Where is the ECM located and can a layman get to it?

There are a number of ECM's in the Prius for various functions.
The previous ECM that was replaced along with the battery was almost certainly the battery ECM that many dealers insist on replacing with the battery for no good reason other than profit.
The sensor after the cat would be my first port of call followed by cat replacement if required. You do not state how many miles the car has covered, and another useful piece of information is oil usage along with your typical daily drive cycle (town, or freeway, or mixture, and daily millage).

John (Britprius)

 

Just checked the workshop manual, and in the fault diagnosis for p0420 it lists.
(1)Leak in exhaust.
(2)Open or short in heated oxygen sensor.
(3)Heated oxygen sensor.
(4)Catalytic converter.
In that order. No mention of ECM problems for that DTC code.

John (Britprius)

 

I can see where your coming from, but this is 13 years down the line. I would think the problem would have shown earlier. How many miles on the car and what is the oil usage as these are major factors in the problem.

John (Britprius)

 

The generic term is ECU (electronic control unit) for all the various ECUs in the Prius ... the term ECM (engine control module) is the old and specific term for the one ECU that controls the engine (and could also be called the engine ECU). It lives right behind the glove compartment, nestled close beside the steering ECU, and they are both easy to access just by removing the glove box.

BTDT hint: The way the manual shows to remove the glove box is to take out the two stop pins that you see in its side walls after you open it. With the stop pins out, it swings all the way out upside down and dumps your stuff on the floor.

Just leave the stop pins in, pop out the hinge pins at the bottom, and lower it so it's hanging by the stops. You can see the ECUs, and your stuff stays in.

-Chap

 

Pretty sure that's the ECM on the left. The giveaway is the fat wires that go to the steering ECU. Both ECUs have a bunch of skinny wires but the steering one also has about four pretty fat ones, so it can send some real grunt to the steering rack. The engine ECU is the other one.

If you want to be by the book, visit the trunk first and unplug the two wires (one fat, one skinny) plugged into the red terminal assembly on the 12 volt battery. Each one just has a tab you squeeze, then pull out the plug.

Once you've done that, unplugging an ECU isn't any harder. There are more wires, but they are grouped into only about four big plugs, and you just squeeze the tab and unplug like anything else.

Your radio and clock settings will be gone when you're done.

-Chap

 

Hi Roy from Canada = I have removed my engine CPU that sits behind the glovebox. Just press on the connectors and pull them off. Remove screws and pull out module (there are actually 2 modules one for the engine and one for the power steering). ( Takes about 10 minutes to remove)
I do not think that is the problem, In my case I replaced both O2 sensors and the code went away. The O2 sensor after the catalitic converter is easy to remove, the O2 that is up on the engine manifold is hard to get at . You have to remove the wipers and a cover to get good access , I had to heat with torch to get it out and is a much longer job.

 

Not so fast ... you're looking at the last 5 digits (47030) but not the first 5. The TSB is talking about the engine ECU, also known as ECM, number starts with 89661-. Your receipt is talking about the battery ECU, 89890. You'll need your high voltage gloves and electrolyte PPE to dig that one out. And nobody would expect your battery ECU to give you an engine, catalytic converter code.

-Chap

 

Not really. If you monitor the values of bought o2 sensor you may be able to tell if one of them is faulty or if there is leak in exhaust or if catalytic converter isn’t working. But even then it can be hard to diagnose. How much oil does your car consume?

 

I have asked that question along with millage on two occasions without results. I hope you get a better response. The reply to our questions is often pivotal to the diagnosis of the problem.

John (Britprius)

 

- Roy from Canada trying to help - we know you are not a person who regularly tries to do repairs yourself and are hoping we will guide you to the most likely cause and effective correction to code 420.

Code 420 is usually triggered by the O2 sensors - one looks at the exhaust before it is cleaned by the catalytic converter the other looks at the cleaner exhaust. The two electical signals from the O2 sensors are compared to see how good a job the converter did - if it gets outside of the specifications it will trigger 420. However that does not mean the O2 sensors are defective. Many things are happening in the exhaust gases that could make the sensors think there is a problem. On this site many have given possible causes for the 420 code - in your case it is coming and going which suggests it may not be the O2 sensors.

Some suggestions I have read about that will cause or cure p 420

Overfilling with oil after an oilchange. (cause)
Clean the air intake chamber (cure)
If car uses a lot of oil between oil changes it may cause this problem(cause)
Leak in exhaust system (cause)
Faulty catalytic converter(cause)

What the above indicates is the exhaust gases are changing and the sensors are picking it up (if they are not defective). some people just keep driving and resetting the code.

 

Even the type of driving you do can have a bearing on this code. If you drive mostly round town "lower speeds" The catalytic converter if marginal in condition will not operate as well as it would if you do high speed runs. With these the cat runs hotter and is cleaned off of soot and oil deposits. Making it more efficient.
This is part of the reason for asking how much oil does the car use as oil use is known to damage the cat. Looking at my previous post it does look a little abrupt, and if that is the way it came across I apologize, but we do need answers to the questions or we are chasing shadows.

John (Britprius)

 

Check Engine Light is the OR of a long list of problems. If the problem clears, the third start clears the Check Engine Light but the control computers remember it. Just you have to use a Prius-aware, OBD scanner to learn what it was.

Please read the stickies and the good folks here will explain what they mean.

Bob Wilson

 

Whenever I see a P0420 code in my shop it's almost always a high mileage car (over 150k) and one that burns oil. The oil gets into combustion chambers then hits the CAT on the way out and over time destroys the CAT. My vote is your CAT is toast, but you could get a newer ECM and temporarily install it (just hang it after removing glovebox by moving ECM connectors but leave original installed), drive a few hundred miles and see if anything changes.

 

I just did this job myself, and now i've got a christmas tree dash and ECM plus Transmission Control Codes, Gas engine won't start. Gas Gauge is flashing. any ideas on what could be the issue? i'm chasing my tail like a madman.

 

You're just toying with us to say you have codes, and not say what they are...

-Chap