Best of Deals Car Reviews Repair Shops Cars A-Z Radio Show

2004 Outback intermittent check engine

My 2004 Outback now has 120K, has always been maintained and repaired at the dealership (except tires and battery), and has always run great! I’ve had the plastick-y parts replaced over the last few years including head gaskets when one cracked and made smoke. In July I got a jump from a AAA tow truck, who said my battery tested as needing replacement. In September my check engine light went on, and the dealer found the code PO420-catalyst system efficiency. They charged $135. and didn’t seem to look any further into causes. The cat converter replacement was to cost $2521. They reportedly tested the battery as OK as part of their routine checkup. I declined the new cat converter till learning more. Meanwhile, my check engine light seemed to cycle on, then off, then on, then off . . . every several days.

In December I had the battery checked at Sears, who replaced it under warranty. So I guess it wasn’t okay after all. After that my check engine light stayed off for somewhat longer than usual, then back on for most of January. I was just about to give in and get the repair done when it cycled off briefly a few days ago.

I’ve lost trust in the dealership because of their battery check which differed from others before and after theirs; and for their lack of specific diagnostics. The light is back on and I don’t know what to do next.

Nobody can possibly advise you until you find out which diagnostic trouble codes have been stored by the car’s OBD system. (The illuminated Check Engine Light is your clue that codes have probably been stored.)

If you go to Auto Zone, Advance Auto, O’Reilly, or (possibly) Napa, they will read the trouble codes for you. Then come back to this thread and post the exact codes for specific advice.
They will be in a format similar to “P0123”.

Edited to add:
I hope that the “intermittent” CEL is not a flashing/blinking light, and is a steadily-illuminated light, even if it is only steadily illuminated for a short time. I say that because a car with a flashing/blinking CEL should not be driven, lest you wind up with VERY expensive damage to the catalytic converter and (possibly) the engine. If the CEL has been on w/o flashing/blinking, it is not of an emergency nature.

Other than agreeing with VDCdriver I might add this about your dealer comments.
The battery testing was done over a 4 month spread and things can change in 4 months along with the ambient temperatures. Good in Sept. doesn’t mean good in Dec.
Battery testing methodogy can vary also.

You might clarify the part about diagnostics. You state they didn’t seem to look into further causes.
What did you want them to do? They gave you a converter diagnosis and a price so is this a matter of not liking the price and wanting to hear that it’s something much cheaper?

You’ve apparently had a head gasket failure in the past and that’s what likely caused any converter issue; coolant entering the combustion chambers. That would be the white smoke you referred to.

Thanks for your help. It didn’t occur to me that subsequent codes might be different from the first, PO420. I will try to get a listing of the stored codes.

‘Intermittent’ refers to how the check-engine light has cycled on (not blinking) and then off again, every several days or longer intervals. I’ve looked online and have not read about this on-again-off-again problem.

There has been, though, quite a bit of talk about how the code itself is not diagnostic as there are said to be factors upstream and downstream from the converters that can set this code. I don’t understand the mechanical talk, but I am a diagnostician in another field, and this general gist made sense to me. Some report that inconsistent battery power can cause the computer to give off a code–which is why I had the battery re-checked after 2 conflicting reports. Also stories about folks who have replaced cat converters one or more times only to have the light reappear. So it was my impression that the code was a starting point, not an end, in the diagnostic.

Thanks again, and I will try to get the suggested information!

While the intermittent CEL could mean that the cat is right on the edge, it could also be a problem with one of the oxygen sensors or their related wiring (e.g. a chafed wire making intermittent contact with the body, grounding it out).

Absolutely right . . . the code is NOT the answer . . . only a starting poing

Thank you. These are good considerations and the first explanation I’ve heard why the light might cycle on and off.

As you suspect LOTS of things set the P0420, and it if is so intermittent I’d guess that you have some other intermittent problem:

In very specific terms the only thing it means is that the computer is getting readings from the downstream (after cat) O2 sensor that look too much like the upstream (before cat) O2 sensor. For simplicity sake, just think of an O2 sensor as a pollution detector that is more active with more pollution. Before the converter you expect a lot of activity because of the pollution level in the exhaust. But the converter should clean up a bunch of the pollution - so there should be a lot less activity on the O2 sensor after the cat. Thus - if the two sensors read too much alike the computer sets the P0420 - and it could mean “catalyst below efficiency” - or any number of other things.