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El Sobrante, CA
Does the car have a guage on the cluster or just a light? If there is a guage, what does it say pressure is? Is the Check Engine light on? Check your oil, then put a manual guage on it and see what it reads. If the dash has a guage and it reads normal, and the manual guage reads normal, then you need to test the Oil Pressure Sensor circuit with a manual instrument panel guage tester.
Is the check engine light on? Do you have a scan tool available to look at data?
In answer to your question of "What else could it be?", lots. The EGR valve may not have been bad. It could be the solenoid to it. Disconnect the EGR vacuum line to the valve and install a vac guage to the end of it. You should use a long piece of vac hose to the guage so you can see it from inside the vehicle. Start the vehicle and put it in drive wirth the parking brake on and your foot on the brake pedal. With your other foot, power brake it slightly and see if the vac guage needle moves. Then go drive the truck and see if the problem still exists. If the problem is gone, and you get vacuum almost instantly at off-throttle, you need the solenoid replaced. The EGR should not open until you get to a cruise speed at about half throttle or so. That could be one issue. You could also have a bad ESC Module (Electronic Spark Control) which is the big square thing next to the throttle body and in front the coil. Also, when you changed the cap and rotor, how much corrosion or "green fuzz" was on the distributor around the control module connectors and the reluctor wheel? Any? Take the cap back off, and using a small screw driver (it must be a metal screw driver, not aluminum) place it on the reluctor and see how much resistance there is when you try pulling it away. If there is little to none, you have a weak magnet in there and need a distributor replaced. I just did one on a 1990 Burb 2 days ago for a similar problem. What does the injector spray pattern look like at idle vs throttle open? Pull the air cleaner and start the truck. Use a bright "whitelight" flashlight and look at the injector spray. Are they both spraying nice EVEN fine mist cones or is there interruptions at idle speed? Do you hear nice continuous hissing from the throttle body or is there crackling (would sound like sucking water through a straw from just above the water where you would hear sort of a crackle every time it sucked up a drop of water). If there is crackling, the injectors may simply be dirty. Check that stuff and let me know what happens.
It may not be a bad battery, but globalhelper also said that it could be a loose cable or connection. This could very well be. There was a common problem found from the factory where the positive battery bolt was cross threaded during installation (or this could happen in the field during battery replacement). Disconnect both cables at the battery and check the bolts/threads. There was also a common concern (which we saw a lot of) where the Ignition Switch has been indexed wrong from the factory or during repairs in the field. You asked about the Security System; does the Security message or light come on? Are there codes in the PCM or BCM? Vehicle Cranking is what gets disabled by the Security System. You will need a scanner (not code reader) to find out what codes (if any) are stored, and also to monitor the VTD fuel disable, Crank request signal, and starter relay command signal when the problem is happening. Give me more info.
Give me the VIN number (just the last 8 digits will do) and I will tell you the complete vehicle build for your reference.
Ok. 2 very different problems. P0141 is an Oxygen Sensor Heater Code. There is a fuse for the O2 Sensor heaters. Check all your fuses first. If all the fuses are good, you'll need a scan tool (not code reader) to check one more thing before replacing the Heated Oxygen Sensor. You need to use a scanner and look at the Ignition 1 input for voltage. You'll also need a digital multimeter to check voltage at the underhood fuse/relay box. Watch the scanner while cranking the engine. If the Ignition 1 input signal drops below 10 volts during cranking, then check the main power feed to the underhood fuse box to see if it is above 10 volts while cranking. If the 10 volts is good at the underhood box but the scanner shows that Ignition 1 input drops below 10 volts, replace the Ignition 1 relay. If those all check good, replace the O2 Sensor. As far as the ABS, codes would be helpful. If the light is on, there is a/(are) code(s) stored. I have personally done quite a few BPMV/EBCM combos. They are expensive parts, let alone the labor. And if you do the repairs yourself, you'll need a scanner that will be able to run the ABS Automated Bleed and Motor Reset features after replacement. Find out the ABS codes and let me know what they are and I can give you further diagnosis.
Is this an S-10 or Full-Size? Which engine? DEFINITELY NOT TRANSMISSION!!!!!
To check the computer(s) you need a scan tool (not code reader). You'll need to look at the inputs to the Engine Control Module (ECM, or powertrain control module (PCM)). See if it is actually agreeing with the Shift Indicator. If the PCM input and the shift indicator agree, and you are sure the Neutral Safety Switch is new and CORRECTLY INSTALLED, then you could have a wiring issue from the switch to the PCM/TCM. I would first loosen the neutral safety switch and rotate it and watch the shift indicator and the PCM inputs to see if they change as the switch is moved. This will give us a better idea as to the problem. Let us know what happens.
It sounds like the code did not clear the first time you disconnected the battery. Use a scan tool to clear it. The O2 Sensor diagnostic only runs during closed loop operation, and the PCM has to see less than 175mV for 50 seconds to be a failure. The PCM does not turn the Check Engine Light on the first failure either. It has to have 2 failures. A simple code reader may or may not have a "clear codes" function. Clear it for sure first before spending money on any attempted repairs.
I agree with pushrod that the P0430 is probably due to the misfires. Random misfires can be caused by lots of things. How many miles are on the vehicle? Do you feel stumbles or misfires while driving? Does the check engine light flash when the misfires are happening? Please give more details.