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OT - Auto diagnostic/repair question....

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  • OT - Auto diagnostic/repair question....

    I know we have some top notch auto repair folks here, so maybe a bit of advice, please:

    The patient is my son's 1998 Chevy Cavalier, 2.2L 4 cyl, single cam. Fuel efficiency has dropped quite a bit, and we got a check engine light. Read the code, and it gave "P0420 Catalytic System Efficiency Below Threshold (Bank 1)".
    A bit of research gives me generic advice that at idle, I should see a fluctuating signal on the upstream O2 sensor (sensor 1) and a pretty stable signal of about .5 or .6 V on the downstream O2 sensor (sensor 2).

    My OBD reader can graph sensor outputs, so I set it up to look at those. The upstream sensor does indeed show a fairly regular signal, fluctuating between about .4 and .8 volts. The downstream sensor is pretty steady but may be a bit high at about .75 volts.

    The major culprit would seem to point to a clogged or failed cat converter. I also know that blindly following OBD codes can occasionally send one barking up the wrong tree. Hence my inquiry here.

    Anything else I ought to check or do before I go and change out the converter? The cat does not look too difficult to change in this car, but I'd hate to do it and not fix the problem.

    Thanks for any advice.

  • #2
    Come up, Saltmine

    He must be out for the evening, but will be checking in at any minute.



    • #3
      You can swap the two O2 sensors and run the test again. They should be the same sensor. If you get the same results, then the sensors are probably good and it points to the cat. But first check for leaks in the exhaust from the head (manifold) to the cat. A leak or crack in the exhaust manifold can cause a 420 but it will also (sometimes) throw a lean code or at least show erratic (elevated) short-term fuel trim as the PCM tries to richen the mixture. (Air gets drawn into the exhaust from a leak) Also check the plugs and wires. A misfire can also cause your problem.

      And, of course, the normal checks for chaffed wires causing intermittent shorts on the O2 circuit. Especially on the drop leads.


      • #4
        An actual faulty lambda sensor should give you a code for it, but if it wasn't for that I'd be going for the sensor rather than the converter. Many people don't seem to appreciate that the lambda sensors wear out, and I wouldn't be at all surprised to find one faulty in a car of that age. It's possible that at least one sensor is not actually worn enough to throw a code in itself, but generate another fault code for a related area. I'd consider the sensor that isn't changing voltage to be highly suspect IMHO.


        Edit: I was hunting for some additional information on Lambda Sensor Life Expectancy and came across this site, I think it gives an excellent overview of the sensors
        Last edited by ; 11-28-2010, 03:18 AM.


        • #5
          Thanks guys. Good ideas and thanks for the link, Pete, good info there.
          I did check for leaks and found nothing obvious. I'll do a more careful check when we get back at it this week. Swapping the sensors is a great idea, I'll give that a try too if they are indeed the same sensor. Plugs and wires are fairly recent, but I'll give them the once-over as well.

          Thanks again for taking the time to reply.


          • #6
            What does the exhaust smell like? Does the car have full power like it always has when you floor the gas peddle?

            There are some possibilities. A plugged cat, bad injector (sticking open), bad plugs or wires or coils. A plugged cat should make the car sluggish and have limited power and the exhaust should smell very bad. A sticky injector will make the exhaust smell as well but the car will not have limited power. Plugs, wires, or coils the engine should have a miss or run bad.


            • #7
              I would check for continuity on the post catalyst O2 sensor heater first, as well as making sure it's getting voltage to it.

              Next stop for me would be to check the coolant temp sensor output to the ECU. (False "cold" input into the computer screws up the internal test parameters.)

              9 times out of 10 a P0420 code alone points to a bad cat vs. O2 sensor problems, but there is always the odd exception...


