Dreaded PO420 Returned

[normanh]

The code has returned again after everything settled in nicely. I have the 420 code - cat below efficiency and Sensor heater issue. Question I changed the post cat sensor last time and everything settled nicely- proper Denso unit from sparkplugs.co.uk, will the heater failure element in the code cause the cat failure? The sensor was replaced several months back not sure if a failure can be expected so soon bearing in mind its a Denso?

Might have to bite the bullet and change the chat and sensor this time?

Norman

[Dev]

 Generally no but its possible if the heater circuit is blown and not able to get up to temp fast enough when the ECU is measuring it just might throw a 420. The 420 has a set of conditions before it determines if the cat is not doing its job.

 If you are blowing though heater circuits like the last one I would first confirm using a multimeter by testing for continuity on the two black wires.  If it is blown then I would clean the contacts on the female connector that is on the car and try again with a new sensor.  If it blows again you for sure have a problem with the wiring that needs to be traced and sorted.

[normanh]

Hi Dev

Just checked the wiring and I have 14 ohms between the 2 black wires which sounds right for a heater circuit, so maybe its the cat, not what I was hopping for but better than tracing a wiring fault on the heater circuit?

Norman

[Dev]

Hi Dev

Just checked the wiring and I have 14 ohms between the 2 black wires which sounds right for a heater circuit, so maybe its the cat, not what I was hopping for but better than tracing a wiring fault on the heater circuit?

Norman

 As long as you have continuity that means that the heater element is intact. However there is a possibility that there is a loose contact after the connector going into the cabin or a corroded plug. If I am not mistaken, I vaguely recall there is a fuse for the heater circuit from reading someones past post long ago. If the fuse is blown it might show up as the heater wire being disconnected. Better to do some research on this.



 

[Ardent]

As well as above. Double check for gas leaks as well.

If possible, are you able to check the pre and post cat sensor waveforms?

Another idea.
Drop into a test centre and let them measure the gases.

If passes. Suggests cat ok. reset and carry on.

[normanh]

Its been reset twice now and had a Catclean dosing in the last week, no leaks as far as I know.

Back earlier in the year when tested it passed the emissions at a local test centre.

Just googled the fuse issue - seems there are no fuses.

Will look into waveforms over the weekend now!

Norman

[Dev]

I did some basic research for you. It appears that the reading  should be between 11-16 ohms which it is. 
There was some discussion that is not clear about the fuse. It appears that it will be in the rear fuse box and it might be a 7.5 amp fuse. I would check for any fuses in that box as a process of elimination. It would be nice if you had the old 02 sensor and have it also tested to see if the heater circuit is intact.

If so one of the possibilities is resistance in one of the wires after the connector causing the CEL to trip as it goes outside the parameter that is set by the ECU. This could be due to a bad connection. When you use a new 02 sensor the problem goes away but over time resistance changes happen as the 02 sensor gets older and if the wiring was borderline it will be realized early.

Most of the time the heater circuit just burns out so this might not be the case at all however I have seen stranger things with these cars that is not so cut and dry.

[Ardent]

Back earlier in the year when tested it passed the emissions at a local test centre.

That is good and suggests the cat is not the issue.
Annoying and frustrating but does not sound too tragic.
Something electrical, which could be bugger to find.
Reset and enjoy.

[normanh]

Just monitored the trims and I have a very odd reaction on the longterm fuel % trim bank 2 its a flatline at around -11 to -12%, its the only one thats not moving with the engine running all others are fluctuating over the scale.

When I googled fuses it said none on sensors will check it tomorrow, I assume its the box behind the battery in the engine compartment?

I assume O2 sensor 1 Bank 2 is the post cat sensor, will check that again in the morning?

The fuse will have to wait now till the morning now.

Norman

[shnazzle]

Longterm ones don't fluctuate. They're basically baseline resets at a particular level.

I.e. If your short term trims are -10% at a certain load for a certain period of time, it will store -10% in the long term trims for that load.

