So, i'm eager to roll a Warforged Barbarian character and in thinking about the character and how I would play them I thought of what is probably quite a common go to, he's old, rusted, forgotten and then somehow reactivated. He may look like junk at first but was actually top of the line, has a tonne of hidden strength / abilities. The usual. 
But then in thinking of him being a bit tired and old, rusted and beat up I thought, wouldn’t it be funny if bits just fall off him every now and again, or an eye stops working, a joint locks up etc etc.
I've been inspired by a friend playing a wild magic character and really running with the chaotic element of that idea so it struck me how similar these two ideas could be.

What if, every time I do X action I push my battered old hulk to its limit and I have to roll on a table for mechanical faults?
Some simple, some more serious, some purely RP, some impactful in combat etc.

I think this could be a great way of taking the edge off a race that it’s generally agreed can be fairly OP, as well as just opening doors for great RP. 
After a day of tossing the idea around I have come up with the following and i'd love to know what you all think and any additions or tweaks you think would work:

Repair Status:
Your repair status represents the current reliability of your physical form. The higher this repair status is the less chance of something going wrong when you push yourself to your limits.
You can raise your Repair Status by spending time and gold in a blacksmiths or a tinkerers shop.  
(how much Gold/time is something i'm hoping people have some ideas on)

You begin as a rusted hulk in a poor state of repair. Either having spent a long time deactivated or having been in use but not maintained.
Your repair Status is 50% to start.
Repair Status has a max value of 90% and will be reduced by 10% each time a Fault Roll is triggered.

ie My repair status is 50%, I do something listed below that pushes my characters body  to the limit.
I roll a D20 (any Die really that can work for that percentage, D10/D100 would probably be best),
If I roll 1-10 nothing happens, If I roll 11-20 then I have to roll a Fault Roll and my Repair Status goes to 40%.

Fault Rolls:
Things that can cause a Fault Roll are generally anything that taxes your system, pushes you to the limit etc
There are some standard events/activities that can potentially cause a fault roll and some that are unique to classes.
Standard Fault Roll triggers: 
Going through a night without resting (warforged don’t suffer exhaustion due to lack of sleep so this offers a potential consequence to you always standing guard etc), 
Being reduced to half hit points
Being reduced to zero hitpoints. (to show some kind of mechanical cost of the damage sustained.)
Falling 20+ foot. (As opposed to DMs sometimes asking for an acrobatics roll etc to see how well you land it).
Each time you are exposed to Extreme heat/fire damage?  (it worked on the T1000!) or possible just any high elemental damage, cold makes metal brittle etc.
Rolling a Nat1
(any other suggestions?)

For my Barbarian build I have thought of: Each time you enter a rage. (possibly also each time you enter a frenzied rage or attack reckless etc) 
For fighters it could be each time you use an Action Surge. etc
Any ideas for other classes are welcome, As you can spend time and gold to increase your repair status having more triggers is acceptable but im not sure how many would be too many...?

Fault Table:
I've taken a D100 Wild Magic table (Credit to 'hillermylife' on the UA reddit) and chopped off anything that was purely magical and couldn't be explained in a mechanical way.
I've made some balancing changes, mostly toning down any damage done, and added back in some new ideas, any suggestions and other ideas are welcome:

