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

Check engine light

I own a 2002 Supercharged Nissan Xterra. It has been such a reliable vehicle. Last month the “service engine soon” light came on and stayed on. I took it to a dealership and they diagnosed a knock sensor failure and also the #4 fuel injector failure. I could not afford the

repairs right away and they said I would continue to see the light and to schedule the repair asap. I have been driving it gingerly and, as they suggested tried not to push it because if the light would start to blink that would “potentially cause greater damage”. If I drove over 40mph uphill was when I would notice it start to blink. Well, for 2 weeks now the check engine light has remained off all together, even at higher speeds. I should say that it never did run rough or anything. Have you ever heard of such a thing? Thank you…I still don’t have the repair money…

Do you have the codes that were scanned during diagnosis?

A bad knock sensor can be bad if the engine isn’t in good health or if you’re using lower-octane fuel than you should. In a supercharged engine… I’d say it’s pretty damned important to have in functioning.

Here’s the deal: your engine uses the knock sensor to detect spark knock. Spark-knock occurs when -for a variety of possible reasons- some of the fuel/air mixture in the cylinder get ignited before the spark plug fires. When the spark plug fires, it ignites the unburned fuel/air in the cylinder. The two explosions (flame fronts) collide and cause that knocking sound. It’s violent and can cause engine damage if it’s very serious.

Spark knock can occur if the engine timing is too far advanced, if you’re using too-low an octane fuel (I assume that your supercharged Xterra asks for premium fuel?), or if your engine is unhealthy (excessive carbon deposits on the pistons, overheating, etc). When the knock sensor detects the pre-ignition (literally by detecting, in the block, the harmonics of these events) it tells the engine computer, and the engine computer typically compensates by retarding the ignition timing until the knock is no-longer detected.

Here’s where that could be a problem for you: a bad knock sensor isn’t correctly detecting spark knock. So if something occurs and you’re experiencing pre-ignition, the computer may not know about it and will go on happily maintaining the advanced ignition timing while a hole gets burned in your piston (something that happens when you let a really bad knock go on for a long time).

So I think… if you have the means, at least try to get that knock sensor replaced. I would maybe say that you could let it go if you didn’t have a supercharged engine. But since a forced induction engine is more sensitive to fuel/air trim and preignition, I am going say, yeah, it’s important.

Of course, there is the question of how they diagnosed this and if they were actually right, or if there is a different problem. It seems odd that a knock sensor and fuel injector would fail together.

Best of luck.

Drive it gently, save your money, and expect the CEL light to return.

They do not “clear” themselves typically.

If it is flashing you are damaging the motor to a degree by continuing operation.

I think that by driving “gingerly” you have possibly avoided triggering the problem (possibly preignition as mr josh said) for enough miles/drive cycles that the computer has cleared the code after not detecting the malfunction for a while.

But do yourself a favor - the next time you start the car watch the dash lights and make sure that the CEL still lights up briefly. It is just a dash bulb, afterall, and could have burned out (not likely but just check).

Either way I wouldn’t assume that the problem is solved. It would be great to have the codes that were read from the engine. See if you can get those out of the dealer.

And…don’t use the dealer. You can have this problem addressed just as well for a lot less $$ by an independent mechanic. Find a good, local, reputable one not attached to a large corporate chain.

And…do inspect what your owner’s manual has to say about the required fuel for this vehicle and follow it. My guess is that you’re supposed to be using premium fuel.

thank you for your reply to my problem…finally getting it repaired tomorrow

thank you for all of your suggestions…very helpful

Take it to an independent shop (even if you don’t know anybody in the shop). Even if they do the same work as the dealership, that work (and parts) will cost you much less than the dealership.
Your engine, especially #4 fuel injector, may benefit from a “fuel system cleaning”. The #4 fuel injector, and knock sensor, may not need replacing.
The shop can scan the engine computer and get a lot of information about how your engine has run, and how it’s running now. The scan looks at the sensors and the information they provide to the engine computer, such as: fuel flow, air flow, engine temperature, and other stuff.

What puzzles me about your description is that you said that the engine never did run rough. If an injector has failed, the engine will run rough, no ifs, ands or buts. But this is easy to check for yourself. With the engine running, disconnect the electrical connection to the #4 injector, and if the engine idle changes, the injector isn’t bad. If there’s no change in the idle with the injector connected or disconnected, the injector is likely bad.