              • #8
                Cats can plug for all kinds of reasons but they can go the other way too, If a cylinder misfire (you stated poor economy) goes undetected its basically pumping in raw fuel (and air) directly into the cat- the cats job is to consume anything in its path but its capabilities are limited in what it can handle, a little four banger that's intermittently dedicating 25% of its power producing capabilities directly to heat ( i.e. the cat) can push a converter over the edge -- the results are a total meltdown and it starts directly in the center of the honey comb/ceramics and blows a path right through,
                One quick check method is to go back to the muffler or even resonator and pound with your fist directly against gravity and then listen -- if it sounds like a bunch of little rocks shaking around inside then you just might have a cat that's had a meltdown...


                • #9
                  First, don't bother switching the oxygen sensors. They each have a specific task, and are not intended to be switchable.

                  The front sensor (Bank 1) is what the engine calibrates from, for fine fuel mixture tuning...While the rear sensor (Bank2) only monitors the efficency of the catalytic converter by comparing it's reading with sensor 1. If both sensors read close to one another, the computer sees this as a loss of efficency. (the converter isn't doing it's job)

                  Loss of efficency doesn't usually impact fuel economy or performance. Aftermarket catalyic converters used to be "low restriction" and would set this code almost instantly. Besides, the sensor 2 oxygen sensor is a slow acting, "lazy" sensor meant only to monitor, not trace the variations of engine load and speed.

                  With an upstream sensor reading between .4 to .8 volts, the downstream sensor would, most likely, read .75 volts. That's telling me there's nothing going on in the converter.

                  Usually one of two things happen in the converter to cause this condition.
                  The matrix which carries the catalyst has fractured, and broken up, ending up in the car's muffler, or the catalyst has become coated and become ineffective. Now...should the substrate clog up the muffler, your performance will drop along with your fuel economy, because of the exhaust restriction.
                  Coated catalyst will simply cause the code to be displayed, unless it's coated so badly it restricts the exhaust.

                  Without any more information, I'd say to take the car to a reputable muffler shop, and have them do a pressure test (or take a backpressure reading).
                  This will determine if there's a restriction.

                  My younger brother has a '97 Cavalier with the same engine, and it just turned 400,000 miles a few weeks ago. Of course, his timing chain wore out around 200,000 miles and he had to replace the catalytic converter twice already...Other than that, he just puts gas in it, and changes the oil once in a while. And yes, he bought it new. He used to be a happy Honda owner but soon realized that Americans can build reliable cars too.

                  A little more information would prove helpful. Like how the car is driven...short trips, parked a lot, long expressway drives, etc. Has the car been serviced and maintained? What is the mileage on it?
                  Oil consumption? Hard starting? Information, information. After all you don't call your doctor and schedule an operation because you have a belly-ache, do you?
                  No good deed goes unpunished.


                  • #10
                    Originally posted by saltmine
                    A little more information would prove helpful. Like how the car is driven...short trips, parked a lot, long expressway drives, etc. Has the car been serviced and maintained? What is the mileage on it?
                    Oil consumption? Hard starting? Information, information. After all you don't call your doctor and schedule an operation because you have a belly-ache, do you?

                    OK, guilty as charged! A bit more info:

                    Car is driven daily, but almost all short trips now, under 20 miles/day, most days around 10. All around town, mostly work-and-back type driving. No hard starting or significant oil consumption. He has only owned it for about a year, I would say it has had OK but not meticulous service in it's previous life. Got new plugs in the last year, gets oil changes and has had the basics like air filter, etc. changed. Over 100K miles but not by a lot, the exact number is escaping me. Have not noticed any obvious rough running or misfiring, but have seen a significant drop in fuel mileage.

                    All in all this has been a pretty good, cheap set of wheels.

                    Again, thanks for any guidance.


                    • #11
                      Originally posted by saltmine
                      First, don't bother switching the oxygen sensors. They each have a specific task, and are not intended to be switchable.

                      With an upstream sensor reading between .4 to .8 volts, the downstream sensor would, most likely, read .75 volts. That's telling me there's nothing going on in the converter.
                      Yes I was thinking the same regarding swapping the front and rear sensors, however not being familiar with that specific model didn't want to comment.

