My daughter is driving across a few states today coming back from a trip in her 2001 Forester and her check engine light came on. At the last gas stop before it came on she noticed her gas mileage was down about 30%. She stopped at a mechanic who read the code and said she needed a NOX sensor. He said she would be OK finishing the trip before having it done. She was an hour from home then.
Where is the NOX sensor located? Is it easily accessible? What should it cost to replace?
She had a new cat put in last October. Would they have replaced the sensor then? Jon
There is a _knock_ sensor. These do fail, and are easily replaced. Then there is an Oxygen sensor. These can go bad as well, and are a little harder to get to.
I don't think there is a Nitric Oxide (NOx) sensor.
ByeBye! S.
Steve Jernigan KG0MB Laboratory Manager Microelectronics Research University of Colorado (719) 262-3101
Thanks for the reply Steve. I checked with my daughter, and it is the knock sensor that needs replacement. That actually makes a lot more sense as it would directly affect gas mileage. Any idea where it is located?
You bet. As Clare says, it is bolted to the top of the engine block., toward the back, just to the left (drivers side) of, and below the throttle body. Look for a round black plastic thingie about 1.5 inches in diameter by
1 inch tall, with a single lead (two wire), and a bolt thru the middle holding it to the block. Think it's an 8mm bolt; 12mm head in any event.
It's a little busy in that area with the cables for the throttle and some misc. wiring, but once you get your eyes to focus on it (bright light helps immensely), it isn't any trouble to put a 12mm socket on the end of a 4 - 6 inch extension on it.
Remove the bolt, and disconnect the lead, either at the sensor, or at the connector into the engine harness. I'm trying to remember what the connectors look like; I think you will want an ice pick or similar to lift either a plastic tab, or a wire retainer. This turns out to be the hardest part of the project, 'cause it's kinda tough to get both hands on the job.
I think these are fairly expensive at the dealership, but probably about $30 at a salvage yard. There is a way to test them with an ohmmeter. I can't recall exactly, but the folks at the yard should know. Perhaps someone here can tell us as well?
Anyway, be sure the replacement is clean and free of corrosion where it hits the engine block, and also that the block is clean. Connect the lead, put a dab of antisieze on the bolt, and bolt it down using only modest torque; 10 - 15ft.lb or so.
It might take a little while running for the CEL to go away, or you could reset the ECU by disconnecting the negative battery terminal for several minutes, or use an OBDII reader to reset the code.
FWIW, this is a fairly common problem, and it will cause your fuel economy to drop.
Pardon any typos that sneak in; our kitten has been "helping" me type this morning :-) Hope this helps.
ByeBye! S. Steve Jernigan KG0MB Laboratory Manager Microelectronics Research University of Colorado (719) 262-3101
Knock Sensor replaced. CEL is gone, mileage back to normal. Job was fairly simple but I had to pick up a new torque wrench in the middle of it the job as mine had mysteriously gone missing.
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