Unresolved Incorrect Bed Temperature Reading on R1+ (Reads 78C Ambient)

[Preamble: this is my first post since getting the printer in December 2017, but I've read quite a bit and want to express my thanks to you for such a great community. I've already upgraded to the E3D V6, built an enclosure and relocated the spool feed, added a cable chain, and other tweaks, often relying on various threads here as a guide, so the knowledge base here has been extremely helpful already. My house is permanently perfumed with Aqua Net, I've got discarded support material littering my desk, and once the current print finishes, I'll be rounding-out a solid 24 days of printing time. I've always been a maker/doer/DIY guy, and this printer is bringing to life *forty years* of ideas and designs that I've never had the time/tools/resources to do.]

As the title states, I'm getting an incorrect reading on my R1+ bed temperature. It reads 78C ambient; room temp is more like 12C. The bed will heat if I set the target temp above 78C, but predictably, doesn't quite reach 80C (more like 60C at 80C target temp). Any ideas on the cause? I presume it's simply a bad thermistor (Occam's Razor and all), but I know there are other possibilities, like the board. I read some older threads on the subject, but the ones that were relevant were pretty old (>5 years), so I'm not totally sure if they're applicable.

Q: Thoughts on the cause?

Q: If it's the thermistor, will any replacement 100K thermistor work? (of course, the unit is still under warranty, being less than three months old).

Other technical details:
- R1+ (purchased December 2017)
- Hot end replaced with E3D V6, and all necessary firmware mods performed
- Using MatterControl

Thanks!

 

Thanks, Mark - I appreciate the prompt and thorough response. I went through the diagnostic procedure as you described and the temp reading didn't change from the ambient reading (in fact, baseline is now up to 82C). With this, I think I can isolate this to a RAMPS board malfunction.

Are there any alternatives to fix this, other than a hardware replacement? I view this as a cut-and-dried warranty claim; however, I always like to solve my own problems before making a claim (as long as it's reasonable for me from a cost/effort standpoint). Is there a way to reset this in the terminal/Arduino?