What is FAILSAFE (Suzuki's name for this, in U.S.A) +  a  Backup mode? 
Scope: 1989 to 2004
The best statement on FAILSAFE, ever, is here , regardless of  names
The newer cars are easy to see Failsafe mode is active, but the old cars, pre 1996 are a pain. (if there are no DTC codes? , it still can be in limphome (kinda).)
My friend calls it Grenade mode, How apropos !

Pre 1996+ cars use this page for DTCs.
The old cars will  blink the CEL lamp to tell you the Failing DTC error number  (on demand)
In 1996+  all scan tools sold in the USA fit all USA cars.  OBD2 magic.  (and as cheap as $10 tool)

No matter what year, check all fuses first, then work the DTCs errors out of your car.  (fix them all, first)
On newer cars 1996+, I look for , the  oddly acting or dead sensors.

1989-95 (USA)
If car is very old, discovery of Failsafe mode can be hard, if there are no DTC errors. LOST? IS it ? Fueling out of control?
What I do, no tool, pre 1996 is to measure every sensor input 1 by 1, and be sure they all work right. (no scan tool) Then  I check all Actuators. "a lost weekend, again?"

In General:
I will not use the slang words Limphome below, only the 3 Suzuki book names.
But there are 3 , Major modes.  NORMAL, FAILSAFE and BACKUP (my friend calls  the  last name  Grenade mode

In  Backup mode the Check engine lamp (CEL) may even be dead, just keyon.  The engine might start, but floods instantly and stalls..
Proving this is hard, (Suzuki doc.'s are no help here) you must kill the ECU some way to prove this,, sorry I refuse.
Backup mode, may cause all commications to go dead with any scan tool . (sure will on OBD2 , seen it 100s of times)

Backup  is a MAIN processor malfunction mode and might use a  back up chip. {historically true for all old EFI cars back to 1988}  (or some magic secret Suzuki logic, or other, but is seriously DEAD ECU)
Suzuki's big secret.
In about 1999 or so, Suzuki moved Backup mode to just a line item in the Failsafe list. (backup (true) is now gone, and is just a listed item under Failsafe, listed as dead processor)
SUZUKI does not cover backup mode at all. (books)
End backup...Back to FAILSAFE.

The PRE 96 OBD1 cars Failsafe can be very punishing,  and may go way rich, retarded timing and limited RPM.
 In 1996 the USA invented, OBD2  had some goals  , are to be smarter and with less punishment. Less melted CAT's (less smog too)

One example ;a dead MAP. 100% dead. CEL glowing,  and DTC errors, shows DEAD MAP.  The FAILSAFE mimic is horrid ! Try it.

In all cases, predictions are impossible,  are there more than one failure?, does one failure try to hide another?, did you know if AFR goes out of spec  and  a tad rich ,you get more engine power?
On the same track, if the injectors are clogged, forcing failsafe mode, can in fact, increase power, as the AFR goes from way lean to NICE ! (better) .
This is one good reason to check fuel pressure and leak down rates on all injectors before condeming the ECU or its limited number of sensors. 
The ECU can not know the injector is weak, or fuel pressure is too high or low , in most cases. (on newer cars , we do have , for example, lean codes, etc)
One other trick is to unplug the 02 sensor (front) if you suspect there are exhaust leaks, causing the 02 lie.  This can bring  the EFI from way rich, to far better. (as  a test)