1. Recurring Fault: Roll on this table every round for 1 minute
2. Random Fault: Roll on this table again at the DMs choosing 1D4 times.
3. Engine Blown: Cast fireball at 3rd level centered on self (at half damage)
4. Both hands malfunction, Drop any held items.
5. Heart Burn: Caster breathes 10’ line of fire next time they speak
6. Memory Leak: Cast confusion centered on self
7. Backfire: Cast thunder wave centered on self
8. Field Repair Module: Regain 1D4+1 HP every round for 1 minute
9. Oil Leak: Cast grease centered on self, treat the grease as oil (ie flammable)
10. Magnetised: Each creature within 20’ must make a strength save to keep hold of weapons, 
    Any creature in heavy armour (or any weapon on a failed save) is pulled 2D10 foot towards you.
11. Erratic movement: Servos malfunction. Effect of taking the Dodge action for free
12. Messed with the Change Language menu: Roll for a random language and only use that language until next long rest.
13. Terminator Vision; adv. on Perception for 1 hour
14. Cramp!: Foot/leg malfunction, an ankle/knee locks up, move speed halved, until next rest.
15. Next turn take no action, vomit oil in a 10’ cone
16. Prototype Functions menu accessed: Teleport up to 30 feet away to unoccupied space
17. Your hands become locked in place, if holding items you cannot let go, if not holding an item your hand is locked into a fist. Until the end of your next rest.
18. Max dmg for next damaging attack/spell (if cast in the next minute)
19. Sounds Like..: Automatically (and perfectly) mimic the voice of whoever you’re talking to for 24 hrs
20. Shockwave:  PC and the next target they hit with an attack/spell are flung 10’ in opposite directions both take 1D8 thunder damage
21. Hit it and it works: Regain 2d10 HP
22. PC is distracted by complex mathematical postulation for next minute, disadvantage on attacks/saves/stealth. Mutters formulas constantly.
23. Prototype Functions menu accessed: For the next minute, teleport 20’ in a random direction each turn
24. Prototype Functions menu accessed: Magic Proof Plating, The next spell to target the PC ricochets to target a random creature within 30’
25. Prototype Functions menu accessed: Hover mode initiated: Cast levitate on self (it lasts the full 10min duration)
26. Eye malfunction – Blind in one eye, disadvantage on perception checks until next rest
27. Bumblebee: Cannot speak, until the end of their next rest
28. Smoke fumes from caster’s ears/nose for 1 minute
29. The ‘Looney Tunes’: Caster’s face blackened by small,loud explosion, no damage
30. Fire Hazard: For 1 minute, PC has Flammable Touch (affects anything not worn/carried)
31. Regain lowest-level expended spell slot
32. Burn your lowest available spell slot.
33. Burn your highest available spell slot
34. For 1 minute, you can only shout when speaking
35. Bad exhaust: Cast fog cloud centered on self, once immediately and once at any point in the future at DMs discretion.
36. Tesla: Yourself and 2 random creatures in 30’ take 1d8 lightning dmg
37. You fall Unconscious until the start of your next turn
38. Faulty coding: You are Frightened by the nearest creature until end of your next turn
39. Resistance to all damage for next 1D4+1 rounds of combat
40. Prototype Functions menu accessed: Graviton field: Everything within 20’ is pulled 10’ toward the PC
41. Overheating?: PC smells faintly like ozone & burning electrics for 1d6 days
42. Eyes Glow brightly for 1 hour. They give light like lanterns..
43. Exterminate: Your next kill causes the target’s body/extremities to fly apart
44. Overclocked: Take one additional action immediately
45. Your fists deal 1d8 bludgeoning dmg, until your next long rest.
46. Prototype Functions menu accessed: Syphon: All creatures within 30’ take 1d10 cold dmg. PC gains HP equal to their loss
47. Prototype Functions menu accessed: Adaptive camo: Become invisible/silent for 1 minute or till atk/cast
48. Messed with system settings: PC’s INT and STR swap for 1 hour
49. Can see invisible creatures for 1D4 days
50. Faulty Coding: PC becomes frightened of a color for 1 hour

Let me know your thoughts?

I get the idea of the repair status, but part of me feels like that's going to quickly become an annoyance to keep track of.

Comparing it to Wild Magic... with wild magic the player rolls a d20 any time they cast a spell, and rolls on the table only when they roll a 1

I think to simplify things I would say that if your character performs any class-specific action, they make that same roll. So anything other than the universal skills such as attack, hide, push, dodge, etc. It's still low odds each time, but it's easier to track across multiple classes that way.

I would also say that rolling a Nat 1 in any skill or ability check should require a Fault Roll as well.

I do like the idea of using this to give warforged a unique form of "exhaustion" to make resting more interesting. I think a simple way to execute this is that for each night they do not rest, they have their odds of rolling a Fault Roll increased by one. For example, after one night of not resting they must roll a 1-2 on a D20 to trigger a Fault Roll, and after two nights it's 1-3, and so on. Lol I could even see a player deliberately choosing not to rest just to increase their odds of getting a Fault Roll.

