At 213K km, there's wear in the valve guides (== more oil consumption), the O2 sensor(s) are getting tired, and the plug wires and plugs are old. All these can cause the jerkiness.
Changing the plug wires would be a good start, followed by the plugs, unless thus has been done. A tired O2 sensor usually sets the occasional 420 error code.
You could also try resetting the ECU. What this does is place it into a learning mode, and the jerkiness may disappear for a while. If it does, it means the ECU is running the engine around the edge conditions. Replacing the O2 sensor may help, but don't do it unless you get an error code.
Also, check for exhaust leaks on the "donut" connection after the catalytic converter. Even a small leak can cause the O2 sensor to report "off" conditions.
SOURCE: Cylinder 1 misfire on my subaru!!
What you should check first is the spark plug for the #1 cylinder and see if there is a white film on it this would be a sign of a coolant leak which would cause a problem with the O2 sensor
SOURCE: 2005 Subaru Legacy: Where are the O2 sensors located (both)
the o2 sensors are locted in the exhaust system. upstream is before cat converter, downstream is after cat converter.
SOURCE: Check Engine Light
Hello engelhard58
I have attached a link that can give you some of the reasons why your getting that code.I hope you able to get your answer out of that.
Mean of the code=P0420 Catalyst System Efficiency Below Threshold.
Link to site
SOURCE: what does error code 55 mean for Subaru leagcy 2005
This is a communication error with the BIU (body integrated unit). Need to take to the dealer, possible that the ECU needs to be replaced to fix this issue. It should be covered under the federal emmission wrty.
SOURCE: subaru forester heater
take back to the dealer,hopefully the will find that the blend door cable is off or not adjusted properly
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