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It is an error code on your ecu. Find a garage with a diagnosis reader our buy one from halfords if they do one. It could be a whole range of things until you find the error code. And there's not much point paying out for dealership prices as far as spanner work is involved as your car is out of warranty
 

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Yeah it should be. Majority of the time there sensor failures or sensor reading out of set levels. Usually most cars it will say stop on the dash, or if the light begins to flash then stop straight away. But be warned, all the time your driving your risking the problem getting worse and possibly ending in a terminal failure. My old car had it's engine management light on because an injector and a heater plug wasn't...."in a state that may cause the engine to run with higher emissions" it still worked and ran fine, it just used a lot more fuel and cost an compleat fortune to get fixed. But had I left it, I could have destroyed the engine. Best bet is to keep your milage to a utter minimum until you've got it to a garage which you should do as a priority.

CHECK BREAK FLUID TOOEdited by: RichieD76
 

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Get a fault code reader, they're very cheap these days and you can use it on any modern car. One of the most common faults on the petrol 03 Riois the O2 sensor on the exhaust (actually there are two, one before and one after the cat). Before you look at changing the upper one though, check the wiring for it, it passes right next to the cooling fan and this can cut through the cable if it comes loose!
 

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<h2>OK, so what does a P0420 code mean anyway?</h2>


As mentioned above, the code's description is Catalyst System Efficiency
Below Threshold (Bank 1)
. OK, now tell me that again in plain English
please.... let's break it down: the catalyst system being referred to is your
three-way catalytic converter; so the catalytic converter is not working properly
(i.e. it's not efficient). Bank 1 refers to the side of the engine which contains
cylinder #1. On inline engines such as 4 & 5 cylinder engines, there is
only one bank. So, to really simply things let's just say that the
efficiency of the catalytic converter and/or exhaust pipe and/or O2 sensor
is not so great
.
<h2>There are no symptoms so why should I fix it?</h2>


Great question! A P0420 code will cause no symptoms aside from the MIL (malfunction
indicator light) will illuminate. The reality is there is still something
wrong and your car needs attention in order to be in excellent running condition.
A poorly maintained car will run less efficiently, burn more gas, and cost
you more money in the long run. Plus, if your check engine light
is on for a P0420 code and you decide not to fix it, another more serious
code could be triggered and you'd never know. The MIL lights up whether you
have one code or ten! While I have your attention, don't forget to do oil
changes as the owner's manual schedules and keep those tires inflated to the
proper PSI.<h2>So what is causing my P0420 code?</h2>


There are a number of things that could be causing this p0420 catalyst efficiency
code. The most common thing is the catalytic converter itself is no longer
functioning properly. The other likely thing is the rear O2 (oxygen) sensor
is no longer working properly. Other things could include exhaust leaks, damaged
exhaust pipes, damaged O2 sensor wiring/connectors, plugged catalytic converter,
etc.
<h2>OK, so I want to diagnose things, what do I do next?</h2>


The easiest thing to do first is a visual check of things. Visually inspect
the exhaust system for leaks, check the catalytic converter for dents, holes,
severe discoloration, and check for a rattle inside. If any of those syptoms
are there, the converter likely needs replacement. Then, visually inspect
the downstream O2 sensor (behind the converter). Check for broken wires, obvious
faults, etc. If all that checks out, you'll want to check the operation of
the O2 sensor. To do that, you'll need access to a scan tool or oscilliscope.
Check that the waveform is pretty steady. If the reading fluctuates then the
sensor is likely bad and will need to be replaced.


On some vehicles such as some Subaru models an ECM reflash will fix the problem.
As you can see a P0420 can be caused by many things, so if you have any doubts
as to whether you can fix the problem yourself, please seek the advice of
a professional technician. Good luck!
 

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Perfect discription. I had the equivalent code on my passat a year or so ago. Left if to long and cost for a full set of injectors as cylinder 4s packed up due to coke deposits, set of heater plugs as cylinder 3 was coked up and burnt out. O2 sensor in catalyst buggered and thanks to leaving it, I got around 10mpg less....add a little blue smoke and an invoice for £1800 and it's a good old kick up the backside to fix the problem NOW!
 

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Discussion Starter · #11 ·
like i said above i got the code read and the guy just switched it off as the car was due for the NCT and i wanted to see if she would pass the emmisions part with the sensor gone.she passed on that part but failed on some other small stuff,splil pin missing and tracking so must fix her up to retest within a month but the fella working on the car doesnt know exactly which sensor and the are something like e80.00 a pop.the light hasnt come back on since by the way.
 

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Discussion Starter · #12 ·
just an update on the EML .the first time it came on was a wet day after i drove through a puddle and the same thing has happened again after the flooding the other night,wonder if the two thinghs are related?
 

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A quick DIY MIL reset, is to disconnect your battery NEGATIVE terminal for 10 seconds - use this wisely- if your light comes back on quickly - please get diagonosed - but my 02 rio 1.3 has a top o2 sensor thats connection plug is loose due to broken clip and as I drive across rough ground it comes on every now and then
 
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