ENGINE CHECK CODE .... HELP, TRIED BOTH

[Guest buickapollo455]

Help, engine check for engine temp sensor / thermostat came up P0128, we have changed the sensor at intake base 3 times now, what now???

help, our CAT plugged due to poor plug carbon. So we changed out the cat with aftermarket CAT, pipe clamps were muffler "mudded" with muffler cement, the front was welded and mudded. would this be a bad o2 censor front or rear of cat? change plugs, cleaned out throttle body, changed the EGR too, HELP, dont want to do cat again. code p0420

car : buick century 2001 3.1, v6

email us help............. c_geske@eartlink.net

[Guest Jim_Edwards]

Let me say right off that I'm not familiar with your specific automobile, but with that said I think I would be checking the cooling fan relay, the fan motor, and if the fan has plastic blades I'd check for blade distortion, which is very common with earlier electric fans.

Jim

[Guest buickapollo455]

fan is good, system says under code , the engine is too cool in exhaust readings. Car runs great, took out cat when plugged, to see if that was it. and it was the plugged cat. did total toon new wires plugs, new temp sensor, new cat, yet still get the signals of cat or bad reading of sensor before and after cat dont read code free.

Anyone have GM code help work?? stumped this time.

[NTX5467]

On some engines, there are TWO temp sensors . . . one runs the temp gauge in the instrument cluster and the other one supplies information to the ECM. I doubt that such a sensor would cause the issue with the converter clogging unless the flaky signals to the computer were not letting the computer go into closed loop (during open loop, i.e. warm-up right after initial start-up) the computer used default "a value" to run the engine until it gets hot enough to go into closed loop operation. PLUS, if the engine was running THAT rich, it should have been highly obvious (poor fuel economy AND visible exhaust smoke), I suspect. I suspect the vehicle still has a spec-195 degree F thermostat in it?

Usually, the exhaust sealant you reference is not used with welded joints, just clamped joints. I like to use it as it's extra insurance of not having any leaks, but if the pipes are a good "mate", then the indention from the clamp should be good enough for an effective seal. I also hope that it was labeled "Safe for All Oxygen Sensors"!

In the world of computer codes and OBDII, the front oxygen sensor(s) is/are used to control the fuel curve/map, but the rear sensor is to ensure that the "catalyst efficiency" is what it needs to be. In other words, the only reason for the rear sensor is to make sure that the catalyst is functioning as designed--period. ANY trouble code for a non-functioning or "out of range" oxygen sensor will detail which oxygen sensor (i.e., "pre-cat","post-cat", "Bank 1", "Bank 2") is supplying the flaky data--no guess work there. A reason that OBDII vehicles don't need the chassis dyno-based emissions test in many states . . . if the "Check Engine" light is off, everything's working as designed.

Also be aware that in the fine print of the aftermarket catalytic converter warranties, you'll notice that they are designed to only last and be effective for about 30K miles--one reason they are significantly less expensive than an OEM-sourced replacement converter (even for the "direct-fit" versions, not to forget the ones which need a "fit kit" for your specific vehicle). We have also encountered a few situations where a few-weeks-old muffler shop replacement converter would trigger the "low catalyst efficiency" code on new vehicles--it happened several times--with the only way to turn off the light and it stay off was to use the more expensive factory replacement converter/pipe assembly. Hopefully, you'll not have that issue!

For some reason, the idea that the temp sensor caused the catalytic converter to become clogged from carbon seems a little unusual to me. We had some issues with the 4.3L V-6 "Central Port Injection" engines having clogged EGR systems from particular brands of fuel and/or leaking injectors, but that's the only thing I can think of in recent history. Usually, the average run time for the engine to get into closed loop is generally about 2 minutes from dead cold, but with the most recent cold weather we've had, it might be a tad longer . . . engines with heated oxygen sensors can be more consistent.

We had some issues with overheated/clogged converters on early 1990s C-20/30 454 FI pickups when they were used as horse trailer tow vehicles. In those cases, the sustained periods of towing would overheat the converter (bead converter) and cause the converter to come apart internally, letting the beads flow toward (key word!) the rear of the exhaust system to be stopped by the rear NOx converter (which was the honeycomb monolith type) . . . where they stopped . . . as exhaust back pressure increased until the engine lost significant power and stopped too . . . no matter where you were at (many times in freeway traffic--GM and FORDS alike had those problems back then.

