2009 Prius Check Engine Light On

I own a 2009 Prius (4 years and 4 months old) with about 33K miles on it. Last Sunday while driving the Check Engine Light (CEL) suddenly came on. I made it home and started digging for possible causes - everything from a loose gas cap to a failure in the water pump for the inverter to a 12V battery nearing the end of its life. I’ve elimintated the water pump for the inverter as the source of the problem since, when I put the Prius into “Ready” mode, I can see movement in the fluid reservoir (when the gas engine is not on) indicating that the pump is working (it had also been replaced under a recall on that water pump a couple of years ago). I’m down to a loose gas cap (which I checked and tightened) or the 12V battery as possible problems. I’ve tightened the gas cap but the CEL won’t go off and I’m about to replace the 12V battery. Will that clear the CEL? I’m going to take it to a mechanic on Friday morning (my first opportunity - until then it’s sitting in the garage) to get a code readout. Could this be something more serious? I mean the car is under 5 years old and has only 33K miles on it! Does anyone have thoughts as to what else could be causing the CEL to light up?

No way to tell without knowing what code turned the CEL on. Many car parts stores will read it for free, call and check. If it’s on, but not flashing, and the car drives normally you can drive it to have the code(s) read. Let us know what they are.

Why would you suspect an inverter water pump failure or a weak 12V battery? For that matter, why would you suspect a loose gas cap? Until you scan the system and come up with a or some specific fault codes you have no idea what system isn’t functioning properly. Whatever you do, don’t disconnect the battery. It may cause you to lose data from memory that your mechanic needs to address your problem.

i have the same problem but my Prius is 2004 and it has 116,000 miles. My 12 V battery is the original one but the car starts just fine. i replaced the gas cap twice, one with a generic one and one with a Toyota one. i paid the local mechanic $98 to tell me the code is 0446. She doesn’t want to deal with it and the dealer said it was the loose gas cap. i too replaced my water pump recently. Nothing works but since the light came on around the end of May, it went off by itself twice. But it is on forever now and the car runs fine. What should i do?

“My 12 V battery is the original one but the car starts just fine.”

The 12V battery does not start the engine, the large battery does. A 10-year old battery may be about done.

The P0446 is the evaporative control system, more info here:

Which 10 year old battery you are talking about? The $10,000 big battery? The car runs great with the check engine light on. Why does my local mechanic doesn’t want to deal with the evaporative control system? The dealer said loose gas cap.

No, I’m referring to the 12V battery.

Evap systems are a pain, lots of places for leaks, cracks, etc. A loose/worn gas cap is the first thing to check, if tightening it doesn’t help, then I’d buy a new one from the dealer.

Do you ‘top off’ the gas when you fill up? That can cause problems with the evap system.

If you get 3 or more years life on a 12 volt battery, you are very lucky.

I once had a car where a few carbon particles in the evap control systems “air” filter got loose and moved around the system and caused an error signal.

Finally got a chance to take it to get the code readout. The code is P1121 Fuel Air (coolant flow air valve) - replace coolant control valve. When they readout the code they also cleared the error and it hasn’t shown up again yet (admittedly I’ve only driven it about 4 miles since I had the code read out so I don’t know if it will show up again so soon after being cleared). The mechanic said I could drive it to the dealer and see if it would be covered by warranty - the car only has 33K miles on it although it is over 4 years old. I know the drivetrain warranty is 5 years/60K miles but is this part of the drivetrain? Thoughts?

I never top off my gas tank. I always use 87 octane and as soon as the nozzle stops, I stop pumping. But for the last 9 years I didn’t tighten my gas cap due to my carpal tunnel. When the CEL started showing up this May, I do the 2 clicks when closing the gas cap and replaced the gas cap twice. I must admit that I never hear the hiss sound when I open the gas cap what normal people hear.

Why do people suggest that I replace my 12V battery if it is good? I never replaced it. You think replacing it will get rid of the CEL?

That CEL (check engine light) is just a kid in class waving her hand trying to get you attention because she has the answer. You need to have the codes read. Some places will read them for FREE. Try Autozone or Advanced Auto Parts. Get the exact code (like P0123) not just their translation into English and post it back here.

The CEL is there to be useful to the car owner, not to cause grief. At least that is the intention. It is there to save on shop diagnostic time, and provide a warning to the owner that something is amiss. Car owners would be wise to at the minimum ask their shop to determine what is causing the CEL to come on. It may be possible, once the cause is determined, to ignore the problem for a while; but never finding out the root cause could be setting the owner up for very expensive repairs later.

The other problem with not getting it fixed is that should another problem crop up later, the CEL is already on, so the owner wouldn’t know about the new problem. And the new problem may be an important one to fix.

If the owner goes to a place that provides free DTC (code) reading, as mentioned above, best to post the actual codes here for an interpretation. The codes are cryptic, and often seem to imply that some part is needed to be replaced, when the problem isn’t actually caused by that part at all.

@hm1731 … I don’t think the experts here would recommend to replace your 12 volt battery if it is known to be good. Batteries do fail though. Fortunately, the state of the battery (the 12 v one anyway) is easily determined by a simple test. There’s no need for any uncertainty in the state of a car battery.


The code which was cleared may not come back. Your Prius is covered by a factory warranty. If the code does come back take your Prius to a dealer for service. These codes are mostly warnings of things which need to be looked at. They are usually not critical and have more to do with the car’s emission system. Again, not an emergency but you should do the right thing and have it looked at.

Will there be signs before the v12 battery goes bad? For the last 9 years i always filled my gas tank before it went more than 1/2 empty. Does that cause the CEL to come on?

As I understand it a weak 12v battery can cause a number of odd issues. You might check out a Prius forum like Priuschat, I bet there’s lots of posts on that. But a simple battery test should a tell you how good it is.

And no, you’re not causing a CEL by filling up that way.

Well, the CEL did light up again this afternoon. I already have an appointment at the dealer for Tuesday morning (tomorrow is Labor Day so can’t do anything until Tuesday). Until then I’ll just rely on the minivan for any transportation needs. The code that I was told was it - I wasn’t told of any other code. I could take it to AutoZone or Advanced Auto Parts to get another reading but at this point I am already planning on taking it to the dealer. I have to think this is covered under warranty (while I’m outside of the bumper-to-bumper warranty I am still within the 5 year/60K miles drivetrain warranty and I suspect this falls under that - although the dealer may try to claim that it doesn’t).

I took the car to the dealer last Tuesday and it was exactly what it was - a bad control flow valve. It was fixed to the tune of $250 (the dealer split the cost of the parts and labor with me on a 50/50 basis). Now, just a few days later the CEL is back on. To say that I’m infuriated is an understatement - haven’t had a chance to have the code read yet (doing that tomorrow) and I’ve already got an appointment with the dealer. My feeling is that this better not cost me a red penny to get it fixed (especially if it’s even remotely related to the control flow valve - e.g. if it’s the control flow valve sensor that should have been taken care of when they replaced the valve itself).

The inverter has a coolant loop? I assume it generates a lot of heat. What fluid is used to cool the inverter? The fluid has a storage tank. How is heat removed from the fluid?

The CEL cleared itself this morning - before I could take it to the auto parts store to get it read. I assume that this code would be stored in the OBD - can I just use a reader to get it just so that I can know what it was? At this point I’m a little mystified.