Limp mode without a check engine light can be caused by a couple of things, most commonly the SCV but it will also do it on an overboost. We were travelling to The Bend Motorsport Park in August and the turbo_outlet-to-Tillix_valve hose had rubbed enough on a hose clamp that it wore through and the valve wasn't closing properly to hold the boost down, resulting in the ECU limiting me to 2,000rpm. Find a rest area, cut a 5cm piece of silicone tubing and replace it in such a way that it isn't going to rub on the hose clamp and we were off and running again.
Sometimes the SCV responds to fuel injector cleaner. On that same trip, we noticed the engine starting to "hunt" at idle (never settling on a steady idle, but surging constantly). A bottle of injector cleaner at a nearby servo - and since we couldn't get the usual Liqui Moly Fuel System Treatment, we grabbed a bottle of Nulon Diesel Injector Cleaner - and within the next tank of fuel she was good again.