Q – I have a 2000 Carrera and the “check engine light” is on. When my car idles, the engine feels rough. When I rest my arm on the armrest, I can feel the engine vibrating and it wasn’t like that before. When it was scanned, the computer stored OXS sensing range codes for all four sensors. I had it reset and it happened again after about 15 miles. I only have about 30,000 miles on it and I wonder if I should replace the sensors. – Keith

A – 30,000 miles for OXS sensor failure is very early. They normally last a lot longer than that. The sensors are reading a fuel mixture anomaly and they are merely reporting the fault, they are probably not causing the fault. The other clue is the rough idle. There are some common, testable components that need to be verified. Fuel pressure for instance, and there are a couple temp sensors that may be sending a signal to the computer that fool the computer into thinking it is much colder outside or that the engine is colder than it really is.   Another possibility is that the air mass sensor is reading a higher air mass reading and fooling the computer into adding more fuel to the engine.  My bet is the air mass sensor.

You are probably wondering that if there is a problem with fuel pressure, temp sensors or an air mass sensor, why is there no code for that? Simple…The computer is not that smart! As clever as modern On Board Diagnostic systems are, sensors that are out of range often fail to set a code and instead cause other components to set codes.   This in turn sends a well intentioned but inexperienced tech on a wild goose chase instead of giving useful information. Goofy isn’t it!

My suggestion: Clean the air mass sensor. This is easy, cheap, and CRC (the company that makes the cleaner) guarantees more horsepower.   You can’t hurt anything for the effort and it may make your car run great again.   When air mass sensors were a brand new component, they were ridiculously expensive but now that the engineering has been amortized over a few million of them, they are reasonably priced and cheap enough that I consider them an exchange test part. In other words, if I think it is bad, I replace it with the understanding that if results are not achieved, I take it back, no harm, no foul, minimal expense. With sky high labor costs, the part seems like a bargain by comparison.

I have had philosophical discussions with many technicians who feel that swapping the part is not the correct way to diagnose a car but even Bosch agrees that a part exchange may be the best diagnostic approach in limited circumstances.

Now if cleaning (or a new part) really makes a difference, the performance will be crisp, the engine will run smoother and after a few miles, the computer will relearn how to produce both power and fuel economy. As far as the OXS sensors, they are probably coated with black sooty deposits and it will take a few miles before they will operate at peak efficiency. My suggestion is to get out your Valentine One and hit the highway for a third gear, 5,000 rpm three or four mile trip to incinerate all that gook in your catalytic convertors and clean the sensors.   The check engine light will be ready to monitor engine functions after a 10 to 20 mile run so you need to wait to see if this clears up all the codes. MC