Howdy,
I've been driving a 2000 s40 for about 5 years now. The od is at 149k. I've been experiencing this random acceleration without driver input nearly every time I drive the car for several months now. At times I'm driving without taking my foot off the brake to avoid reckless driving. It will also rev high for brief periods when in neutral. More recently I've been experiencing trouble starting the car, though it eventually will start if I keep trying. I'm beginning to think the two symptoms are related. The car is also giving me two check engine codes: p0420 (catalyst system efficiency below threshold), and p0128 (coolant temp below thermostat regulating temp). I imagine a sensor of some type is on the fritz but I don't know where to start. Anybody else had an experience like this/ have an idea what needs fixin?

 

your P0420 is saying the rear O2 sensor is showing too much unused oxygen. This can be caused by a bad cat, intake air leaks, fuel trim issues (caused by one of several sensors) or an exhaust leak. The P0128 can be caused by a faulty thermostat or engine coolant temp (ECT) sensor. Since the ECT may be giving bad input to the ECU, I'd probably start by testing or simply replacing. The ECT screws into the thermostat housing so most people do both the ECT and Thermostat at the same time. Follow the upper radiator hose back to the cylinder head and look right under the end - you'll see the ECT and under the domed cover is the thermostat. The cover is simply two torx screws, the worst part of the ECT is figuring out how to unclip the wiring connector (no kidding - it works kind of backwards). I've done this on an S40 and only lost about a half pint of coolant so it can be done without draining the system. Once you have a new ECT, then I'd listen for any exhaust leaks (S40s are prone to the flex pipes failing - my S40 is on its third one) and then for intake air leaks and vacuum line leaks If all looks good you may try and replace the rear O2 sensor but there's still a reasonable probability that its the cat, particularly if the car is consuming some oil.

 

I have replaced my flex pipe twice as well and more recently the oil pan. These things just weren't built to rally I am slowly losing oil and haven't noticed any leaking so perhaps it is the cat. I'll start with the ect/thermostat and check for vacuum leaks. Thanks for the solid tips!

 

Well, I replaced the ECT sensor, and all is well. No more random acceleration and no more check engine lights. I’ve been driving daily for a week now and haven’t had the light come back. Thank you for the tips.