just after some ideas, Iv got code P0420 keep reapearing catalist inefficiency threshold bank 1. but it comes on randomly sometimes after 10mins of driving or doesn't come up atall on journeys. could this be due to the de-cat exhaust or is it a sensor on it's way out?

A code P0420 may mean that one or more of the following has happened:

• Leaded fuel was used where unleaded was called for
• A damaged or failed oxygen sensor (HO2S)
• Downstream oxygen sensor (HO2S) wiring damaged or connected improperly
• The engine coolant temperature sensor is not working properly
• Damaged or leaking exhaust manifold / catalytic converter / muffler / exhaust pipe
• Retarded spark timing
• The oxygen sensors in front and behind the converter are reporting too similar of readings
• Leaking fuel injector or high fuel pressure
• Cylinder misfire
• Oil contamination

Possible Solutions

Some suggested steps for troubleshooting a P0420 error code include:

• Check for exhaust leaks at the manifold, pipes, catalytic converter. Repair as required.
• Use a scope to diagnose the oxygen sensor operation (Tip: The oxygen sensor in front of the catalytic converter normally has a fluctuating waveform. The waveform of the sensor behind the converter should be more steady).
• Inspect the downstream heated oxygen sensor (HO2S), replace if necessary
• Replace the catalytic converter

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.

We strongly recommend that if you need to replace the catalytic converter that you replace it with an OEM unit. Second choice would be a high-quality replacement part. There are many stories in our forums where folks replaced the cat with a cheaper aftermarket one only to have the code return shortly thereafter.

One thing to note is that many vehicle manufacturers offer a longer warranty on emissions-related parts. So if you have a newer car but it's out of it's bumper-to-bumper warranty, there still may be warranty on this type of problem. Many manufacturers give a five year, unlimited mileage warranty on these items. It's worth checking into.

thanks for that gambit, I shall do some more testing on it. maybe run it with the tester in see what the sensors are doing

No problem mate

Hope it helped in some way it didn't really narrow it down though did it

Just to piggy back onto this, I've just had an engine warning light come on which Ive had before. It went off after a few days, but to pre-empt it not turning off what diagnostic tools would you guys recommend?

:( I use a Bluetooth adapter from home I can use with either my phone or tablet/Ipad has not let me down so far and when it's been double checked with my friends £2000 bit of kit so far has been the same, so would recommend these.  

https://play.google.com/store/apps/details?id=org.prowl.torque&hl=en

 

http://www.ebay.co.uk/itm/Android-Torque-Car-Mini-ELM327-OBD2-OBDII-Bluetooth-Diagnostic-Interface-CAN-BUS-/181124903208?pt=UK_Diagnostic_Tools_Equipment&hash=item2a2be2b128

 

And you can clear the codes to see if they return too :)

myself I use a snap-on ethos tester. not cheap but able to diagnose any system on the car and monitor the ecu datas.

although now I'm getting disheartened with my impreza. Iv now lost power above 4000rpm, could this be caused by the P0031 fault. or because my gasket from turbo to cat pipe decided to start blowing this morning. any idea guys as I'm really starting to loose hope with it

Adam on another post just said if the exhaust is blowing can result in the turbo stuttering so it is possible :( that is the problem of lost power

result. I showed a m8 who has a pro drive wrx and we noticed the bolts for the turbo had come loose. them now tightened once again it pulls like a train. hopefully they won't keep coming loose

Woop-woop result mate glad to see some good news on here for a change :D

NIce - a simple resolution for a change!!