EDIT: Okay, I've been thinking about this a lot, because the idea is really fun to me. And here's what I'm thinking...

Rather than have a Repair Status, I would say that there should be Disrepair Points. Each Disrepair point would function as I mentioned before with the faux-exhaustion concept... each point increases the chance of a Fault Roll by One.

Some of the examples you listed for triggering Fault Rolls I think should instead give the player Disrepair Points. So dropping to 0 HP gives a Disrepair Point, entering an area of extreme weather grants a disrepair point, and falling 20 feet or more as well. I would say that a player could spend 8 hours (the equivalent of a Long Rest) in self-maintenance to remove all Disrepair Points, or alternatively they could spend Gold in town with the help of a blacksmith/tinkerer to get Disrepair Points removed in 1 hour (the equivalent of a short rest).  Maybe 50 GP for each Disrepair Point? I think it might also make sense to say that this repair could be done in the field if you have at least one ally who is proficient in blacksmithing/tinkering (or any other feasibly related skill)

This... all sounds fun. :D

I have a Warforged in a campaign I'm running, so this is awesome, thanks for sharing.

I also had a player rebuild a Steel Defender and they didn't QUITE nail the rolls, so this is a great starting place for me to build a Steel Defender Malfunction Table! Thanks a second time!!

so I saw this a while back and thought it fun but didn't put much mind to it at the time but I started playing a warforged and remembered this so I've been tinkering with your original idea and added some things what ya think

WARFORGED FAULTS

COWBELL

Repair Status:
Your repair status represents the current reliability of your physical form. The higher this repair status is the less chance of something going wrong when you push yourself to your limits.
You can raise your Repair Status by spending time and gold in a blacksmiths or a tinkerer’s shop.   (at player’s judgement or ref. Rate of Repair)

Your repair Status is 69% to start.
Repair Status has a max value of 95% and will be reduced by 5% - 20% each time a Fault Roll is triggered.
Once your Repair Status reaches 10% or lower you go into critical mode and rage uncontrollably for 1 hour and then black out (most likely a defense mechanism to avoid danger). 

If it ever reaches 0% you shutdown until someone can repair you.

Rate of Repair:
To understand or calculate how much you repair yourself at a smith or magical repair etc. is going to be mostly subjective. But, here are some guidelines you can follow: after repairs are made you can analyze and determine (at the DM’s discretion) how good the repairs are; amateur, mediocre, good, and expert. The quality of repair can determine the rate you are repaired per 5-10 gold. Ex. for every 10 gold you spend you are repaired 5%

For a more advanced chart you could also add on how difficult the repair would be. For example doing some armor plate repair on the surface can yield more repair per gold while wiring and hydraulics require more gold to have the same repair quantity. But of course you might think that well then the only repair i want to do is the easiest so if that's what you want ignore this section this is for the more detail oriented player.

Fault Rolls:
Things that can cause a Fault Roll are generally anything that taxes your system, pushes you to the limit etc.

To roll for faults first roll whether the fault happens with a d20. Then roll d100 to pick what happens.  Depending on your repair status it will determine if you succeed the hit roll. (ref. Fault Hit Table)

Fault Roll triggers ex:
Going through a night without resting (warforged don’t suffer exhaustion due to lack of sleep so this offers a potential consequence to you always standing guard)
Being reduced to half hit points
Being reduced to zero hit points. (to show some kind of mechanical cost of the damage sustained.)
Falling 20+ feet. (As opposed to DMs asking for an acrobatics roll to see how well you land it).
Each time you are exposed to Extreme heat/fire damage?  

Each time you enter a rage.

Rolling a Nat1
etc.

