Spouse and I bought our second Prius, a 2005 package #5, six months ago. It has only 39K miles on it (common for Oahu) and is usually only driven 10-20 miles at a time. It'll probably only log 5000 miles the rest of this year. It's out of the 3/36 and 5/60 warranties but still within the 8/80 emissions warranty. Looks like a 2005 with P1121 has been mentioned in another thread: http://priuschat.com/forums/gen-ii-...oubleshooting/47116-2005-prius-dtc-p1121.html Any new info since then? Two weeks ago, the "check engine" light came on and stayed on for about an hour. Spouse finished her other two stop-and-go errands and headed home, and the light cleared on the way home (about five miles). The car was probably barely up to operating temperatures, had not been driven faster than 60 MPG, and not for very long. There were no other symptoms other than the light. Whenever an alarm lights up on her she's subjected to extensive interrogations about driving conditions and other visual/aural/aroma/tactile symptoms. No other problems, no other clues. I can't remember when I refilled the coolant reservoirs on our two Priuses, but it was within the last few months. I think I noticed an empty reservoir in our other Prius and topped off this one too, but it had coolant. So I'm pretty sure it wasn't low on coolant at the time. The MFD "energy" display claims that so far on this tank she's getting 50 MPG, which is about her usual. (She pays no attention to fuel economy.) I gassed up the car a week later (she actually got the one-bar beeping on the gas tank level) but I was only able to put eight gallons in the 12-gallon tank, so that's probably not a good check of the MPG. Yesterday she was finishing another round of stop-and-go errands and began the two-mile drive home when the "check engine" light came back on. It stayed on all the way home. Now, the next morning, it's still locked in. It's not blinking, just on steady. My OBDII reader (CarMD) pulled primary & secondary codes "P1121". Possible causes could be a stuck coolant flow control valve, faulty valve wiring/harness, a clogged cooling system, or a faulty ECM. This is a four-year-old reader on a seven-year-old car, so it's possible that P1121 has other definitions & causes. However the CarMD website thinks the coolant flow control valve should be replaced. The radiator and the overflow reservoir were still both full. CarMD does a few other "Vehicle health monitor" checks like the catalytic converter and the O2 sensor. Those all completed satisfactorily. CarMD also popped up a bunch of other data like "Engine coolant temp 183.2 deg F". I can't tell what data is relevant so I won't regurgitate all in here, but let me know if there's some other info I should look for. The previous owner used to scamper to the dealer at the first whiff of trouble, so the car is probably good on most of the VIN's 45 TSBs and four safety recalls. However the car is eligible for the "hybrid electric water pump" replacement (for the inverter cooler loop), so this seems like a good opportunity to get that done. It seems that this code starts out as more of an annoyance than an immediate problem and then gets worse with time, but I have no reason to tempt fate. I'm going to make a dealer appointment for later this week or early next week. I'm probably going to drive the car the ten miles to the shop (downhill all the way) without any issues. Any other problems or checks or maintenance that I should consider?