2006 s40 t5 awd 160,000 miles a great car for my university kid 45 minutes away. Transmission and oil fluids are good, battery old.
Yesterday he drove it one minute and entered limp mode. All warning lights flashing, power steering disengaged and 1300 rpm max.
now that I researched this, a warning sign for fuel rail sensor was present. Last fall on warmish 50f/10c day, he was home on a day visit. When he left, the car did a slow starter sound a wha-wha-wha then started up. I checked the battery at 75% and charged it before he left. Now he says that on a super cold or cold day, it sometimes does a normal start sometimes a slow start.
last few weeks it’s gotten -20f/-30c and will start and drive fine as long as he uses it every week. Yesterday was warm, almost freezing when it hit limp mode.
my questions. Could battery cause limp mode? Will a scanner be helpful, I’m reading not too helpful. Do I just replace sensor? Try something else?
not looking forward to fix it in a snowy parking lot, but that beats university town rip off mechanics or an expensive tow home
thanks in advance!

 

A bad battery is going to cause all sorts of problems. Sounds like this one should have been replaced quite a while ago. A bad (shorted) cell can cause a low-voltage condition, even when running.

The el cheapo method to diagnose this is to have your son get a multimeter (often free, or very cheap anytime at Harbor Freight - perfectly useable if "minimalist"). Or, if he can't be trusted with having to deal with putting probes on mechanical bits (a common malady among our current crop of youths) you can get
Under $7 delivered the next day, and a couple USB charging ports to boot.

If the battery is sitting much under 12 volts (not running), it's probably toast. The alternator should bring the voltage up to near 14 volts with the engine running. If it's not, you need a new alternator (or it could be a wiring issue).

 

Thanks habbyguy. The battery is a cold weather monster- 84 month 980 ca/ 790 cca. It’s 60 months old but sits in a university parking lot a lot then does occasional short city trips. My new plan is to get the battery home and charge overnight and bring multimeter back. If that gets it out of limp I’ll have him check status for a couple days. Leading up to problem it was daytime high below 0f and -20f all week. This week it’ll get up to 40f. If that’s not fixing it, I’ll try the fuel sensor

 

60 months is approximately twice the average life span of Arizona batteries. ;-) I'd say you're money ahead swapping it (if only to save the tow charge from the local bar - errr, library - when it finally does die).

 

Thanks habbyguy. Got up to check out car, battery at 10.1 when resting and no codes popped up so just replaced battery and no more limp mode! I’ve never experienced limp mode in any vehicle I’ve ever owned and all info said fuel sensor.
You saved me time and cash- appreciate it!

 

I love a happy ending! ;-)