Need advice: P1151 and red triangle light after heat storage tank replacement

Hello and welcome to PriusChat!

For your warning light, is it just the red triangle, or do any other ones come on as well? This light can turn on if there's problems such as a door is open when the car is in gear. Or if engine oil pressure is low, coolant temperature is too high (there are other small icons that pop up on the central screen as well).

Or it can come on if there are faults with something in the hybrid drive system - but that usually turns on a number of additional amber warning lights as well.

First thing to do is to check the engine fluid levels. Oil and coolant. For coolant, when the engine is "cold", confirm the level is full in the radiator, not just in the plastic overflow tank.

Also look at the coolant in the inverter system in the tank on top of the transmission. (When the car is "ready", the coolant in there should swirl around from the electric pump)

I do not know how much detail you want. If you're not DIY then here's the basics. Most generic scantools only check the engine computer (ECM) for codes. Diagnostics require using a much more capable tool to check all 15-16 systems on your Gen 2 Prius.

I would like to think that a competent shop has such a scantool, but not always. I prefer to see a diagnostic report that lists each of the systems & whether it had any codes present or not.

For the coolant heat storage (CHS) system. It's a thermos tank that keeps coolant inside hot for a few days. Pump coolant in on shutdown, pump it out before startup. This speeds engine warmup & reduces cold start emissions.

There are 2 sensors to track coolant temperature - 1 for the engine and 1 for the CHS. P1151 sets because the ECM doesn't like what it sees from those 2 sensors when using the CHS.

Anyone diagnosing this needs a capable scantool to monitor (graph) those 2 sensors during a cold start. To see what the system does. Is the pump actually working. Are both sensors accurate and do they go up (and by how much) when the pump runs.

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the CHS rarely fails, if you can hear it running after turning off the car then it's working fine. Most of the time when it's low in coolant or wasn't bleed properly that it causes the red triangle (RTOD) due to overheating. I don't know how competent your mechanic is when diagnosing but blaming the tank (CHS) quickly doesn't seem ideal. I have the P1151 code on my 425k miles Prius and it was the relay that caused the issue. Driving the car is not safe when the RTOD is showing up without diagnosing the cause and it will lead to major problems if not addressed quickly (head gasket or engine failure)

 

Right anything in that circuit can be to blame relay temperature sensor the main temperature sensor the pump funny thing is though you always hear the pump running and when you get in the car in the morning and turn it on you can hear the pump pump in that warm cooling or whatever's in the CHS tank into wherever they want it to go into the system whatever I mean it sounds like the whole fender's about to rattle off the car and for a good second too I don't want to say a minute because it isn't that long and then it cuts off and everything is quiet and then after shutdown 5 hours it's supposed to sample the water for some reason and I guess if it doesn't like what it sees at that sample when you boot the car up next it'll tell you so but none of this should keep the car from running and driving just fine and all other systems working okay and your gas mileage should be unaffected this is strictly for the first few seconds your car starts up I'll say in the morning whenever your morning might be and that's that so I guess you could drive with this like this for essentially ever because my other half was driving the silver car like this for a year or so and then it actually stopped coming on the code for the 11:51 and nobody had changed anything cars never overheated never been low on coolant and either of the systems and all of that It used to set the 11:51 more in the winter than in the summer and now it's been over a year since it has said it at all and nothing has changed nothing the coolant heat storage tank is never been damaged on the car and never been hit squished nothing the pump runs fine

 

without you knowing the RTOD is a recipe for disaster. My experience with p1151 code is that the previous owner hacked the cooling system when the 3 way valve failed. The he disconnected (removed) the relay for the CHS tank. Once I have everything fixed (new 3 way valve) and restored the coolant connections the RTOD resurfaced due to coolant being low. Upon investigation why it's getting low is because of bad head gasket.