I have a 94 850 turbo that will not start. There is spark and fuel and it turns over nicely but it will not start. The codes read 1-4-3 (front knock sensor signal missing or faulty). Other than the code (which has been present for several months now), it runs fine.
I recall about 6 months ago that there was an occaision that it did not start right away but it did start after leaving it for an hour and trying it then. I'm due for a tune-up but I was wondering if the knock sensor is enough to keep it from starting?
Many thanks for any replies.

If I undertand well a knock sensor is a crankshaft position sensor. If it is, yes this may prevent your engine from being started. I had this problem on an Opel Corsa last week and it would not start. We tried everything (fuel pump, etc..) and it was "only" a bad crankshaft sensor. I suppose same thing may happen on a 850.

A knock sensor has nothing to do with crankshaft position, those are two different sensors. I've yet to see a Volvo that would not start due to a bad knock sensor. I don't think that any "decent enough" car would decide not to start because of a bad knock sensor. In fact, I don't think most ECUs make any use of knock sensor input during cranking and startup. It wouldn't make too much sense.

Now, with a bad crank sensor you can't start the engine as there will be no spark. I doubt how one could get a spark w/o an operational crank sensor. Maybe the computer has some "educated guess" limp home mode that would not be enough to start it... I doubt it. It needs to have some sort of crank position information just to time the spark properly. If there's no crank position input, the computer cannot know when to fire the spark and when to fire injectors (in most cases). And you don't really want firing sparks at random points in time. It can have some nasty consequences. Your intake definitely wouldn't agree more, it being mostly plastic and the burning mixture ejected via the open intake valve being quite hot.

If one has a decent spark I doubt that the crank sensor would be dead. Maybe dying but not dead.

If you have fuel *and* spark I'd suspect (roughly in that order): fouled/flooded plugs, disconnected plugs somewhere in the wiring harness, plugged exhaust (usually the CAT converter or a "funny" kid stuck a potato up your exhaust pipe), a bad manifold air pressure (MAP) or air mass (AMM) sensor, maybe also bad TPS. That's all assuming that the engine is mechanically sound, i.e. that the hydraulic valve lifters didn't overfill and that the valve is right.

If the fuel is sprayed out of injectors and there's spark the car should start. It may run crappily due to bad fuel pressure (worn pump, bad FPR), dead/slow O2 sensor, exhaust misperformance (plugged cat or resonance due to a hole) or a host of other things, but it should start.

I've never had a problem with e.g. my 93 '940 starting up, even at -30F. That's fifferent than an 850, sure, don't know if they have such problems. I've had "slow starts" due to a leaking check valve in the fuel pump, but that's not really a no-start. You turn the key to position II a couple of times, waiting every time till the fuel pump stops, and then it starts right up.

Cheers, Kuba Ober

Don't think a bad knock sensor will keep it from starting but a bad rpm sensor will. The engine needs this signal to start and run although it will run ( not very well) while it is in the process of failing. Usually the braided wire just slowly falls apart. ALso, a bad fuel pressure regulator will prevent it from starting. Just some thoughts

bl

could also be the CMP sensor, as it was in my case. also, check the fuel pump relay. i've heard those have a tendancy to fail.