Diagnosing error codes P0420, C0042, C0071

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Diagnosing error codes P0420, C0042, C0071

Postby RX7_BEN » Mon Jun 17, 2019 9:59 pm

Hi guys, I realise this topic has probably been done to death but I've been searching extensively and can't seem to find any conclusive answers.

I'm brand new to Subaru, I've just picked up an 09 Liberty Sport Edition (2.5) Gen 4 and whilst driving down the highway I momentarily flashed my high beams and instantly the traction control light and engine light came on solid and cruise control light started flashing.
I pulled the codes up and got P0420, C0042 and C0071. I tried resetting the codes by disconnecting the battery for half an hour but it all remains.

Are there any recommendations of what I can look at to remedy this issue? The car is still driving absolutely fine, you wouldn't know anything is wrong with it.

I've seen a bunch of threads regarding the P0420 codes relating to O2 sensors and disabling it in the tune, but very little regarding the other two codes.

Any help would be much appreciated. Cheers!
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Re: Diagnosing error codes P0420, C0042, C0071

Postby bigBADbenny » Tue Jun 18, 2019 3:45 pm

The diagnostic trees are in your cars FSM, link in my signature.
The cels might or not be contraindicative, eg one or more in combination can point to an issue upstream or related to those sensors.

Use the cel code as a keyword with my google this forum link in my signature, too.
Eg https://www.google.com.au/search?biw=32 ... ani4LAKDAc

OBD II C0042
OBD-II Code Diagonostic Trouble Code is defined as a Brake Pedal Position Sensor "Circuit A" (Subfault)

Google p0420 $5 fix.
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Re: Diagnosing error codes P0420, C0042, C0071

Postby RX7_BEN » Wed Jun 19, 2019 9:24 am

Thank you very much for that insight mate.

I've just managed to put a snapon scan tool on it. Cleared the codes so we'll see how long it takes to trigger again.
It looks like I'll have to either get the ECU modified to ignore the P0420 code, or give the $5 trick a go.

The C0071 came up as a steering angle sensor power supply malfunction and the C0042 came up as power supply voltage failure. So not sure what the go with that is. I'll see if it comes back I guess.
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Re: Diagnosing error codes P0420, C0042, C0071

Postby bigBADbenny » Wed Jun 19, 2019 9:44 am

Yeah to extrapolate, being both “C” type cels, it could be useful to check the FSM wiring diagrams to see if they have a common loom plug or relay, and check operation/continuity/condition.

P0420 is tricky, eg it can be sensor condition (maf, af, o2 etc) , cat condition, exhaust leak, inlet leak, spark or fuelling related, etc.
Best to read up on the issue on forums, extensively, as firing the expensive parts cannon at it can be a huge waste of time and money for no result.

Personally, I use Btssm to log and LV my car, so I’m always informed of how it’s running.
Eg LV can reveal inlet leaks etc, if and where it’s knocking, logging can reveal everything eg per cyl misfires etc.
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Re: Diagnosing error codes P0420, C0042, C0071

Postby RX25SE » Wed Jun 19, 2019 9:02 pm

You could try removing the front O2 sensors (with a hot exhaust) and spraying some UEC ( eg. Three Bond Engine Conditioner)at the front of the cat and let it soak before restarting the engine. Be prepared for exhaust smoke until the cleaner burns away.

In some cases oil residue coats the surface of the cells, so they are unable to react with the passing exhaust gas.
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Re: Diagnosing error codes P0420, C0042, C0071

Postby RX7_BEN » Fri Jun 21, 2019 11:38 pm

Thanks again to both of you for your input, I appreciate you taking the time to help me.

After clearing the codes on Monday morning, it hasn't misbehaved although I'm sure it's only a matter of time. In the meantime I'll be looking into purchasing my own diagnostic gear, this is actually my first OBD II equipped car.

*update; I just purchased an $8 VAGCOM KKL cable so I can interface with my laptop to clear codes and whatnot. I don't need anything fancy, it's just a bog stock daily afterall. I plan to use that with FreeSSM, although I did read your fantastic thread about BTSSM from a few years back Benny. Great job!
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