New Purchase with two codes showing up

Discussion in 'Chevy C/K Truck Forum' started by murray92589, Mar 19, 2012.

  1. murray92589

    murray92589 New Member

    I have two codes showing on my new 96 1500. they are P0300 and P0420. The check engine light comes on intermittently, so I'm not sure if they are old codes that never got cleared or not. I'd appreciate any help!

    1996 1500 W/T V6
  2. Red Z71 Max

    Red Z71 Max Rockstar 4 Years ROTM Winner 1000 Posts

    I believe this is related to a misfire. You may need to replace plugs and wires.
  3. redreatta

    redreatta New Member

    p0300 code,The crankshaft position sensor and camshaft position sensor detects a misfire. The Vehicle Control Module (VCM) monitors the speed of the crankshaft. The VCM detects a deceleration of the crankshaft that is not associated with normal engine speed reduction. In order to determine if a misfire occurred, the VCM compares the deceleration information to the engine speed and engine load. If a misfire event is determined, the VCM compares the crankshaft position to the cam sensor signal in order to determine which cylinder misfired. The VCM stores the information in separate accumulators for each cylinder. Upon completion (or failure) of the test, the VCM evaluates the number of misfires in each accumulator. If the accumulators are somewhat even or if 3 or more cylinders are misfiring, then this determines that a random misfire has occurred. The VCM also utilizes the input from the ABS wheel speed sensor in order to determine if a rough road condition exists which could cause a crankshaft acceleration and deceleration. If a rough road condition exists, the diagnostic will not run. This DTC is a type B DTC.


    • 25 cycles have elapsed since Transmission shift (automatic only).
    • No Throttle Position (TP) sensor DTCs.
    • No Mass Air Flow (MAF) sensor DTCs.
    • No Ignition Control (IC) sensor DTCs.
    • No Vehicle Speed (VS) sensor DTCs.
    • No Crankshaft Position sensor DTCs.
    • Rough road is not detected.
    • The Engine Coolant Temperature (ECT) is between -6.75°C and 95.2°C.
    • The engine speed between 300 RPM and 5600 RPM.
    • The system voltage between 9 volts and 16 volts.
    • The positive throttle position change is less than 4.9% for 100 msec.
    • The negative throttle position change is less than 2.9% for 100 msec.
    • A misfire is detected.
    If the VCM determines that the misfire is significant enough to have a negative impact on emissions, the VCM turns on the Malfunction Indicator Lamp (MIL) after the misfire has been detected on 2 non-consecutive trips under the same operating conditions. If the misfire is severe enough that catalytic converter damage could result, the MIL flashes while the misfire is present.

    The VCM turns the MIL off after 3 consecutive driving trips without a fault condition present. A history DTC will clear if no fault conditions have been detected for 40 warm-up cycles (coolant temperature has risen 40°F from the start-up coolant temperature and the engine coolant temperature exceeds 16O°F during that same ignition cycle) or the scan tool clearing feature has been used.

    The Misfire Index counts the number of misfires. The scan tool can monitor the Misfire Index. There is a current and history misfire counter for each cylinder. Use the current misfire counter in order to determine which cylinder is misfiring.

    Many different conditions could cause an intermittent misfire.

    Check for the following conditions:

    1. Check the spark plug wires and the coil wire for the following conditions:

      • Ensure that the spark plug wires are securely attached to the spark plugs and the distributor cap.
      • Check the wire routing in order to ensure that crossfiring is not occurring.
      • If the misfire occurs when the weather is damp, the problem could be due to worn plug wires.
      • In order to test for this condition, spray the wires with water and with the engine running, watch for spark to jump from the wires. If a spark is visible, replace the wires.

    1. Check for contaminated and a low fuel level and the following conditions:
    2. Check the fuel condition and quality. Dirty or contaminated fuel could cause a misfire condition.
    3. If the fuel level is low, contaminants in the bottom of the fuel tank could enter into the fuel metering system.
    4. For more information, refer to fuel supply system.
    The numbers below refer to the step numbers on the diagnostic table.

    1. If DTCs P0337 (Crankshaft Position Sensor Circuit Low Input) or P0338 (Crankshaft Position Sensor Circuit - High Input) are set, this could result in a misfire condition.

    1. In order to duplicate the conditions under which the misfire occurred, it may be necessary to drive the vehicle and monitor the scan tool DTC Set This Ignition Cycle variable.
    2. When checking the spark at the spark plug wires, the spark should be consistent. A few sparks then nothing is no spark.

    1. At this point, the ignition system is OK and the problem may be in the fuel system. Fuel System Diagnosis must be performed in order to determine the cause of the problem.
  4. redreatta

    redreatta New Member


    In order to control emissions of Hydrocarbons (HO), Carbon Monoxide (CO) and Oxides of Nitrogen (NOx), The system uses a three-way catalytic converter. The catalyst within the converter promotes a chemical reaction which oxidizes the HC and CO present in the exhaust gas, converting them into harmless water vapor and carbon dioxide. The catalyst also reduces NOx, converting it to nitrogen. The Vehicle Control Module (VCM) has the capability to monitor this process using the HO2S (Bank 1, Sensor 1). The HO2S (Bank 1, Sensor 2), located in the exhaust stream past the three-way catalytic converter, produces an output signal which indicates the oxygen storage capacity of the catalyst; this in turn indicates the catalyst's ability to convert exhaust emissions effectively. If the catalyst is functioning correctly, the HO2S (Bank 1, Sensor 2) signal will be far less active than that produced by the HO2S (Bank 1, Sensor 1). If a problem exists which causes the VCM to detect an excessive HO2S (Bank 1, Sensor 2) activity outside of an acceptable range for an extended period of time, the VCM sets the DTC P0420. This DTC indicates that the three-way catalytic converter's oxygen storage capacity is below a threshold considered acceptable. This is a type A DTC.


