Question about 1999 Honda Civic

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Trouble Code p0420 and p1399

The problem occurred suddenly while accelerating. Now sputter badly. Ran fine before this.

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Code po420 refers to cat converter system bank1 - efficiency below threshold===causes -- cat converter---wiring---HO2S 2
code p1399 refers to cylinder no 6 misfire===causes --fuel/ ignition---VTEC system---MAP sensor--wiring---mechanical fault.

Posted on Jan 01, 2014


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SOURCE: I have an OBD 2

Posted on Jan 02, 2011

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What means catalyst Efficiency below normal

      Got it scanned and it's P0420: Catalyst efficiency below thresholdbank 1. They said it could be a bad catalytic converter or oxygen sensor. I borrowed a scanner and tried to measure the oxygen sensor readings over time.Sep 8, 2011

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Jan 26, 2016 | Cars & Trucks

1 Answer

P0420 p1399

99 civic? ever tune it up, or run the cam belt fore ever?
you do now, ignoring the bent, ends in bent valves, right?

which engine option, cylinder count and displacment. USA 1.6L? I-4?

misfiring. ever tune it up this decade? or 60k miles? with cam belt.
1399 is misfire (let me guess, the CEL flashes )
do not skip the belt if due, or you will lunch the engine.

420 is: (ID IGNORE THIS, till the MISFIRE ENDS !!!)
Possible causes
- Three way catalyst converter Bank 1
- Exhaust tube
- Intake air leaks
- Fuel injectors may be faulty
- Fuel injector leaks
- Spark plugs may be faulty
- Improper ignition timing
- Engine Control Module (ECM)

(the bar room answer for 420 is, the CAT is weak, )
the autoshop class>? ASE.. if the cat is or was tortured (misfire)
then fix that first and pray the cat bakes dry and passes 42x codes
if bake many dry, and 420s cleared.
what happens is you overload the CAT with raw fuel, ignore it.
the CEL flashing (USA) means you are KILLING THE CAT.

now we cover, misfire
there are huge possibles.
in a big shop we connect 5 gas to tail and start there .
in a lesser shop, plenty. we check closed loop at idle
if not in closed loop at idle, we fix that first. (easy ,car is parked)
(later closed loop at 40mph light flat cruise, steady right foot) no hills.
Misfire, :
Bad engine, bent a valve> do to ignoring the belt. got vacuum bounce?
bad spark
bad injection rates, or injectors clogged ,dead or weak or leaking.

me, i watch pendings to see of P0301 , to P030x (x is cylinder num)
if the scan tools shows any misfire codes, i do a leak down
on the cylinder that misfires, 300s is random.

lots of ways, EGR stuck open
AFR runing lean, scan too sees that...
AFR stuck rich, same.
the best order is this. (no clues with extra DTCS
compression test. and leak down. if smart.
spark tests.
last fueling, rates, (fails closed loop at idle spark good , engine good.
i do the the injector leak down tests. and balance,

i also do the cAT melted test. cat back pressure test
less that 2psi at 2500rpm.

Dec 31, 2013 | 1999 Honda Civic

2 Answers

1986 f150 4x4 5.0 manual runs and ides fine but when you accelerate and shift gears it jerks and sputters

I am curious to know what the scan told you when you originally checked it? A lot of people spend way more money than they should changing parts and guessing at the fix. Tell me a little more about the what the scan results.

Dec 09, 2013 | Ford Cars & Trucks

1 Answer

My 1993 honda accord sputters when accelerating

If you can't answer when the last time you changed the fuel filter it might not be a bad idea to change it. It could also be a vacuum leak as older cars ran on vacuum controls for emissions and things like that. listen for a possible whistle when accelerating or air being sucked in like a vacuum cleaner. Could also be a bad map sensor, or O2 sensor.

Sep 18, 2012 | 1993 Honda Accord

1 Answer

Sputtering jimmy

Have you ran trouble codes yet? Most auto stores have a reader you can use now a days. Problem sounds like fuel overload at lower rpm and can be caused by bad throttle position sensor or mass air sensor.

Jul 29, 2012 | 2001 GMC Jimmy

1 Answer


P0420 isnt a misfiring code; this code P0420 is refer at Catalyst System Efficiency Below Threshold (Bank 1).

Feel free to contact us if you need additional details to volve it clicking the link.

Oct 24, 2011 | 2002 Volvo S60

1 Answer

I have a 1997 Jeep Cherokee Sport 4.0 I6 with about 155k miles that is having intermittent issues. I just recently had to replace my muffler because of the car backfiring. This problem doesn't happen...

Have it checked with a computer diagnostics. It sounds like it could be a bad sensor, quite possibly your oxygen sensor. A bad sensor could cause the computer to think your engine needs more fuel.

May 27, 2011 | 1997 Jeep Cherokee

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