94racing

yes i know its only my second post but i decided i need help pretty quick because im driving on of the fleet trucks to school everyday
so my silverado has a miss and its throwing po420 (bad cat effeiciency) usually means cat is bad i know. the motor is a 5.3 m code. so far it has had spark plugs, and wires placed in it. i took the old sparkplugs and set them in the wires and i had spark on all cylinders which means coils are good enough to make spark. so now i think its the injectors in all honestly but i dont want to go off blowing more money on injectors if i dont have to or i could take it to the dealership (no warranty this is a rebuild flood truck....) and spend all the rest of the money that i dont have. (im a 20 year old in college)
if you have any questions for anything i left out ill be looking around the site and ill reply.
thanks

EinST

Care to elaborate? What is this "miss" you speak of? Single cylinder, multiple cylinder, flashing CEL? What are the drivability symptoms?

94racing

Seems to be a single miss but in all honesty i cant tell.
Check engine light is solid. Po420 is the only code that shows up. Its more than drivable but i dont want to melt down my cat if i dont have to if its not junk yet. And you can tell its missing by touching the truck or listening to it

94racing

Also has preignition over 4k rpm. When it finally shifts to second and hard at that i might say it says service traction control and stabilitrac and flashes. Then returns to function after a few seconds. This truck is a flood truck btw but has new wiring and tranny. Ive drive it for 20000 miles without an issue till now

EinST

Okay, so it's not a P0300 something misfire code but it just idles not as smooth? You can pull the spark plug wires one at a time to see which cylinder is not contributing as much. Anyway, what sometimes happens with P0420 is that the catalyst substrate breaks up and blocks the converter outlet pipe. This creates an abnormally high back pressure and results in the engine bogging down above certain RPMs.

MDTAHOE

Bulletin No.: 06-06-04-046

Date: September 12, 2006

INFORMATION

Subject:
Information on Engine Misfire MIL/SES Light Illuminated or Flashing DTC P0300, P0301, P0302, P0303, P0304, P0305, P0306, P0307, P0308, P0420 or P0430
Models:
1999-2007 Cadillac, Chevrolet, GMC Full-Size Pickup and/or Utility Trucks
with 4.8L, 5.3L, 5.7L, 6.0L or 6.2L VORTEC GEN III, GEN IV, V-8 Engine (VINs V, C, T, Z, B, 3, M, 0, J, R, U, N, Y, K, 8 - RPOs LR4, LY2, LM7, L59, L33, LC9, LH6, LMG, LY5, L31, LQ4, LQ9, L76, LY6, L92)

with Active Fuel Management(TM) and E85 Flex Fuel

If you encounter vehicles that exhibit the above conditions, refer to SI for the appropriate DTC(s) set. If no trouble is found, the cause may be due to an ECM ground terminal that has corroded with rust over time. Inspect the main engine wiring harness ground terminal (G103) for this condition.




The wire terminal (G103) attaches either to the front or to the rear of the right side cylinder head, depending on the model year of the Full Size Pickup and/or Utility Trucks. If the ECM ground terminal has been found to be corroded, then follow the service procedure outlined in this bulletin to correct the corrosion issue.




Remove either the nut or bolt securing the main engine wiring harness ground terminal (G103) to the right cylinder head. Refer to the above illustration to determine where the ground is located on the vehicle (1).

Remove all rust from the ground terminal, the cylinder head and the retaining nut or bolt.

Position the main engine wiring harness ground terminal and install the nut or bolt.

Tighten:

Tighten the retaining nut or bolt to 16 N.m (12 lb ft).

Apply some type of electrical moisture sealant to protect the harness terminal from further corrosion.

94racing

update:
codes
po300-random/multiple cylinder misfire
Po420-catalystic efficiency below threshold-bank 1
Po420-Pending- " "-" "
Po121-Pending-Tps/Pedal position sensor A CKT Range/ perf

ok so i did as MDTAHOE said and it ran good at first start and then started stumbling over itself again. i had my dad confirm is has a miss also just so we know its a miss.
and i checked the spark again have spark on all cylinders.

would the next step be having the dealer check the injectors?

also tested the injectors after up to engine temp and drove 7 miles. both cats max temp was 222 dergrees farenheight.
thanks

EinST

Once at normal operating temperature, the upstream O2 sensor will normally bounce back and forth between .2-.9 volts. When the catalytic converter is doing its job, the downstream O2 sensor will read steady at ~0.45 volts. P0420 says it bounced around instead. If you have access to an OBD scanner that can monitor O2 sensors, you can verify that it does bounce around. As for the abnormally high back pressure, the only reliable way is to connect a pressure gauge usually in place of the O2 sensors (measure both pre- and post-converter). P0300 and possibly P0121 can be caused by it.

94racing

I will se we are currently looking into getting a obd2 device so i can keep it connected while the vehicle is being driven. It normally takes about 50 miles for the cel to come on.

94racing

Ok so i stopped at advanced today after the light came back on and i got
Po303 cylinder 3 misfire. Ordered and injector to adress this
Po420- cat deffeicency bank 1
Po420- "" " pending
Po121 pedal ckt blah blah blah
Cat temp driver side 964 Pass. 912


02 voltage reading was spanning on
(v)
Df.0.25-.9 Pf 0.18-.76
Dr. 0.73 Pr. 0.72

Going to put an injector in like what the dealer mechanic told me and see if the miss disapears