Would've thought fuel filter would affect all injectors rather than just one bank but won't rule it out. Though after saying that it's a commonly overlooked component that can cause lean mixtures, yet changing the fuel filter in my Gen2 Lib was super easy and cheap for a good name brand via eBay.
I agree with Duncanm; if can pull the spark plugs in the bank and check the colour of the electrode or even if there's an electrode left on each plug. Plenty of sites will tell you how to read the colour of a spark plug electrode whether it's lean, rich, or spot on.
Given you don't mention if it's miss-firing on one cylinder therefore it's a bit suspicious. Generally a misfire is easily found when idling the engine by pulling each spark plug lead off one at a time (then reconnecting before moving onto the next lead) to find which cylinder isn't firing properly. Of course you're dealing with very high voltages in the spark plug leads, that'll jump rather large gaps and give you a nasty zap, so lots of clean rags / insulation of some kind between the lead and your hand if trying this method. And watch out if & when the cooling fans kick in.
I presume the mechanics did the old reset the ECU and then run it after reset to see if the CEL error comes back?
