07 Toyota Downstream Heater Circuit. No PCM Activation

Hi guys,

So I got stumped when I saw this and in fact I quickly hooked up the Modis to a 2010 Rav4 as well to check the downstream when it came in for a state inspection to try and verify this. The 2007 Toyota Corolla I was working on I had real trouble getting the downstream to function. I did replace the downstream O2 (Kinda shot gunned that part, didn't do much testing on it and I should have). Minutes later, 5-10 minutes into the engine running and O2 still pegged at 0.05v. Only after it gets hot (reving the engine) does it start to work obviously.

At stone cold (winter 8*F) I started the car and I was expecting to see a PWM signal or any circuit activation on the ground ctrl side of the heater circuit immediately after starting the vehicle. Well minutes later I still had no ground coming from the PCM and only after the vehicle was warmed up did the PCM start an odd timed, random PWM signal to the heater circuit, basically the amount of time it was turning on the circuit, it was basically off. I do have saved images from the modis, I apologize I'll have to put those up later.

While driving the car, I did noticed a 100% activation of the circuit (no pulsed signal) and once in awhile it would pulse off, but then switch back to deactivation and and once in a great while and random times, pules the circuit active. I believe I am remembering that right, the modis images will correct me if I am wrong.

I did verify at the PCM as well to make sure there were no breaks in the harness. I also tested the heater circuit on the upstream AFR and that works as soon as you start the engine. Though sometimes it does drop out (PWM stops for a quick moment - circuit off)

The 2010 Rav4, after starting and having it run for maybe 1 minutes before hooking up the scope, unless I didn't get a good connection, I saw no heater circuit activation on that either.

Questions:
Is Toyota doing something odd with these downstream O2 heaters? (Narrow Band Sensors)
Does Toyota use downstreams for fuel trim correction?

I want to add that this car was in originally for a p0171 code. No O2 sensor fault codes and no engine performance problems. Replacing intake gasket, cleaning all sealing surfaces out to the air box and cleaning the throttle body helped fuel trims to get lower, possibly acceptable to the PCM for now, I want the customer to bring it back so I can dive into it some more and get those trims to zero out

I do remember reading on ALLDATA that the downstream sensor should start reacting no more than 20 seconds after engine start. Which is usually the time frame I notice for all these narrow band sensors (5-10 usually but sometimes 20-30)

If the downstream O2 is putting out a normal 0.05 volt at idle and you can drive it rich by working the throttle both the Cat and the downstream O2 are working. I think what you are seeing is a normal condition, the readings are an artifact caused by the way you are hooked up to the wiring.
the downstream O2 is only fitted to monitor the cat is cleaning up the exhaust, on 99.99% of cars it has no function other than to flag up a P0420 or P421 code . When the cat is up to temperature and the downstream O2 is up to temperrature and working a flat 0.05v or close to it is normal as long as the PCM sees that it has no need to heat the downstream sensor but it will periodically check the heater circuit integrity.

I don't have graphics for a car with a wideband upstream sensor so ignore the blue trace of the upstream sensor on these jpegs

Thanks for the reply Andy.

I do understand the use of the down stream O2 and the fact the cat and o2 need to be hot to work, Issue I see is the 0.05 (50mv) for over 5-10 minutes. I've never seen a downstream O2 take so long to become active. Though every other car's downstream I have tested all these years have the heater circuit active as soon as the engine is started, while it seems toyota doesn't activate S2's heater circuit for a very long time regardless of the temp.

Now I do know that it takes a while for the cats to heat up to the point of proper function, but even after the engine is at normal operating temps and idle, the heater circuit is still off and the O2 is barely readying anything, 0.15 - 0.25. Only time it gives me normal voltage for a good cat is if I rev the engine (2500 rpm) to get everything hot while at normal operating temps. Over the years I have concluded anything over 650mv steady is a good cat. Under 650mv and you may need to be thinking about a cat in the future

I did read something about "intrusive heating" which toyota talks about and I am guessing this is a term for the heating of the O2 with the exhaust gases alone. Maybe Toyota only cares about Cat functionality when the downstream O2 is naturally hot from exhaust thinking that is when the CAT is functioning at its peak.

I'm obviously looking to hard into it but I guess I am just trying to understand Toyota's method for why they don't activate the heater circuit and will surely keep this in mind next time I see a downstream O2 pegged lean at 50mv on the scanner for an extended amount of time. What I do question even more, is on ALLDATA, toyota has a document, I'll upload it here later, stating the downstream O2 should start reacting within 20 seconds... though if that were true I would imagine it would through a code.

I also forgot to mention how I was getting my readings in my earlier post.
The O2 sensor signal I was getting from the scanner using a custom pid list to speed up the refresh rate. The heater circuit I had my modis hooked up to the ground ctrl side at the O2 connector and at times the ground ctrl side at the PCM. If I saw battery voltage on that wire, I knew the heater was off, if it dropped to 0v, I knew it was active.

0.05v is active a dead zirconia O2 reads 0.5v Toyota have one or two wide bands sensors upstream and a big close coupled Cat with everything good the downstream will take a lot to move off 0.05 to 0.13 volts. If ever in doubt do a reserve oxygen test using propaane to drive the fuel rich then lean.

Hi Andy,

Is this specific to Toyota? Forgive me for saying, maybe I am just being stubborn , but that goes against everything I have seen in the field. I can see it dropping to .1 or so when performing an oxygen storage test, driving it rich and then driving it lean after shutting propane off but with most other cars idling i've seen, S2 has always been above 0.6 volts with a good warm cat as its consuming the oxygen. If it was lower during normal idle on any other car I would suspect a bad cat, bad o2 or bad heater or harness problem as a lean O2 would lead me to believe possibly the cat is no longer absorbing Oxygen and performing its function.

I have never seen a downstream O2 sit at 0.05v for more than 30 seconds. Afterward it starts to oscillate and as the cat warms up it steadies out above 0.650v

A little more history on the car as memory comes back, After the downstream O2 heated up during a drive, at idle it stay constant around .650 - .750 volts, the entire time which is what I would expect. Sometimes .55 if the fuel trim got too lean.

My Toyota 05 CamryI4/ O7 Tacoma I4, after cat O2 sensor normally read above .5 when idling but on occasion will slowly go down to .2 and back up to .5-.8v

Wideband pre-cat sensors are a game changer, the input to the cat is so tightly controlled that the gases coming out the cat rarely go rich for long.

I don't have wide band on my own car it only meets Euro3 a 14 year old standard the upstream cat is 12 years old and the downstream aftermarket cat is 5 years old but even on that it takes hard acceleration to get the downstream to rise over 0.1v .

Here is some scanner data from the 07 corolla. Once B1S2 is hot, its working. I really thought I saved the data from the stone cold start up with the O2 pegged but apparently I didn't. This is all after I drove it.



Green, Throttle Position from Throttle Body
Yellow, MAF
Blue, B1S2 Heater Circuit Ground Control Side
Red, B1S1 AFR Heater Circuit, Ground Control Side
This image was while driving. The drop in the Yellow trace while at WOT is a gear shift

When blue is at 14v, the circuit is off. Interesting how the computer controls it. Really really wish I had a full time sweep of cold startup to show how long the heater is off for and how long the sensor is pegged at .05v for. Obviously if there is no code, than my logical conclusion is thats just the programming in the computer.