Mot Failure Emissions; Previous Had A P0420 Error Code; New O2 Sensor?

[Mr Rossi]

Hi all

My 2001 Celica Gen 7 (140bhp) has just failed it's MOT on the emissions readings; apparently it should be between 0.3 and 1 and mine was 1.05, 1.06 and 1.03 on several tests - he couldn't quite get it under.

The garage thought the most likely reason to be a faulty O2 sensor. I had a P0420 error code (Catalyst System Efficiency Below Threshold (Bank 1)) 6 months ago so that could indeed be the case, although I did find this on a search:

Overall probably the biggest mistake vehicles owners make when they have a P0420 code is to simply replace an oxygen sensor (H02S). It is important to do proper diagnosis so you're not wasting money replacing parts unnecessarily.

However, I'm short on time & want to spend as little as possible as I'm only getting it's MOT so I can put it up for sale...

I found these very good posts about O2 sensors:

http://www.newcelica.org/forums/showthread.php?t=238830

http://www.newcelica.org/forums/showthread.php?t=222448&highlight=bank1

http://www.toyotaownersclub.com/forums/index.php?showtopic=60799=http://toyotaownersclub.com/forums/index.php?showtopic=60799

I was thinking I might be able to replace myself or at least buy online cheaper 'universal' sensor(s) rather than what the garage would charge.

From this code though is it possible to tell whether it's likely to be the downstream or upsteam sensor, or should I replace both? I know there's a bit of risk/guess work here, but the garage aren't sure themselves, plus I've only got so long for the re-test, I'm working away from home a few days, then on holiday, so it's a bit of a rush...

My understanding is that the upstream sensor (in front of cat) looks like a spark plug sticking out of a hole, whereas the downstream (after cat) sensor 'lives inside a hole' - I take it they are still the same type of sensors though?

Cheers

Ross

[Mr Rossi]

Scratch that last bit about the hole - realised the pic I was looking at already had the sensor removed

[Mr Rossi]

Another quick clarification question - on this post:

http://www.newcelica.org/forums/showthread.php?t=238830

It mentions using a 22mm wrench.

But the parts here, which apparently fit my registration (comes up as Toyota Celica 1.8):

http://www.carparts4less.co.uk/cp4l/c/Toyota_Celica_1.8_2001/p/car-parts/car-body-parts-and-car-exhaust/exhausts/lambda-sensor/?710822805&1&10044fdb20fba9a4b68d815dab48d748da9efb15&000210

-are M18. That's 18mm.

Any ideas??

[edwardio]

Important information you should know about O2 sensors.

• Due to thermal aging and O2 sensor design and construction characteristics, their efficiency will deteriorate over time.
  
• O2 sensors should be check every 20,000 miles (Febi quote)
  
• O2 sensors should be replaced between 30,000 & 100,00 miles (Bosch quote)
  
• O2 sensors should be replaced if the CAT is replaced (Febi & Bosch quote)
  

So in other words, you should see your O2 sensors as serviceable items around the 60-80k service period, as well as if you switch to a Sports Cat or Decat. This will ensure your car is efficient and well placed for emissions testing during MOT’s.

I work for Blue Print, so you can get these (Shoulkd cost you around £85 max each from a Factors or Garage):

Front - ADT37065

Rear - ADT37081

When was the last time the MAF was cleaned?

What brand fuel do you normally use?

[Mr Rossi]

Thanks for the info... my car has done about 110,000 miles and I don't have much history, so do not know if these have ever been replaced. I do not know if the MAF has ever been cleaned... however, I did have a 'full' 100K mile service by Inchcape Toyota last year - would they have done that?

I normally use cheap crap Sainsbury's 95 unleaded fuel and occasionally Asda's too. The only time I used 97/99 was when I first had the car and it was a little sluggish and I was told a good high speed long run with decent fuel would help; it did indeed seem to.

If I put 'decent' fuel in now, do you think it might help me get below those emission's theresholds?

As regards the 18mm vs 22mm question: I guess the 18mm is fine as that's the THREAD size, the bolt probably would be 22mm in that case :)

[edwardio]

For the age of the car, you should have BOTH your O2 sensors changed. Unless your car has been dumping loads of unburnt fuel down the exhaust, the CAT should be fine.

I don't think MAF's are cleaned during services. If you had a full service at 100k & you're on 110K now, I'd service it as well.

This is what I'd do:

1. Get new O2 sensors & fit
   
2. Clean Maf sensor very gently with WD40 or carb cleaner & a cotton bud
   
3. Reset ECU - Just pull the fuse or disconnet the Battery for 30 seconds (more than long enough)
   
4. Run the car for a least 2 full tanks of V-Power
   

If you can't afford the sensors, at least do the other stuff.

Oh I'd just take the O2 sensors off with an adjustable spanner.

[Mr Rossi]

Great, thanks again Edwardio

It seems my local garage can get sensors at a very reasonable price (less than I can find online unless I get generic ones I have to wire myself from the states but then have to pay shipping and there's the delays etc) and aren't taking the p1ss on labour charges (said if they're not too hard to get off he'll only charge the half hour labour - most garages will change you the full hour regardless I find) so have it booked in for Monday :)

I'll clean the MAF sensor then (will do some forum browsing as no idea how to do that but I bet there's something here!), reset the ECU and will try and do the fuel BUT only filled up a full tank last night of Asda crap and not sure I'll use that up in time... may have to do some spirited weekend driving !!

[dimejar]

Same code on mine turned out to be catalytic converter worth looking into mine has 210,000+ miles and runs as good as the day I bought it with only 30,000 miles on it

[ThePermanentProject]

Had the P0420 code here too - turned out to be intake manifold gasket that dried up, fractured, and started leaking air - throwing off the mixture. (common fault - there is a toyota technical service bulletin for this). £11 gasket from dealership and an hour of elbow grease.

Regards O2 sensors, was told by an ex-MOT guy a cheap fix is to soak the tips of the sensors in white vinegar ( - so you can see how much rubbish comes off them i guess?) for 48 hours, then brush them off.

Astonishingly good results here. 6K miles since then - and no hint of a fault.

[edwardio]

Had the P0420 code here too - turned out to be intake manifold gasket that dried up, fractured, and started leaking air - throwing off the mixture. (common fault - there is a toyota technical service bulletin for this). £11 gasket from dealership and an hour of elbow grease.

Regards O2 sensors, was told by an ex-MOT guy a cheap fix is to soak the tips of the sensors in white vinegar ( - so you can see how much rubbish comes off them i guess?) for 48 hours, then brush them off.

Astonishingly good results here. 6K miles since then - and no hint of a fault.

All this will do is clean the sensor. It will not alter the fact that the sensor has degraded since day 1. O2 sensors accuracy degrades with age & not just much much muck that sticks to it.

[ThePermanentProject]

This is true edwardio - 02 sensors do degrade from day 1.

All this was intended to do is decifer if it is surface contaminants causing a poor reading, or if it is the actual (zirconium is it?) sensor that has terminally gone south.

Perhaps i should have said 'test' rather than 'fix'.

Good point though - well made.