Fault Hit Table:

The repair % rounds down

Fault Table:

1. Recurring Fault: Roll on this table every round for 1 minute
2. Random Fault: Roll on this table again at the DMs choosing 1D4 times.
3. Engine Blown: Cast fireball at 3rd level centered on self (at half damage) (15%)
4. Both hands malfunction, Drop any held items. (8%)
5. Heart Burn: Caster breathes 10’ line of fire next time they speak (5%)
6. Memory Leak: Cast confusion centered on self (6%)
7. Backfire: Cast thunder wave centered on self (8%)
8. Field Repair Module: Regain 1D4+1 HP every round for 1 minute (+5%)
9. Oil Leak: Cast grease centered on self, treat the grease as oil (ie flammable) (5%)
10. Magnetised: Each creature within 20’ must make a strength save to keep hold of weapons,
    Any creature in heavy armor (or any weapon on a failed save) is pulled 2D10 foot towards you. (10%)
11. Erratic movement: Servos malfunction. Effect of taking the Dodge action for free (8%)
12. Messed with the Change Language menu: Roll for a random language and only use that language until the next long rest. (5%)
13. Terminator Vision; adv. on Perception for 1 hour (10%)
14. Cramp!: Foot/leg malfunction, an ankle/knee locks up, move speed halved, until next rest. (10%)
15. Next turn take no action, vomit oil in a 10’ cone (10%)
16. Prototype Functions menu accessed: Teleport up to 30 feet away to unoccupied space (12%)
17. Your hands become locked in place, if holding items you cannot let go, if not holding an item your hand is locked into a fist. Until the end of your next rest. (15%)
18. Max dmg for next damaging attack/spell (if cast in the next minute) (18%)
19. Sounds Like..: Automatically (and perfectly) mimic the voice of whoever you’re talking to for 24 hrs (8%)
20. Shockwave:  PC and the next target they hit with an attack/spell are flung 10’ in opposite directions both take 1D8 thunder damage (10%)
21. Hit it and it works: Regain 2d10 HP (+5%)
22. PC is distracted by complex mathematical postulation for next minute, disadvantage on attacks/saves/stealth. Mutters formulas constantly. (5%)
23. Prototype Functions menu accessed: For the next minute, teleport 20’ in a random direction each turn (10%)
24. Prototype Functions menu accessed: Magic Proof Plating, The next spell to target the PC ricochets to target a random creature within 30’ (10%)
25. Prototype Functions menu accessed: Hover mode initiated: Cast levitate on self (it lasts the full 10 min duration) (12%)
26. Eye malfunction – Blind in one eye, disadvantage on perception checks until next rest (15%)
27. Bumblebee: Cannot speak, until the end of their next rest (and everytime the try a cowbell noise plays) (5%)
28. Smoke fumes from caster’s ears/nose for 1 minute (5%)
29. The ‘Looney Tunes’: Caster’s face blackened by small,loud explosion, no damage (5%)
30. Fire Hazard: For 1 minute, PC has Flammable Touch (affects anything not worn/carried) (8%)
31. Regain lowest-level expended spell slot (10%)
32. Burn your lowest available spell slot. (5%)
33. Burn your highest available spell slot (10%)
34. For 1 minute, you can only shout when speaking (5%)
35. Bad exhaust: Cast fog cloud centered on self, once immediately and once at any point in the future at DMs discretion. (5%)
36. Tesla: Yourself and 2 random creatures in 30’ take 1d8 lightning dmg (10%)
37. You fall Unconscious until the start of your next turn (10%)
38. Faulty coding: You are Frightened by the nearest creature until end of your next turn (5%)
39. Resistance to all damage for next 1D4+1 rounds of combat (15%)
40. Prototype Functions menu accessed: Graviton field: Everything within 20’ is pulled 10’ toward the PC (15%)
41. Overheating?: PC smells faintly like ozone & burning electrics for 1d6 days (10%)
42. Eyes Glow brightly for 1 hour. They give light like lanterns.. (5%)
43. Exterminate: Your next kill causes the target’s body/extremities to fly apart (5%)
44. Overclocked: Take one additional action immediately (20%)
45. Your fists deal 1d8 bludgeoning damage, until your next long rest. (10%)
46. Prototype Functions menu accessed: Siphon: All creatures within 30’ take 1d10 cold dmg. PC gains HP equal to their loss (10%)
47. Prototype Functions menu accessed: Adaptive camo: Become invisible/silent for 1 minute or till atk/cast (10%)
48. Messed with system settings: PC’s INT and STR swap for 1 hour (5%)
49. Can see invisible creatures for 1D4 days (10%)
50. Faulty Coding: PC becomes frightened of a color for 1 hour (5%)