                      Typically when people work on their cars they're not at operating temperature and this will cause unusual outputs on the sensors that may indicate a problem that isn't actually there. Just confirm the voltages you were providing were from a hot engine at normal operating temperature?

                      If it were me, I'd firstly look at the sensor outputs with a dead cold engine, they should follow each other a lot more closely as the converter hasn't lit up. If the rear sensor is flat-lining as you suggested, there's your problem as the converter isn't doing anything at this stage. Then try it again with a hot engine, in this case the rear sensor should be relatively stable but the front sensor will still be varying quite a lot. Assuming that checks out the next thing I'd do is take it to a garage and simply ask them to do an exhaust gas analysis. It should be quite an inexpensive step. If it's ok then chances are your converter is ok. If the exhaust gas isn't coming out clean then do a back-pressure check on the converter.

                      Assuming the converter was faulty, the question is why? They don't just fail for a laugh and a giggle because they're having a bad day. The reason I suggested to look firstly at the lambda (O2) sensor is because they most definitely do have a limited life span. Indeed 100k would be pretty much right on the money. On the other hand if the converter is blocked then something has caused that and you'd need to look at what/why before simply throwing in a new one. You should get more than 100k out of a converter before it throws up a fault unless there's something that's caused it to fail.

                      Hope that helps.


                      PS Do NOT start checking continuity of the sensors. If not done correctly you can damage them. A failed heater will definitely throw up a lamda sensor code so it's pointless to risk doing this anyway.


                      • #12
                        "...and has had the basics like air filter..."

                        Changing the air filter on a modern (i.e. fuel-injected with computer control) car is a waste of money unless:

                        a) you drive the car at WOT, and want max power.
                        b) the filter is so dirty that the pressure drop is almost enough to collapse the filter.
                        c) you just want a new filter.

                        Any products mentioned in my posts have been endorsed by their manufacturer.


                        • #13
                          Not that this has anything to do with the OP`s original thread but...speaking of air filters...awhile back a friend had a customer come to his garage business complaining her car was running very poorly. What he found was that she had been going to one of those quickie-lube paces for a very long time, and they dutifully checked the air cleaner filter every time. Only trouble was, when they popped the filter housing apart, they didn`t bother to notice that the side they were looking at was the engine side. That side was very clean of course, but the other side...well, it couldn`t hold any more dirt. It was packed! I`ve seen a couple of other filters that he has replaced that were that bad, one had even been sucked off the sealing surface.
                          Location: Saskatoon, Saskatchewan, Canada


                          • #14
                            I got that same error code on my 1994 GMC serria, Reset it, came back a little while later...

                            My first reaction apon seeing the code was "Oh good, Its just the catalyitic converter, And here I thought it might be something to worry about.. Yaknow, Something my car actualy needs"
                            Play Brutal Nature, Black Moons free to play highly realistic voxel sandbox game.


                            • #15
                              If you are getting bad fuel economy then the first thing i would look at is the COOLANT temperature sensor ..

                              if this goes up the spout will :-

                              Make the engine run rich all the time will probably notice that the tick over is high

                              This in turn will coat the cat with soot ..
                              coat the lambda probes with soot.

                              Hence other problems / fault codes arise from a minor one .

                              Note there are two temperature probes ..single wire for the car temperature gauge ..and two wire one is the one that is linked to the ecu.

                              also worth checking the thermostat.........

                              do a run in the car from cold ..dont have your heater on ...turn it off ..completly ..not blowing and on the cold setting .

                              note that the gauge should move progressively up ..for the first mile or so ...then move up above half way mark ..then drop down to about a 1/3 of the gauge...........this "above half way" .."then the drop to 1/3", is an indication that the thermostat doing its job.

                              well thats how its worked in every car Ive owned .

                              all the best.markj