FAIL-SAFE MODE: (OBD1 and 2 vary in methods and crudeness)
See more OBD facts page:
The ECU discovers a dead critical sensor . (in most cases or is lost)
  • Emulates (or mimics ) the dead sensor 
  •  I have unpluged every sensor 1 by 1  and scanned the ECU to see the results, but never pull them all that  dont work.
  • At fail time, it Stores the DTC code for that sensor. (a dead sensor)
  • Fail-Safe can operate the engine way rich, and or retarded timing and/or top RPM speed is limited, and/or no more idle controls (ISC is dead)  .
  • The CEL (lamp) may glow running.  (and limphome trips for dead sensors) 
  • The ECU can get lost, if it  knows  it is out of control, but does not know why, (for example, fuel pressure at 60 PSI)
  • True closed loop  MAY END,  but the 0 may still  swing correctly , LTFT + STFT  might be DEAD , proving true loss of closed loop, status.
  • While it , mimics any bad sensor inputted. ( the results can be poor, fair or good, who knows?, some are fixed values, other sensors dead, use tables or RPM)
  • The only clue might be , that FAILSAFE is enabled, is loss of idle controls. (and/or  poor fuel mileage)
  • No matter what?, a real scan tool is the best tool, for you to discover all these possible symptoms and ECU reactions.
  • Lost the ECU can get LOST, case in point fuel pressure at 60 PSI (say a stuck FPR) and now the ECU no longer can control fueling, it will be lost. (it will just stick way to rich (AFR)) Ther is no DTC for LOST. It acts like FAILSAFE, (rich) but is not really.  Like Kite with no tail, or a farmers water pump wind mill with no tail. (just flaps about. aimlessly)
 DTC = Diagnostic  trouble codes.

Starting  In 1996 most of all the above is moot, after all you connect that $9 OBD2 scan tool and see all the PID and DTC information and it's all 100% clear what is going on ECU wise; for sure.
The 96 and newer cars can flash  the  CEL lamp driving (nocode), for a CAT burn up warning. (the operators guide , in Oh'bummers English, says "to drive slow and park your tailer and get service soon"
Starting in 1996 (Many more DTCs were added, and many new OBD monitors and way less punishment "features"{sic})
My new Jeep car has 1000+ DTCs  the old Suzuki  had 31.

The hard thing about OBD1 cars is working blind. No scan tools. (ok ,1  hackers tool ,sold in the whole world , and off shore to boot)
Is the ECU seeing bad sensors  (CEL Glows? running?) or is it lost?, or is engine so messed up, that the ECU can not control it? (eg. a dead cylinders)
If the 8v ECU has  dead injector, the  engine stops, period. (same with zero fuel pressure)
There is no DTC to tell you that. No injector pulse read back, not on the old dumb system, nor any fuel pressure sensors. (a bad idea from day one in my opinion)

If the 16v has a dead injector you have a 3 cylinder engine, it runs ratty, misfiring , shaking, and the 02 sensors, see tons of oxygen and the ECU goes nuts and injects too much fuel to the good injectors.
Same deal, No DTC telling you the injector line is dead. but may get oxy sensor errors, and rich errors, if car is 1996 and newer and misfire errors.
At no time, will most any old ECU tell you a sensor is out of calibration , usually only DEAD sensors. ( one execption on USA cars, is the  weak EGR flow code 51)
That is why, I look at the live data and compare it to good data.
To see if the sensor reads wrong, (as the ECU sees it).
Case in point, coolant temperature is really at 180F, but the ECT reads 150F.  (the ECT is bad or air pockets are there, RADiator NOT FULL)
The ECU will never leave  warm up mode at 150F  MPG (fuel economy) goes horrid. 15mpg is common.
The old ECU will never ever tell you this happened. (unless the wires fall off the ECT)
My newer 2004, H25 engine does show, coolant late to proper temp.'s. a DTC just for this error.

With OBD1, you have to do all this work by hand with meters and gauges. 1 by 1, checking it all.
But do consider all DTCs thrown, seriously.

These old ECU just hate: (go nuts or gets LOST)
Too low /too high fuel pressure,(bad FPR) dead injectors, leaking injectors, dead cylinders (compression) and the most common of all, never changing the Cam belt and x4 cylinders at 80PSI compression.
The old ECU will not show DTC errors with many catastrophic errors in the engine, or for example a ECT sensor reading, -30F in Key West.FL..   (it's just clueless)

Factoid 1:  The 4sp TCM Tranny brain  has  failsafe too, It skips 2nd gear. and steady glows or flashes the OD lamp, as one more clue.