[normanh]

I am learning here.

norman

[shnazzle]

I am learning here.

norman

We all are. All the time. Your whole case is a learning experience for a good few of us


In a nutshell;
Your MAF reads how much air has come in, based on that your ECU looks at a table to see how much fuel to inject. It then looks at long term fuel trims and applies that to the result from the table and sends the pulse to the injectors.
Combustion happens, and the o2 sensors read the resulting oxygen level. If too high one way or the other, it adjusts the fueling using the short term fuel trims.
If the short term fuel trims are consistent for a while, it saves it into the long-term. Rinse repeat.

[Dev]

Just monitored the trims and I have a very odd reaction on the longterm fuel % trim bank 2 its a flatline at around -11 to -12%, its the only one thats not moving with the engine running all others are fluctuating over the scale.

When I googled fuses it said none on sensors will check it tomorrow, I assume its the box behind the battery in the engine compartment?

I assume O2 sensor 1 Bank 2 is the post cat sensor, will check that again in the morning?

The fuse will have to wait now till the morning now.

Norman

I did some more research. It appears there is no fuse. I checked the fuse diagram and its not listed. I read it from a post and unfortunately it can be a source of misinformation.

 What I did find out is when you have both of these codes together the 420 is possibly a false code due to the heater element blown or the circuit  not working and therefore cannot bring the 02 sensor up to heat fast enough for the 02 sensor to register the correct voltages so it doesn't trip the cat below efficiency. 
   
  However you have an intact heater element in your 02 sensor so there might be problem on the other end. The best way to check is to find where the black wires terminate on ECU plug by using the ecu diagram. Disconnect both the battery and ECU wiring harness. Connect the two black wire terminals using a paper clip or small wire to bridge the connection. At the engine end of the plug use your multimeter and take an ohm reading. If it is open or has high resistance that could be the problem.

I would also clean all of the contacts with electrical cleaner and check to make sure your battery terminals are tight.

[Carolyn]

Just because the cat was good a few months ago, doesn't say it is now!  Just sayin'.  If it's the factory original, it's got to be close to its sell-by date.

It might be worth popping your MOT tester a tenner and checking it on the analyser. 

[Ardent]

True.
Just erring on the optimistic side.

[M.Dub]

I have the same code at the moment, I hoping it's because I fitted a sports cat?

I'm sure Iv read somewhere that the voltage on the cat 02 sensor should be constant at around .45v? If it's jumping around then it's most likely a dodgy o2.

[Ardent]

Not constant but a slower softer curve than the sensors pre cat.
Jumping around would def. not be right.

[Mook]

I have the 420 code

16 replies and not one joke... what's wrong with you people? 

[normanh]

Original Cat @ 125K miles, so I guess in all honesty its that. It was borderline earlier this year with a just pass not a clean pass as in previous years, had the car since 2005.

Norman

[Carolyn]

Original Cat @ 125K miles, so I guess in all honesty its that. It was borderline earlier this year with a just pass not a clean pass as in previous years, had the car since 2005.

Norman

There you go. It's funny how a sensor tells us something and we always want to blame the sensor!

[Joesson]

Original Cat @ 125K miles, so I guess in all honesty its that. It was borderline earlier this year with a just pass not a clean pass as in previous years, had the car since 2005.

Norman

There you go. It's funny how a sensor tells us something and we always want to blame the sensor!

An up to date take on " Don't shoot the messenger" ?

[Carolyn]

A Cats2U replacement will do fine.  You can get a fitting kit to go with it, but I would recommend adding genuine Toyota crush ring gaskets.  The aftermarket ones just don't cut it.

https://www.ebay.co.uk/itm/130353313042?epid=249139622&hash=item1e59a9b512:g:M8MAAMXQ1PNRZbGg

[Ardent]

Or reset  enjoy and see if it passes again at the next mot.
Nice and hot before you get there mind.

[normanh]

Anyone tried B&M catalysts they seem to be an alternative to Cats 4 U?

Norman

[Ardent]

Purely anecdotal. But B&M seem to be viewed as lower quality.