So, I think I'd make sure which sensor was being replaced and that some other codes might be hidden in there that might not have triggered anything just yet. Might also need to pay the money for a dealership diagnosis with their GM-approved scanner--in many observed cases, that's the least expensive way in the long run, with all due respect.

Got to be something flaky in the fuel supply system or ignition system (what do the spark plugs look like?) that's not particularly related to the engine temp per se. To me, anytime you have a situation when the fuel mixture curve is obviously out of whack, the oxygen sensor would be one of the first things to change (a default mode of sorts) to ensure that the ECM is getting good signals, then look elsewhere for what would be the root cause of the situation and get that fixed.

In the world of computer diagnostics of vehicles, you STILL have to know and understand how the various powertrain control systems interact and work, just like in the "old days" before computers and diagnostic codes! One failure code can generate other failure codes and you have to understand "what causes what" in order to get to the root cause rather than just chasing codes and chunking parts at the problem and NOT fixing the problem (but having changed many "maintenance items" in the process). NOT to forget that some diagnostic codes are MODEL YEAR and Manufacturer SPECIFIC, in some cases (which might not be applicable in this case, but something which might be applicable in other situations).

Just some thoughts,

NTX5467

[Larry Schramm]

Here are the requirements to turn the light on and set the code.

A predetermined amount of engine run time(programmed into the engine calibration) and a corresponding amount of air flow has been used by the engine while running. At that predetermined time the engine coolant temperature must be at least 80°C (176°F). If these conditions have not been met together, then the light will come on and set the code.

The engine coolant temperature sensor is a three wire connector with org/black, dk green, and a yellow wire so you will know you have the correct sensor. Also look at the intake temp sensor along with the mass air flow sensor. I think that you have an upstream problem and not a converter/ down stream problem.

Did you change the thermostat first? If not, then I would change it now.

Additional thought. What did you use to install the coolant sensor. If teflon tape, remove and use pipe sealant with teflon. It is a Loctite product that you can buy at auto parts stores.

Edited February 20, 2011 by Larry Schramm (see edit history)

[Reatta Man]

You didn't say how many miles are on your engine, but a plugged cat is likely caused by a leaking intake manifold gasket. The 3.1 engine had a LOT of intake manifold gasket failures. If it fails, the computer reads a lean condition, and dumps gas into the engine. The unburned gas then clogs the cat.

One quick way is to use a can of WD-40 or carb spray cleaner. Spray it around the base of the intake manifold where it bolts to the block, while the engine is running. If the RPMs change, you have a leaking gasket.

The other suggestions about checking sensors, thermostats and coolant temps are also very valid. You may be working two problems; a coolant temp sensor giving a false reading (and keeping the engine in open loop mode, which runs the engine very rich) or an actual cool engine temp and the sensor giving an accurate reading that is too cool to go into closed loop.

Keep in mind that an engine running in open loop for an extended period will kill a catalytic converter, so you need to find out what is causing this before it kills the next converter. Either a faulty sensor or a leaking intake manifold can cause this to happen.

A shop with a good scanner can tell whether the engine is in closed loop and whether the sensor is faulty. They will also likely have an infared gun to check the actual temp of the coolant once the engine is warm. If it is hot enough but the sensor says it is below 176, that will tell you what is wrong.

Good luck.

[Guest Jim_Edwards]

In the world of computer diagnostics of vehicles, you STILL have to know and understand how the various powertrain control systems interact and work, just like in the "old days" before computers and diagnostic codes! One failure code can generate other failure codes and you have to understand "what causes what" in order to get to the root cause rather than just chasing codes and chunking parts at the problem and NOT fixing the problem (but having changed many "maintenance items" in the process). NOT to forget that some diagnostic codes are MODEL YEAR and Manufacturer SPECIFIC, in some cases (which might not be applicable in this case, but something which might be applicable in other situations).

Just some thoughts,

NTX5467

Unquestionably the best and most accurate comment on the reality of computer diagnostics ever. Makes no difference how many codes a computer can generate there is no escaping the fact one has to fully understand the relationship of the codes to engine function. It is not unusual for the computer to throw out a code that may be totally inaccurate and generated on the basis of not one but maybe two or three sensors reacting to a failure of something totally unrelated to their condition reporting function. Good old basic mechanic common sense does not go out the window with computer controls and code analyzers.

We are living in a world where those who write the computer diagnostic firmware needs to be more than just an engineer or computer programmer to get it right. They should have about five years as a line mechanic to gain the knowledge needed to deal with the common issues that can develop with internal combustion engines exposed to a variety of operating circumstances and maintenance (or lack thereof).

Jim