05 Prius throws P1121 but my question is about radiator & coolant

Discussion in 'Gen 2 Prius Care, Maintenance and Troubleshooting' started by whodeytink, May 31, 2024.

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  1. whodeytink

    whodeytink Junior Member

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    I have a '05 Prius with about 85K miles. Just had the hybrid and 12v batteries replaced (Greenbean). All was fine for a few days then a P1121 check engine error light came on. I have had absolutely ZERO problems with this car for 19 years and wasn't surprised that the batteries needed to be replaced. I assume the new error code is just an unhappy coincidence. Checked with OBD reader, cleared. If I took short trips the light didn't come on (5-10 miles). This morning I drove for 45 miles and it never came on - it was cool this morning 50s/60s. I have been clearing it to see if I can see a pattern. Tonight it came on after about 10 miles - only in the low 70s. The car runs great - no issues other than the check engine light.
    Now the question. I checked the coolant reservoirs and they were a little low so I topped up with the proper red coolant. Also checked the radiator filler cap - it was filled to the top. The question is this: the coolant hose off the engine block is hot and the radiator is hot at the input site. The main part of the radiator feels cool and the output hose is not hot - barely warm. Also, after driving about 20 miles the coolant was barely warm when I checked the radiator filler cap. I'm NO mechanic so I don't know if this is normal but it doesn't seem right. It has been mild here - temps in the 70s and low 80s. I'm concerned about an overheating issue. I've read that the P1121 error does not mean anything very serious in itself but the coolant issue concerns me. Any thoughts? I've searched the posting here but didn't find anything that addressed this specific issue. I'm dreading a trip to the dealer for repairs if I have some sort of coolant flow issue like a radiator blockage. I've never had the coolant drained and replaced - it looks fine - just like what I used to top up the reservoirs. Any help would be truly appreciated!
     
    #1 whodeytink, May 31, 2024
  2. Tombukt2

    Tombukt2 Senior Member

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    20 mi in the morning when it's 70 and heading towards 80° out You haven't warmed up anything in a Prius You've just barely got it started basically give it another 15 miles or so and don't try to go so fast do this in town anyway those are just coolant control valve codes generally external heat source things coolant control valve The other one that matches this is the coolant heat storage tank none of those things are generally going to cause you any strife unless they're leaking coolant on the ground The three-way coolant control valve can click and make noise and all of that but generally generally doesn't cause an overheat condition It's common for the one hose to be warm and the other one to be cool things like that so you've had the car running for 35 minutes or so it's not really worth talking about cool hoses and what have you what's happening when it's 90° out and you've been running the car 30 minutes I doubt anything is overheating because something's cool and some things a little warmer overheating is well above 212° with things running It is pretty rare for a generation two to be overheating not impossible.
     
    #2 Tombukt2, Jun 1, 2024
  3. BiomedO1

    BiomedO1 Senior Member

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    Looks can be deceiving, you need to do a PH test and use one of those cheap floating balls tester to see if the coolant concentration is correct. High or low PH will allow the internal engine components to corrode; so when things stops working properly - it's telling you something.
    Have you checked the coolant switch valve assembly? A old, outdated OBD2 would point to throttle position sensor, but it's actually the coolant switch valve on a Prius.
    Hope this helps...
     
    #3 BiomedO1, Jun 1, 2024
    Last edited: Jun 1, 2024
  4. pasadena_commut

    pasadena_commut Senior Member

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    The coolant should be around pH 8.5. Usually it is coolant with low pH (<7) that causes corrosion in car motors. I have never heard of high pH being a problem. However, hypothetically if somebody put lye in the coolant and got it up to pH 12 or so it might start eating the plastic and rubber. Coolant left to its own devices in the car won't ever do that though.
     
    #4 pasadena_commut, Jun 1, 2024
  5. mr_guy_mann

    mr_guy_mann Senior Member

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    You should use your scan device to monitor Engine Coolant Temperature while driving. ECT should increase (while the engine is actually running) from ambient to about 80C. From there it will increase slower to a max of around 90C or so. The ECT (engine temperature) matters more than the radiator or hoses.

    95+% of the time, a P1121 is caused by a faulty "3-way" coolant control valve. I would replace the valve (I prefer a Toyota part as most reliable) and see what it does.

    Posted via the PriusChat mobile app.
     
    #5 mr_guy_mann, Jun 1, 2024
    Brian1954 likes this.
  6. whodeytink

    whodeytink Junior Member

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    Thanks very much for your reply. I would now have to agree with you regarding the engine/radiator temp. I used a remote temp sensor yesterday. After driving for about 35 miles the highest reading I got off the engine block was about 160 and as low as 140. The radiator input was about 160 and then slowly decreased as I scanned across the radiator and was about 90 degrees on the output end. So, it seems overheating is not an issue.

    Thanks for your reply. I now think the engine temp is not an issue based on scanning with a remote temp sensor. I am going to take to the dealer to be sure a genuine Toyota part is used to replace the control valve. I'm seventy two with bad arthritis in my hands so I can no longer do this type of work - I would have attempted it in the past.

    This is a very good point and something I just completely forgot about. I'll get one of those testers and check it out. Thanks for your reply!

    Yes, you are correct, I do need to do a PH test. I completely overlooked that. Thanks for the input - I appreciate it.
     
    #6 whodeytink, Jun 3, 2024
    Last edited by a moderator: Jun 3, 2024
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