    Converter Warm Up Test

    • A Closed Loop.
    • The commanded Air to Fuel ratio = 14.7:1.
    • The Mass Air Flow (MAF) is greater than 15 g/s.
    • The predicted Catalyst warm up temperature is greater than 450°C.
    Converter Warm UP Test Passed

    • No Vehicle Speed (VS) sensor DTCs.
    • No Throttle Position (TP) sensor DTCs.
    • No HO2S DTCs.
    • No misfire DTCs.
    • No Manifold Absolute Pressure (MAP) sensor DTCs.
    • No fuel trim DTCs.
    • No Intake Air Temperature (IAT) sensor DTCs.
    • No Engine Coolant Temperature (ECT) sensor DTCs.
    • No MAF sensor DTCs.
    • Engine coolant temperature is above 75°C (167°F).
    • The vehicle is in a Closed Loop fuel control.
    • The above conditions met for a period of time in order to ensure a warm catalyst (at least 2 consecutive minutes).
    • The calculated engine load is steady.
    • Vehicle speed is steady between 20 mph and 70 mph.
    • The IAT is at least -9.75°C.
    • The MAF sensor is less than 50 g/s.
    • The engine speed is no more than 4700 RPM.
    • The TP sensor is more than 1.9%.
    • Bank 1, Sensor 2 HO2S average is ±39 mV from Bank 1, Sensor 1 average voltage.
    The VCM will turn ON the Malfunction Indicator Lamp (MIL).

    The VCM turns the MIL off after 3 consecutive driving trips without a fault condition present. A history DTC will clear if no fault conditions have been detected for 40 warm-up cycles (coolant temperature has risen 40°F from the start-up coolant temperature and the engine coolant temperature exceeds 160°F during that same ignition cycle) or the scan tool clearing feature has been used.

    Difficulty running the OBD II status DTC P0420 test may be encountered in areas where the test conditions cannot be maintained easily, especially in urban areas.

    The numbers below refer to the step numbers in the diagnostic table.

    1. This table includes checks for conditions that can cause the three-way catalytic converter efficiency to appear degraded. Inspect and repair exhaust system as necessary. Inspect the HO2S (Bank 1, Sensor 2) pigtail and engine harness for any damage that can cause an intermittent fault in HO2S (Bank 1, Sensor 2) circuits.
    CAUTION : In order to avoid damaging the replacement three-way catalytic converter, correct the engine misfire or mechanical fault before replacing the three-way catalytic converter.

    1. If the three-way catalytic converter needs to be replaced, make sure that the following conditions are not present to caused the catalyst to be damaged:

      • Misfire.
      • High engine oil consumption or coolant consumption.
      • Retarded spark timing or weak spark.

    hope this helps some
  5. murray92589

    murray92589 New Member

    Sounds like I need to check plugs and wires, replace anything thats bad, then if that clears the code, replace the catalytic converter? Also I am burning or leaking coolant, my heater isn't working, and I'm hearing gargling under the dash.. Is this related or could it be a bad heater core? Is there a ignition coil or distributor on these trucks? I just bought mine, and haven't worked on one before.
  6. redreatta

    redreatta New Member

    i am not a "master mechanic" but.........i would look at plugs, wires, and distributor cap and rotor button. AC delco pulgs for sure! i bought a tune up package from a well know aftermarket race parts catalog. make sure you put anti-seize on the plug threads and some dialectic grease( not sure it is spelled correctly) on the plug boots_(inside the boot end )

    for the 420 code it could be a number of exhaust things. usually it revolves around the O2 sensor. go to a auto parts store and they can read the code with live data and tell you if it is bad.

    my truck used alot of coolant and gurgled and no heat so i replaced the heater core,radiator,thermostst and blower motor. cleared up all the issues. (blower motor was hosed and i had no heat)
  7. Dana W

    Dana W Rockstar 3 Years 500 Posts

    The gurgling could just be from the coolant being low enough for air to circulate around with the liquid. The heater core may not be completely full which will reduce the heat output significantly. There may not be anything wrong with the heater itself but you still have a leak somewhere.

    I once had a crack in my recovery tank that took a while for me to even think of checking.
  8. murray92589

    murray92589 New Member

    Im pretty sure the plug wires were replaced recently, I'm going to try plugs and a hole distributor, not just the cap... I was told the stock distributor isn't the best, because its made with mostly plastic, and I should upgrade to one made from aluminum.

    I don't think there is an actual leak, I filled it up and it seems to be staying that way, is there anyway some of it could be burning off somewhere? I still am not getting heat.

    I'm going to take it to a shop and see if they can diagnose the 420 code, the parts stores weren't able to tell me much.
  9. redreatta

    redreatta New Member

    sure, if you have the money to replace it than i would do it.have you checked your oil? hopefully there is not a gasket leak. and the 420 code.....i would take mine to the shop also, to many things to check, save yourself the aggrivation.
  10. murray92589

    murray92589 New Member

    oil looks like it was changed recently. We'll see what the shop says on friday... I hate buying parts and not fixing the problem.

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