FUEL TRIM !: The bread & butter of EFI.
means short term fuel trim and LTFT means long term fuel..... trim.
The whole purpose of good fueling rates, are a working fuel trim system. for lowest smog, highest MPG and max engine life.
I check LONG fuel trim , every time I look. (that it is running and working and now is in  the center of it's range) Plus closed loop status,  at idle and light steady cruise.

The OFFICIAL SUZUKI  pages, annotated in blue and red below:  (very brief, for such a profound impact !)
Below is the anemic Suzuki doc's on Failsafe  and backup ,

The left side image below,  is old  OBD1 cars (89-95)                                                                   
Translating, old sensor names,  Pressure sensor means MAF/MAP  (air meter/baro sensor)  (stuck at 0v or at 5v the are deemed bad.)
WTS  (water temp sensor)= ECT  (called engine coolant temp, today)  (deemed dead, if open or shorted only, both temp sensors)
 ATS (air temp.)= IAT (called Intake Air temp , today)
 The TPS throttle angle pin,  is deemed dead (below 0.2volts or above 4 volts) approx.  (it shorted or open,)
See the word "Certain"(to whom?) below, they really mean REDUCED. (in some cases, grossly reduced)
Add MAF to the word MAP below, "MAP/MAF" as the case may be..
The backup mode , we think the CEL lamp goes DEAD totally.
The TPS is on the throttle angle side failsafe goes active, the idle switch does set a DTC but does not go to failsafe mode (I think)

Photo 1: from FSM.         USA BOOKS!                                                                                              Photo 2: FSM. (actually copy and pasted)

Click the above image to zoom in.
1997 list of fail-safe  Active DTCs  , OBD2  (96+)

MAF = P0102/103
IAT    = P0112/113
ECT   = P0117/118
TPS    = P112/123
VSS1  = P0500  ( speedo vss) Vehicle Speed sensor = VSS
TR    =   P0705  (A/T Range shifter output  in conflict)
VSS2 =  P0720 ( 4sp Vss tail speed sensor dead)
SS#1  =  P0753   (shift solenoid 1 bad)
SS#2  =   P0758   (shift solenoid 2 bad)
TCC   =   P0743   (torque converter clutch malf)

there are no optional or extra sensors, each must work correctly.
The above is revealed in the Suzuki suppl. serv. manuals.
More on Mimic modes: (tests I've done, etc)
The 16v ECU ,does run with the MAF unplugged (with DTC warnings for each, the 8v runs with the MAP unplugged (both horridly)
For fun , on my 8v (91) I unplugged all sensors 1 by 1  and found out when I last unplugged the TPS, car died.   The TPS is used to mimic the MAP so... it dies.
Tests that I've done show that a dead MAF (or MAP on TBI 8v) the ECU goes to limphome and uses the TPS and RPM as the backup air metering device.
It  can emulate most the  sensors, but not all sensors at once.  (seems logical for sure)
In some cases (ECU) a pulled ECT , the ECT now uses a fixed temperature, or on some cars a table based on time .  Time from startup .
Pulling he CMP sensor kills spark dead , (and injections 2nd)  and sets a DTC for that. The ECU even reports CMP drop outs of 1 in 10. (even cranking, !!!) Cam sensor.
The 16v (96+) can run with a dead CKP (crank sensor), but kills dead the OBD2 "MISFIRE MONITOR", and that fails you for smog test. "dead monitor and DTC for CKP:
CKP = crank sensor, below front bottom of oil pump. Starting in 1996.  (USA)

This is Not bad, really, these False-safe features, but Beats walking home in the snow. ? or worse?
On new cars the car goes to RPM limits and Speed limits (limphome extremes) at the first sign of idle speed issues. (my car has 4 TPS sensors, a 2008 JEEP or the throttle motor sticks !)

