On Board Diagnostics

Welcome, John. Toyota's codes can only be read by their very expensive device. The ScanGauge II works for me. I bought an extra cable to use on other cars.

 

Have you been properly tightening the gas cap?

 

:welcome: Welcome to PriusChat, John!
The CEL will usually clear after a few on-off cycles, but a code is set and retained in memory for a while longer. You can use a ScanGuage II (available through the PriusChat shop at a good deal) to read codes and many other parameters and information about your Prius. As Jim said, Toyota private codes may not show up. Best bet is to take it in to the dealer. Of course, you've checked all the fluid levels and been changing the oil every 5K miles, right? And a loose gas cap can set the CEL, as firepa suggested.

 

No. Many go beyond the 5000 mile recommendation. The service light will start staying on for some time at 4500 miles then stay on at 5000 miles past the last reset.

 

Let's make sure this is clear

The "MAINT REQ" light comes on every 5000 miles and stays on till reset. (500 miles before the 5000 mile point, it will come on for a short period after starting the car.)

The "CHECK ENG" light comes on when a whole series of various triggers occur. It is required to come on when any out of spec emissions condition occurs, as well as a variety of Toyota specified conditions.

Please note that a number of auto parts stores will read the codes for free and most Toyota dealers will charge you quite a bit for "performing diagnostics".

 

My understanding is that regular OBD scanners can read only the engine ECU (sometimes called ECM) codes. That's all that SAE have standardised. You have to know the other addresses to read from to get codes from other ECUs, and Toyota haven't published this information, therefore only their scanner can read the other ECUs.

In addition, some codes have a sub-code or information code (Toyota calls these INF codes) which add extra detail, in some cases leading to a very different diagnostic tree.

Finally, only codes where the numeric part starts with a 0, from 0000 to 0999, are standardized; others have different meanings between different manufacturers or potentially different models.

Code P1121 on a Prius is Coolant Flow Control Valve Position Sensor Stuck, and this seems to be an Achilles heel of the Thermos system. Basically, the valve has three positions: sending coolant to the thermos from the engine, or vice versa (position A), sending from the engine to both thermos and cabin heating system (B) or from engine to cabin heater (C). There's a sensor to monitor which position it's in: the code is flagged if the car tries to move the valve but the sensor says it's in a different position, or if the coolant temperature doesn't change.

The code can also indicate that there is clogging, preventing the hot coolant from reaching the engine.

The diagnosis tree says any other codes should be resolved before this one, which includes codes from other ECUs. It then says to use the tester to set the three positions, and check the resulting voltages; if wrong, replace the coolant valve assembly. On another thread the dealer was reportedly hooking the new valve up incorrectly, so if you get the code repeatedly, get them to check this. If that's OK or doesn't help, check for clogging. If none or fixing the clogging doesn't help, replace the ECM.

 

Mike already covered P1121, no need for me to expound on it

Regarding the general condition of the OP with a 2005 Prius, what shape is the 12 vdc battery in? Once the little battery poops out, you can have all sort of odd problems

 

Are you still on your original 12v battery?

 

Hi Everyone,

It's been a long week for me and my 2007 Prius. Brought it to the dealer after receiving my first ever CEL. I have 222,000 miles on the vehicle. They tell me that it will be $58.00 to diagnose issue. They call me an hour later and tell me that the error was related to the ECU and that they would need to "pin point" troubleshoot more and that it would cost an additional $164 to continue diagnosing. $222 later in diagnostics, they tell me that the error is P1121 - Coolant Flow Control Valve Position Sensor Stuck. The cost to fix - $533.00. The a/c was also blowing warm when the CEL came on. They checked that and told me that the compressor was "breaking up." Cost to replace $1,600. Then they call me back to tell me that my 12V battery died. Cost to replace $300. They asked me if I had problems with starting the vehicle. I said never. Never had any issues with the car in 222,000 miles. Well, told them to replace the battery (no choice) and the car is 5.5 years old in Florida heat so it was ready to die. Very suspicous that it died while they had the vehicle, but what can I say. Told them to not repair the coolant flow valve or the a/c until I could investigate further on my own. CEL is now off. The a/c is blowing cool again. Still had to pay the $222 diagnostics since I didn't have them repair the coolant valve. So $570 for a battery and diagnostics.

Two questions:

Is a $222 diagnostic charge reasonable? I mean could Auto Zone have scanned for free and received the same error code of P1121 in a matter of minutes or did the dealer really and truly have to commit more time for a "pinpoint" scan, etc.? I feel like I screwed up bad ($222 error) by not going to AutoZone first!

Second, do you think the Coolant Flow Control Valve Sensor being stuck could effect the cooling (A/C) system? Since the light went off, the A/C blows cold. I should add that about three weeks ago, the CEL came on one day and went off (the very first time ever). The A/C was blowing less than cool at the time. They purged the system and added dye to check for leaks. They said that it needed freon (or the new substance - don't know the name). Cost $90.00. No leaks. Now, three weeks later, they say the compressor is "breaking up." All sounds crazy to me. What does "breaking up" mean?

Sorry for such a long post, but whoever has read this far, has the knowledge to add thoughts, and feels like doing so, could you please post a response? I am thinking of going to trade mine this weekend for about $5,000 and get a 2012. I am worried that this is the start of problems for me on a vehicle that has an excellent service history!

Thanks!

 

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Possibly, possibly, a DIY methodology would have kept you better attuned to the peculiarities and ordinary maintenance procedures required by the Prius? In less than 5 years 222,000 miles shows some intense use. Was $222 too much for diagnostics, Yes, but, so was $300 for the 12Volt battery! You could have easily replaced the battery for around $100, depending on which battery. Your first mistake was going to the dealer. But what can one do? Next time, at the very least search Priuschat and ask questions first. This is not in anyway a critisism of you. As for mechanical devices, coolant valve, pump, thermostat, tire rotation, oil changes sparkplugs etc.it is sometimes fun to investigate and do it yourself, definitely cheaper. Should you buy a 2012, definitely get the PIP.

 

You are right "breaking up" sounds like typical dealership circumlocution.

 

Great points. I have always taken it to the dealer for the past 5 years because I was nervous about having non-hybrid mechanics work on the vehicle. Having said that, I have kept up with all Toyota (not dealer) recommended service intervals. I have had great luck with the car. The very first repair being the A/C blower motor at about 200,000 miles. After that, the 12v battery this week with the diagnostics check for $570. So, I have only spent about $800 in repairs. Service has also been fairly budget as well. It's been a learning experience this past week. No CEL since last Thursday. A/C is running fine.

 

Great