Would you worry? Long trip planned and....

I would be perfectly confident to make a coast-to-coast road trip.

There is always some minor roll-of-the-dice chance that any roadtrip could result in a breakdown. Such is life.

 

While a breakdown is always possible, I think the OP was concerned about the engine coolant code that got thrown (check engine).

The question is given the environment (extreme cold, car sitting for several weeks), code gets thrown, gets cleared, and doesn't return in an hour, whether the code was probably a glitch or is a probably legit and something to be worried about on his trip.

I don't know the answer to that, but I bet someone here does.

 

Can you please give a specific temperature for "bone-chilling" ?

 

How many miles on her and has that electronic coolant pump ever been changed along with the OEM coolant change interval. You should probably monitor coolant temperature during your trip; so you don't end up popping a head gasket....

Good Luck.....

 

Why is it that so many people think clearing codes is the most immediate action to take once CEL illuminates? The car is perfectly capable of clearing the code itself if you fix the issue. I would try to postpone the trip for a couple of days and take the car out for a long drive first to see if the code returns. During the drive, continuously monitor coolant temperature. Most obd scanning apps are capable of monitoring that parameter.

2000 miles with that code in the back of my mind? Hell naw!

 

What was the voltage of the [nominal] 12-volt battery after "sitting in the garage for several weeks"? If you're lucky, low boot-up voltage confusing the computers might explain code. Otherwise, having either coolant pump die during your trip wouldn't be much fun.

 

Your data points are kinda irrelevant, since your driving in freezing cold. It's a fairly economical engine that will self cool when moving below 70F. Just because you crossed the Allegheny Plateau in freezing weather doesn't mean your out of the woods. Did you monitor your ECT while you were doing that climb; because your temperature lamp doesn't light up until 248+F, nominal is 180F-210F. Your car might hit 210F on that climb, if it was 100+F with your AC going. I'm assuming it was cold when you did that, running your heater to remove more heat.
There's been many reports of the electronic coolant pump getting lazy and or intermittent past 120K miles, I'd certainly look at it before it does some major damage to the motor. Many just install a temperature gauge or monitor it through torque app. It's a silent killer and you've got your first warning.

It's your car, do whatever you want. Just don't be surprised if it blows up on you..... Just saying......

 

My concern about the engine water pump is that it is what makes the coolant move.

When the coolant moves, it can carry engine heat away from the engine, to the radiator and cabin heater.

Just as importantly, when the coolant moves, it evens out the temperatures between different places in the engine. And it makes the coolant temperature reading (the sensor is just on the head where the coolant comes out) an accurate picture of the temps inside the engine.

When the coolant doesn't move, it does none of those things, and your engine temperature reading is just telling you how hot a particular corner of the cylinder head happens to be.

 

Welcome to Florida, You didn't bring any more cold with you did you????

Cold can have strange effects on parts. It could have just been a fluke.
But it's always best to check thing to be safe.
The coolant pump might be on it's limits. How is the coolant level? Is the heat working?

 

Every code has its own detection conditions. Some are one-trip (light comes on right away if the detection conditions are met), some are two-trip (code is 'pending' when the conditions are met the first time, light comes on if the conditions are met again while 'pending').

P261B is a one-trip code that means the water pump RPM dropped below 900 while the pump should have been running.

So, that happened once, on February 1st, and turned your engine light on.

And it hasn't happened again yet.

Does that make it "a true issue" or not "a true issue"?

 

I would get a water pump at a dealer asap to make a possible emergency easier to resolve. Hopefully you have towing via your car insurance or AAA.

I would hope you brought a scanner with you and can monitor both engine temperatures fulltime.

An ideal monitor for this situation is a P10 obd2 with a user set early audible alarm since you are running it while sleeping.

 

Every time I see the title of this thread, I think of:

 

If you are monitoring your coolant temperature as suggested, stay the course.

The reality is many water pump problems don't code but bidirectional testing can usually diagnose.

In the same manner, it is very common for engines to be running hot, even dangerously so, and not code. Sometimes we hear the light flashed once and never came back but overheating was occurring.

 

It's true; what you don't know - you don't know. That's why they also say ignorance is bliss - If you don't see it coming; it's someone else's fault.
We've already told you to actively monitor your ECT - then you'll KNOW if it's a phantom error message or not. It's your car, so you can just keep playing the what I don't know game - Again ignorance is BLISS....

Peace out....

 

It may depend on what the condition is.

It seems to me that the right number of times for a Prius water pump RPM to have dropped below 900 when it should be operating, is zero.

This one has already done that more than zero times.

I could see maybe letting it go, if you had thought of a plausible explanation for why it did so on that one occasion, and that explanation also covers why it hasn't again, and convinces you that it won't, going forward.

I'm not sure what such an explanation would look like.

Have you checked the freezing point of your coolant mixture? Could the coolant in the bone-chilling cold have begun to gel, and slowed the pump down that way?