02 Subaru Forester Catalytic Converter Issues?


Wrench. I help when I can
@jd, fuel trims are a neat thing to look at for this type of code.

Since you wrote that it is a term you don't understand. I thought maybe you would like to know what that term means.

Fuel trims are a measurement that the ECM needs to lean out or the enrichment of the fuel to air ratio to maintain a balance. I think maybe this video will do more justice than me and my sloppy wording.



Staff member
"Fuel trims" are how much the PCM modifies the injector pulses, either rich or lean, from the programmed values. The PCM is altering fueling from the programmed value in response to what the upstream O2 sensor is reporting on how the air-fuel mixture (AFR) has burned. That's why getting live-data readings on the O2 sensors is so important here, we are just guessing without that info.

There are both "long-term" and "short-term" fuel trims. Long-term account for gradual changes in things, like a fuel-pressure regulator slowly drifting with age, or compression changing due to ring wear or carbon build-up in the cylinder. Short-term is in response to intermittent issues, like a vacuum leak in a hose that only occurs when the engine moves to a certain position. I'm kind of generalizing, but I hope you get the idea...

Excess fuel can wash oil off the cylinder walls, but generally is not much of an issue if the engine will run, even pretty rich. The excess fuel is mostly vaporized and little really gets onto the oiled surfaces to dilute the oil film. Especially since most of the oil film is below the piston rings, not very exposed to the air-fuel mixture, whether it is too-rich or not.

Edit: added not!
Last edited:


Wrench. I help when I can
Yeah, what Bill said. He has a better way of saying things than I do.

I will repeat myself, BLACK smoke is raw un-burned fuel. When a cat goes south, the chemical reaction can no longer get hot enough to burn the NOX's and such. Hence the in-efficiency of the cat/s.

In a region like mine, this would be a moot issue, we don't have emission testing up here.

When the programmed algorythm is not seen and under or over valued, a P0420 code will set and this is basically the ECM/PCM asking(begging) you to check the exhaust system in its entirety. Most often a cooked cat.

Now, the question is why did it get cooked??

That is the 800$ new cat question.

So this is why fuel trim and o2 switching readings are so important.

Who wants to spend so much money on a cat when possibly it is only a 2$ gasket leak?
I downloaded the OBD Auto Doctor software and got the trial version going to test. I have it connected to the vehiclee, but the Oxygen Sensor Monitors tab says that it is "Not supported by the Electronic Control Unit". It does have fuel trims, but they're not available in the trial. I'm trying to decide if it will be worth it to pay the $50 for the software without the O2 sensor info.
I'm going to check the PCV and exhaust system today - will report back.

Thanks nickb2! I'm a total newb but I'm going to keep on going at it. I appreciate the support from everyone here.
Rear of the car is to the left. Catalytic converter on the right of the pic.

Also found an oil leak in vicinity of the oil filter. The oil filter let me tighten it by hand about 1/4 turn. Looks like the oil loss is unrelated to the P0420 code.



Hero Member
Update on that little Subaru Forester Biter: This is the one I just spent hours on with Headlight Wiring...

It had been running "OK", about Average, Uses more gas than it should (did that on my 8,000 mile trip back from Alaska in 2011, too, but no Check Engine or Oil Loss), black sooty tailpipe end.

Till last Saturday. Started up extremely rough. Check Engine Light FLASHING. Overheated! Engine Hot, Radiator Cool, Coolant FULL. MikeD took it to a shop. Shop's a good one, but couldn't duplicate the condition. Wrote an estimate for miscellaneous fluid leaks and power steering rack problems bu nothing addressed what happened Saturday. Ran OK to and from the Shop.

Sure seems like a Sticking Thermostat. Replaced Thermostat and made up the little bit of coolant it lost. Ran OK. Saw evidence of the oil filter leak MikeD noted above. Also noticed oil was full, which it's rarely been.

Till Tuesday. I'd warmed it up, checked for leaks, all OK. Pulled out into the neighborhood, Bad Shakes, Flashing CEL... Mike told me I'm not allowed to drive it any more and we parked it. I hope he'll take it back to the shop.

We were swapping cars. Leaving our Ford Aerostar (1997 with 75000 miles) and taking the Forester the 700+ miles back home. We made the trip yesterday in a 2017 Hyundai Accent rented from Budget.

Shop tested some engine management/emissions items, said both O2 Sensors were Good.

I doubt I'll ever see that Forester again...


Wrench. I help when I can
Good, cuz for the few times he came here to BAT, I found he was solid. He must be cuz he is family now for you. In french we say "beau fiston" slang for son in law, I think. The Innu use a different term. "Innuatsh" The innu language is quite complete when you start to listen.

I mostly will miss the elders and there easy way of just being/existing. Very wise ppl. Hats off to them for sure.


Hero Member
Solid Indeed, My Friend! We're blessed that our daughters have married stand-up men, and that they freely include us in their lives.


Hero Member
It's been awhile, but MikeD started the Suby up today with one of our cheesy OBD senders and TorqueLite. It shivered and shook like I'd experienced, and threw Codes P301 and P302, Misfire #1 and Misfire #2.

I think that ignition uses "waste spark" where 1 and 2 are actually connected to one coil in the pack, and 3 and 4 are connected to the other. When 1 is on power stroke it gets spark but so does 2 even though not on power stroke, from one of the only two coils in a four-cylinder pack. Firing order is 1-3-2-4 so no spark to 1 and 2 would produce an engine running on Miss 1==>Fire 3==>Miss 2==>Fire 4. A relatively smooth running two-banger like an ONAN welder engine, just with no power.

I also found that only one timing belt drives cams, oil pump, and water pump plus a few idlers. That tells me the overheat we had must have been a sticking thermostat. That, since coolant stayed full, none was lost, and overheat never came back.

I conveyed all this to MikeD, but whatcha'all think?
Last edited:


Wrench. I help when I can
If no presence of overheating, a t belt kit is fine for these horizontal engines. The heads on those are very durable.