Fail-safe and for sure Backup mode:
The engine performance is greatly reduced and smog emissions will be horrible and the CAT may burn to a crisp.

ONLINE information from any of the 3 top USA sites, Alldata/Michell's or Chilton's commercial data bases:
If you go on line to FSM live data base and watch the live video:  (on line docs for pre 99 Suzuki's are very limited, but Alldata.com is the best., last is Chiltons) IMO.

It says,  1999 and newer Suzuki's,  "The RPM is limited to 2500 RPM (to save the CAT from destruction !)  Timing is fixed, injections are fixed and the ISC will be Inoperative."
By fixed, they mean very crude fixed values , timing and fuel tables  based (indexed) off simplistic RPM readings.
ISC = idle speed control solenoid.

No idle controls, is a dead ISC, and when dead the RPM drops, to the wrong RPM with accessories loads, Head lamps and blower fan?
When they say tables, that means, it's not using live parametric, from live sensors, but now, instead, uses crude fixed tables, indexed by RPM, etc. (very crude)
Some time the  IAC and ISC names get mixed, this is because some cars have only one actuator and they call it IAC.
But this car has 2, the ISC is electric and the IAC is thermal only. (mixing up the names is ok,  but the electric, idle controls is what controls idle speed hot)  See jargon here (SAE)
Suzuki changed the names 3 times here, too.  1986 to 2004

My 04 Suzuki Grand Vitara states. Page 1a-24 FailSafe table. (in short) (Can enter Failsafe and exit at will, on the fly)  NOTE THE DATA HERE, a new thing for SUZUKI , FACTS !!!

Dead sensors.  ( 1 at a time?, but for sure no all.) I found this by reading all FSM ,all books on Suzuki)
  • MAF,  now TPS based.
  • IAT,  set to 40F.
  • ECT,  set to 80F fixed  A/T set to 31F.
  • TPS , is set  8 degrees
  • VSS ,  set to 10km/hour. IAC limited air.   (this means the ECU mimic's speed at a fixed speed, limiting power for sure.)
and lists 5 more A/T transmission failures,  I skip them. It is  covered the FSM, on your tranny and varies by year and by tranny model.

Grenade mode: (a.k.a.) also is "Punishment" mode.
  • Drops MPG from 30 mpg to 15,  so the gas bills double.   (yes, grenades your wallet )
  • Fouls the spark plugs fast.
  • Burns the CATALYST to a crisp or melts it, kiss off a $600 CAT.
  • Washing the cylinders down with raw fuel ,causes fast wear of the piston rings and walls.
  • Causes bad air for the humans to breath. (at CAT death soon to happen as it glows cherry red.)
  • RPM limits. Can't pass logging trucks, now?
  • Lost 1/2 your engine power.  4speed  trans, seems stuck in 2nd gear.?
  • Exhaust manifold cracks? due to its glowing red hot ?
  • RPM controls were suspended, now car stalls? or engine races, or you get rear ended, at this stall?  (ISC IS now DEAD)
  • I have to turn off A/C or the Engine stalls?
  • I fail Smog and now my state smog station lists my car as gross polluter, and now I must get mandatory smog checks every year as a door prize.  (Calif, blues?)
  • Engine now back fires, due  to a small crack in me. muffler, I ignored, so now it exploded and is now just metal chunks on highway 1. yes, she grenaded.

For sure Suzuki does a poor job of documenting FAILSAFE, and for sure BACKUP mode,  they only just hint at the rules and actions taken.

The only good thing about this, mode, is you are not walking home,  That be IT.

The only true way to FULLY understand fail-safe is ECU/PCM processor ROM code dis-assembly, but in USA, that is illegal.
But, some have done this on early ECU;s and is really helpful. Thanks to them!

ver 14   new page 2-22/13 , revised 6-28-2014 , major edits. I find the OBD1 issues, very interesting, due to Suzuki's penchant to secrecy. (near zero documentation)