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New engine with failing sensors


I’m hoping someone will be able to help me with this question as my mechanics are making me crazy!! About two weeks ago I had the engine replaced in my 2000 Subaru Legacy. As I was driving it home from the shop, I noticed a lack of power. The car was having a really hard time getting above 40 mph. The next day the check engine light came on and I took it back to the mechanic. He first told me that I wasn’t getting any power because my floor mat was stuck up under my gas pedal (very strange answer that I knew was garbage) and that my check engine light was on because of a failed air sensor. Finally, he told me that I might have a failing transmission and to take it in to the dealership, he couldn’t diagnose the true problem himself.

So I took it to the Subaru dealership, who told me that I had a failed MAP sensor. Once they put in a new MAP sensor, they are now telling me that I have a failed air sensor. The original mechanic says that these are the sensors from the original engine, that he didn’t put in any new sensors with my new engine. My question is: how could it be that RIGHT after putting in a new engine (and spending a good chunk of money on that) could I now have all of these failing sensors? Is there a connection here between the installation of the new engine and the failing sensors? Has anyone seen anything like this before? They are telling me the new sensors will be another $600.

Please, any information or ideas will be much appreciated!!


Replacing / rebuilding engines in modern cars is SELDOM recommended on this board for the very reasons you are discovering…These projects frequently turn into a black hole for your wallet…The problems just never end…

It’s normal to use many of the old sensors and peripheral components on a new engine.

In order to form an opinion, I’d first want to know
the year of the vehicle
how many miles are on the vehicle
why the engine was swapped out
what the history of the vehicle is
and the current symptoms

Frankly, it bothers me that a shop that can’t diagnose an operational problem is swapping engines. But that’s a done deal now. Now you have to deal with what the current problems are.

It’s standard operating procedure to swap all of the peripherals over onto another engine when it is changed out. You got a “new engine”, not a “new engine and everything attached to it”.
Everything attached could have been replaced at the time the engine was changed out but would you have agreed to double or triple what you paid for the engine?

Just to clarify something; does “new” engine mean remanufactured or rebuilt or does it mean new to the car, as in a used motor procured from whoever?

Did your check engine light come on, and if so, what were the codes read by the mechanics?

Thanks everyone for your info and questions.

Moutnainbike: It’s a 2000 Legacy with 180,000 miles on it. The engine died after an oil leak- the car was running on no oil (without the oil light coming on until the very last minute) for about a half a day, I think. My car has had very few problems in the 6 years I’ve had it… the only serious work I’ve had on it includes a new alternator and new brakes. I am ALSO very bothered that this guy is replacing my engine but now doesn’t seem to have a clue what the problem is with my car. The only symptom happening is a lack of power- the car struggles to get above 40 but no noises or hitching or anything like that. Doesn’t seem like a transmission issue to me.

jtsanders: Now, the check engine light does come on. The first mechanic says his computer reads only a failed air sensor. The dealership reads both a failed MAP and air sensor.

ok4450: The original mechanic says that all of the sensors are from my original engine, that the new engine didn’t come with anything more. The “new engine” is used, with 32,000 miles on it. The dealership said that they sensors might have become contaminated when they were switching out the engine but the original mechanic maintains that there’s no way that could have happened and will not pay for the repairs unless we can prove that it’s his fault the sensors failed.

caddyman: It just seems very suspicious to me that they would fail immediately after the new engine was put in. I guess I’ve learned my lesson about replacing engines… but repairing it is still cheaper than buying a new car at this point.

Unless you get in the habit of raising the hood every week or so to check the fluid levels you are going to go through the same thing again in the likely not so distant future. The purpose of the exercise is to make sure that red oil light never comes on. By the time that light comes on you’re already up the proverbial creek without a paddle.

There’s not enough detail know to make much of a guess as to what’s going on with the car. If the sensors are bad I see the odds of the mechanic being responsible for this as being very remote. There’s also the issue of aged wiring and connectors because a code showing a failure for a certain part does not mean that part is bad. It could be a poor wire connection due to scale or corrosion, etc. and the act of changing the engine tweaked them to some degree.

Generally speaking, an engine that is totally gutless and lacks power, has a low top speed, etc. has a problem with abnormally low fuel pressure, a clogged exhaust converter, worn engine (theoretically not likely on a 32k miles engine), slipping clutch on a manual transmission, or a slipping automatic transmission.

Does the car buck and jerk pretty badly or is this a case of just a general, steady sluggishness?


The sensors are still 12 years old. They are prone to fail at some point. Nothing lasts forever. Using the original old sensors is common practice. Replacing all these sensors with the engine could easily push the engine swap price up over an additional $1000 dollars easy. Some of these sensors are not cheap. I hope this helps.

You need too find a new machanic. Subie are kind of like old VWs, there is a cult around them. You need to find the cult in your area and their best mechanic. He loves Subies beyond reason and will keep yours going with out you going broke.

it’s general sluggishness, no hitching… subaru says a new MAP sensor will solve this…

When an engine is swapped there are all kinds of grounds that must be disconnected. It could be that when the replacement engine was installed one or more grounds weren’t reconnected.