more. After three goes of penetrating oil and leaving it overnight, I still couldn't get that lower fitting loose -many have reported similar.
If you need to remove the EGR itself, the triple fitting on the main EGR pipe is also extremely difficult to loosen, but that one usually eventually lets go with enough penetrating oil, a bit of heat, letting it soak overnight, rinse and repeat, then use the biggest wrench you can get on it while squeezing in your hands against a counterhold wrench. Some combination wrenches are a bit too long to get in there and be able to turn them. I used a large Crescent wrench as the counterhold. Stupid thing is you have to remove the EGR valve to get at the oil separator box that's mounted behind it, which is why I was doing this. Many people simply resort to leaving the vertical tube connected to the EGR valve and if needed to remove it on the bench using heat.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now
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