@ddelahay59 is this 2020 a new Forester? Your last 2 threads show 2019, as is displayed under your avatar?

Bobby...

['07 FSXT Member Journal] ['03 X Member Journal]

 

@ddelahay59 is this 2020 a new Forester? Your last 2 threads show 2019, as is displayed under your avatar?

Bobby...

['07 FSXT Member Journal] ['03 X Member Journal]

My mistake door pillar states Aug 18 build Australian plate 2019

►Moderator edit - I fixed it for you! Bobby...

 

On the four cylinder Subaru, there is only one "bank".

This is one of the more common topics, P0420.

P0420 comes on as a result of the information from both O2 sensors. The computer compares readings from both and comes up with an opinion. If the readings don't create a satisfactory solution, the code pops.

Could be anything related to that system including holes in the exhaust pipes. So you have to troubleshoot everything related to find the problem. There are two simple tests to examine catalytic converter function, a back pressure test and a differential temperature test. You can clean the connectors for the O2 sensors. Pull the sensors and examine them. Get a full time OBD monitor and evaluate the signal from the sensors. Stuff like that.

 

Thank you for the attached information I am slowly working through the troubleshooting whilst learning the system on the Subaru. Next step is to test the O2 sensors.

 

Update on progress,
Could not find any damage to the exhaust, 02 sensors and wiring ok.
I ran a generic cat cleaner through the fuel (added to tank). Connected my OBD reader to the port, taped it to the dashboard and completed a 35K trip combined city and freeway drive one direction with O2 sensors set return with cat temp set all functioned as it should
I have run 3 x tanks of 98 premium fuel (I usually use 95 or 98) no further fault codes.
As the first fault occurred on a tank of fuel from an outback town I am now assuming I had some sort of contaminant or bad fuel.
The car is booked at the local Subaru dealer to repair a tyre pressure sensor fault as I cannot identify TPS faults with my basic reader it will be interesting to see if the dealership mentions any history re PO420 faults when they interrogate with their software.
Thanks again for the responses and suggestions

 

Update after approx 4000 Ks fault repeated interestingly this time it faulted within 500 meters of home on a cold start (blue temp light was still on) The CAT would not have been at normal operational temperature at this time. I am wondering if this could be an engine mixture rich at cold / warm up running temperatures. I checked the sensors on my basic OBD2 reader both ok. Has anybody experienced PO420 on warm up and identified a cause?

 

Have you checked your engine grounds are in good shape?

 

That's a good point I will check them out at the 150k service. I had a TCV replaced under warranty maybe they disturbed the earths during that dealer visit.
thanks for your help

 

Update- I completed the 150K service during the service I removed the airbox / intake plenum from the throttle body to provide access to the front differential fill point, change air filter etc. The airbox / lntake plenum lifted straight off the throttle body. On reinstallation I noted that the hose indicated in attached photos was very difficult to push back on being a very firm fit this was certainly not case on removal as I did not have to pull the hose off at all. I am now considering the possibility of a vacuum leak from this hose attachment point Causing my intermittent PO420 code.

Additionally I removed the lower eng cover and thoroughly inspected the visible exhaust areas with no visible damage. I checked the 02 probe heater resistance with the CAT sensor reading 1.6 ohms and the exhaust exit sensor reading 5.9 ohms. Although the O2 sensor connectors were very clean I cleaned and added some de-oxidize spray as a precaution. I will update should I get the code again.

Thanks again for all the suggestions and assistance

 

Well after a long road trip and 5,000 kms since the last PO4020 code my forester has done it again. I had purchased a TOPDON scanner in anticipation and am still learning how to understand what I can get out of this unit. As I was on a highway trip I reset the code now (350 kms ago) and have today experimented with the TOPDON unit. I am unsure of exactly what parameters to check or monitor and have attached some photos of screen shots taken off the unit. Im not sure if I have found No 1 (upstream sensor) readings If any members could advise what are the best parameters to monitor live are that would be appreciated.
Attached some photos of my results so far for info and comment or suggestions.

Thanks to all

 

I recommend you get freeze frame data whenever a code pops.