Check Engine Light on Suburu

Dear Car Talk,

I have a 2002 Subaru Imprezza that we bought used in April 2006. OK (full disclosure), so it was previously used by a business, was in a flood apparently, and left for junk. This used dealer salvaged it and we bought it for what we thought was a very good deal. It?s been a great little car for us until this Fall 2008 when the Check Engine light came on and off intermittently. All diagnostics pointed to a catalytic converter problem so we replaced that, at about $500, so it could pass inspection in Maine. But the light came back on less than 24 hours later and this time it said it was the Knock Sensor. The mechanics went through several songs and dances with us (cleaning connections, replacing the computer ?brain? and the sensor) but the light would continue to come back on (Knock Sensor indication) after leaving the shop. I gave up, saying I couldn?t afford labor fees indefinitely. In their defense, they didn?t charge for quite a lot of labor or the replacement parts since they didn?t do the job. Have you got any suggestions?

Linda in Portland (on the other coast)


Hopefully your just about or have already paid this car in full. You may need to drop back and punt this one away. Trade it in on another car, and get a good mechanic to look it over before you purchase. Maybe the one that tried to work on this car, because they seem honest. Also, get a Carfax report for the car your interested in before you buy.

I run away from cars that have a hint of flood damage. Today’s cars are full of electronics, and are basically rolling computer networks. Flood damage causes water to infiltrate into areas that water should not go, and that damages some very sensitive components. Electrical gremlins ALWAYS pop up in flood damaged cars because of this. Even if the mechanic were to fix that problem, another one would pop up right after it. For the sake of your wallet, you may want to let this one go, and try for another.

I agree with BustedKnuckles.

A “flood car” made prior to the late '70s might have been able to escape ongoing electrical problems, but a modern car laden with onboard computers, other electronic devices, and LOTS of wiring and connectors is almost always a money-pit following a flood. It’s just a shame that you did not know this reality before buying what you thought was a bargain.

As BK stated, get rid of this car, as the electronic gremlins that are plagueing it will continue to get worse over time.

It sounds like a bad diagnosis to me and the onetime flood may be a red herring.

Find a Subaru specialized shop to look at it or versed Subaru mechanic to diagnose this. Occasionally fixing a CEL is unrelated to the direct problem but may something else causing it. Hence the need for good diagnosis.

Selling it is not the best option as the car is a really tough sell unless your bought this for a third of book price on a undamaged car.

Knock sensors are “tuned” piezoelectric (or piezoresistive…but nomatter) devices that send a signal only when a specific frequency is sensed that corresponds to a pinging cylinder.

The problem is either that you have pinging going on OR that the knock sensor OR ITS WIRE has been damaged. The sensors can be tested but not by everyone. Tapping on the head does not a test make, as it does not necessarily replicate the frequency to which the sensor is tuned.

Unfortunately I cannot guess with more specificity from here, but I’d suggest trying another shop. Or perhaps a local school that offers an automotive technology program…they’ll have the training, the equipment, and the technical references to properly diagnose the problem…and this will be a great lab project for the kids.

My 2001 Subaru Legacy Outback AWD Wagon has a check engine light issue. The catalytic converter, front and rear oxygen sensors, and knock sensor have been replaced, along with tires, brakes and other maintenance. There’s no flood damage - the car was a one-owner with a clean Carfax report. The car runs fine, and I think it has many miles left. The light will go off usually when the gas tank is full, then come as fuel decreases. Could it be a wire, some kind of fuel gauge, a gremlin or what that’s activating the check engine light?

My best guess is that you have an issue with the evaporative emissions system. This occurs most often when car owners insist on forcing more gas into the tank after the pump clicks off. Does that describe your way of filling the tank? Even if you do not do this, the previous owner(s) may have had this bad habit.

However, rather than relying on my guess–or anyone else’s guess–you need to have the car’s OBD system scanned for trouble codes that will localize the problem. Many auto parts retailers will do the scan without a fee. After you get the code(s), start your own thread and report those codes for guidance.

If my guess is correct, and if you are very lucky, then the problem may just be an inexpensive valve in one of the evap emissions hoses. If you are not lucky, then you will need to replace the expensive carbon canister. The cost of this unit usually convinces people to stop forcing more gas into the tank after the pump clicks off.

Also, you should be aware that we have had quite a few reports on this forum from people whose car had a “clean” CarFax report, but later it turned out that the vehicles had been in a serious accident or a flood. CarFax does have some value, but it does frequently omit vital information, and thus can provide a false sense of security.

Incidentally, it is not considered good internet etiquette to “hijack” a thread, which is what you did by posting your question in someone else’s thread. When you have a question, you should click on the big red button on the upper right of your screen that is labeled Submit a Car Question. If you begin your own thread, you are more likely to get a good number of responses.


It would help us greatly if you could tell us exactly what the trouble code was. It should be something like P0325. It sounds like the shop was trying to do the right repair work. The problem could be as simple as a bad wire connection to the sensor but I assume the shop would have checked that first. The knock sensor circuit is a very basic and simple circuit and shouldn’t be difficult to fix.

Sorry about the hijack - I’ll repost my own thread. I always let the pump click off automatically when gassing up. Thanks for the insights.

Wow, thanks for all the input. I don’t know what the code was but the mechanic had the car over several days. They said they checked the wiring and even replaced the knock sensor with another one. The light still came on within 24 hours and they said the code was indicating the knock sensor again. This is when I gave up. I’ll digest all your suggestions and come up with a game plan. Thanks to all! Linda/grayshoal

To my way of thinking at least, the shop should deal with this. You paid to have it fixed and they tried but you still have the problem. They need to fix this for you. At least for a reduced price.