1999 Yaris 1.0 - failed NCT emissions

shietpilot

Right lads, I was spamming the motors chat thread with my shoite so it's time to make a thread, because I haven't a clue of what's going on right now.

Here's the relevant details from the NCT sheet:


It has had the P0420 - Catalyst System Efficiency Below Threshold (Bank 1) code up for years but only failed yesterday.

Yesterday I plugged in the diagnostics and the P0420 code was not there but a P0136 - Oxygen O2 Sensor Circuit Low Voltage (Bank 1, Sensor 2) took its place. Today I plugged it in again and this code was not present any longer, check engine light was off.

Looking at live graphs of both O2 sensors, the post-cat sensor (2) is maxes out at about 0.45 volts but usually sits close to 0.15 volts. There are times when it's showing a complete flat 0.00 volts for half a minute or so, like it's not even plugged in.

The pre-cat sensor seems to hover at around 0.70-0.85 volts. I googled about and I read that the pre-cat sensor should be oscillating from high to low and back to high non-stop rapidly and that the post-cat sensor should be sitting at a stable 0.5v (roughly).

I took out the spark plugs and put it new ones now. The old spark plugs looked absolutely perfect, no oil deposits, no soot deposits, nice brown/grey looking plug like from a picture book.

I recorded the O2 sensor action after changing the plugs. The car is running in closed loop.

What to do now? Is the pre-cat sensor forked?

Interslice

Could be worth a look at the post cat sensor wiring and connector if its an intermittent fault like that?

shietpilot

Yeah I will lift the car up today and check it out. I'm considering just putting in 2 new Denso O2 sensors, maybe it will sort it out.

I just hope it's not the cat as all the spurious ones are rubbish and an OE cat is uneconomical.

nd

from watching this

high hc means the mixture is rich, because of either too much fuel or too little air. High co means unburnt fuel.

Is the air filter fairly new?

What software/interface is that btw?

shietpilot

nd wrote: »

from watching this

high hc means the mixture is rich, because of either too much fuel or too little air. High co means unburnt fuel.

Is the air filter fairly new?

What software/interface is that btw?

The strange thing is that HC is low on the high idle. Air filter is newish and I changed the spark plugs after it failed the test. Previous spark plugs looked great.

Wurth WoW! 5.00.8 with the Delphi DS150E scantool.

Waterson

The P0420 code usually flags because the catalytic converter is no longer functioning to acceptable parameters. It is common enough to have to change the CATS out from Toyotas of this vintage.
The next step would be to check the function of the oxygen sensors. Force the engine to run lean. Disconnect a vacuum line, e.g. a brake booster line to force a lean condition and observe O2 sensor Bank 1 Sensor 1. It should show a low voltage immediately, which may rise as the fuel trims are adjusted to compensate. This will verify sensor plausability. Short Term uel Trim figures will provide an idea of what is going on in the engine, i,e. you appear to have a rich running condition so I would imagine seeing a STFT or LTFT of -15-20%. This would have to be investigated prior to replacing CAT as a new one will fail very quickly if not addressed. Common causes of rich conditions would be excessive fuel pressure, faulty injector(s), blocked air filter/tract or faulty EVAP system.

shietpilot

STFT was sitting between -4% to -8% if I recall correctly. Post-cat sensor is definitely dead.

shietpilot

Problems have been fixed by changing pre and post cat O2 sensors. I changed the post cat sensor first and it improved emissions slightly but they would still have failed. Pre-cat sensor made emissions perfect again Thanks for the help everyone.

THEFRIDGE

shietpilot wrote: »

Problems have been fixed by changing pre and post cat O2 sensors. I changed the post cat sensor first and it improved emissions slightly but they would still have failed. Pre-cat sensor made emissions perfect again Thanks for the help everyone.

Was it an expensive fix? Do you have part numbers or links?
I'm having the same issue.

Thanks

shietpilot

THEFRIDGE wrote: »

Was it an expensive fix? Do you have part numbers or links?
I'm having the same issue.

Thanks

I ordered 2 of these: http://www.micksgarage.com/ProductDetails.aspx?pid=1712082&carid=10466

Took the old sensors out, cut the connector and crimped it onto the new sensors, put them into the car and it passed with flying colours.