I have a 1996 Chevy G10 van with 14X,XXX miles on it. It has the 5.0 engine. I do not have a scanner myself, but, according to a shop, the van was throwing code P0420.

According to my research, this is not the most helpful of codes when it comes to pinpointing the root cause. For example, according to this article, an imbalance in the fuel-air ration (either too lean or rich) could trigger the code.

The van idles rough, seems to lack power, and has occasional power surges. While it seems as the most common cause for the code would be a defective 02 sensor or catalytic converter, the driveability issues seem to indicate a problem elsewhere. I suppose the catalytic converter or one of the 02 sensors could be faulty, but I do not believe either one, based upon the symptoms, would be the only issue. On the contrary, I suspect the van is running lean.

I understanding that faulty sensors, such as the MAF, could result in a lean condition. However, many (if not all) of the sensors have a corresponding trouble code (like P010x for the MAF). Therefore, my question is as follows. Is it possible for a fault with a specific sensor to trigger this P04020 error code, rather than a code specific to that sensor? Or, is it more likely that I am looking at something like a vacuum leak?


  • Before you go chasing non-existent gremlins, you need to eliminate the cat and O2 sensors as being the cause of the P0420. This code is specific to one of these two being the culprit. To assume they aren't and bypassing them is a foolhardy move which will most likely yield you huge amounts of frustration, along with a lot of energy spent on chasing your tail. Sep 3, 2017 at 19:11
  • Thanks and fair enough. I know what the code means, but is it possible for a lean/rich mixture to cause it? To elaborate, my concern is how the van is running, not getting the light off. The code was meant to help diagnose the issue (for example, eliminating particular sensors). I do not think the 02 sensors would cause such problems. While I suppose a clogged cat could be the cause; I reckon such a failure might be itself a result of another issue. Given its cost, I am very hesitant to replace the cat so readily. If it needs done, it needs done, but I am curious what else might be possible.
    – KellyM
    Sep 3, 2017 at 20:14
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    A bad O2 can most certainly cause a P0420 code to pop. If you haven't changed them out in the last 90-100k miles, you may just want to go for it. Check the car through temp differential, front to rear. I have a post on here which described it (I'll post a link when I'm not on my mobile). Sep 3, 2017 at 21:05
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    The other thing you aren't taking into account is this could be multiple issues. Try doing all of the little things which might cause your other symptoms, like cleaning the MAF, throttle body, and checking the plugs. Do all the cheap stuff first, then worry about the other. Sep 3, 2017 at 21:13
  • Thanks much. Sorry, I meant that I I did not think an 02 sensor would cause the driveability problems (power surges, rough idle, sluggish acceleration). It does seem quite possible that the van may have multiple (hopefully) mild/moderate issues. The van is a recent purchase (I have not even put 5,000 miles on it). Certainly, not knowing the history does hurt a bit, and I am not sure how well it has been maintained. Thanks again.
    – KellyM
    Sep 3, 2017 at 21:13

1 Answer 1


Your question is a bit confusing. "Is it possible for a fault with a specific sensor to trigger this P04020 error code, rather than a code specific to that sensor?" I think you are asking if sensors can work improperly, causing a P0420 code, without actually producing a code themselves.

If so, the answer is "yes." Sensors like MAF and O2 often get dirty, causing them to not work properly while technically they are not broken and produce no code of their own.

You are correct in assuming that you likely have multiple issues. Overall poor combustion in an engine, which is in need of a tune-up, will cause the erratic problems you describe. Eventually, after thousands of miles, the catalytic converter can be damaged beyond repair.

You are also correct in suspecting air/vacuum leaks. They are common and will cause the engine to burn excess fuel, which can damage the catalytic converter. The other most common cause is leaks in the exhaust system, which have the same effect.

Follow the advice of Paulster2. Clean the MAF, throttle body & change the plugs. I would add replacing the air filter, crankcase oil, PCV valve & hose, and at least test, if not replace, the EGR valve and clean its ducting. Check for regular spark on each spark wire with a test light & look for arcing at night (in the dark).

The other possibility is that there is nothing wrong with the engine -- it is just dirty like an old fireplace. On many cars I have owned, the engine runs very poorly after a hard winter, local driving and stale fuel. Come Spring, acceleration is weak, with ping, backfire and stalling; emissions are foul, with black and white smoke; Check Engine light glowing; rough idle, etc.

With a dirty engine, the solution is Marvel Mystery Oil (MMO) followed by Berryman's B12 ChemTool. Start with 1.2 ounces per gallon of MMO in the gas with a full tank. Drive the vehicle hard at a wide range of speeds and RPM, revving it frequently in neutral multiple times in a row, then shut off immediately and allow to soak for 20 minutes. Drive the vehicle on an open highway and "floor" the engine, 50-75 MPH, at least six times consecutively. Shift to L1 and floor the car up to 30 MPH, at high RPM, for 30 seconds. Cycle repeatedly for a 1/2 tank over the course of days, then fill with gas and add Berryman's at 2 ounces per gallon. Run hard immediately for as long as possible (the Berryman's evaporates a lot after 24 hours when the weather is above 70 degrees F.).

It may be necessary to use these products for three tankfuls on a very dirty, neglected engine. Perform all other tune-up procedures first, changing the crankcase oil, coolant, and transmission fluid if necessary.

I have fixed many cars in this manner (GM, Ford, Mercedes). Carbon and other deposits are softened by the MMO and vaporized by the Berryman's B12. Pistons, rings, valves, spark plugs, O2 sensors, EGR valve and cats are largely self-cleaning with enough heat, pressure, and air flow from the engine, so drive aggressively.

  • Your assumption is correct. I was curious if I a fault with a sensor could throw the more "generic" P0420 rather than a sensor specific code. I already had the wires, plugs, fuel and air filters, and PCV valve replaced. Today, I plan on looking for leaks and maybe cleaning my MAF and/or EGR. Thanks so much for the detailed response.
    – KellyM
    Sep 4, 2017 at 12:33
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    You're welcome. The EGR valve is a common source of vac. leak. Test it by removing the vac. hose & attaching a spare piece of hosing (engine off). Suck on it and you should hear the metal diaphragm move, with no leaks. Reconnect hose and start car, raise idle to 2k rpm, detach vac. hose -- there should be suction coming from the hose. Also, at 2k rpm, there should be a noticeable change in rpm when the hose is removed from the EGR valve.
    – Carguy
    Sep 5, 2017 at 7:43
  • Many people clear P0420 with a product called CataClean, which is very similar to Berryman's. Add a 16 oz. can to a 1/4 tank of gas. Drive on the highway for 15 minutes, then floor the car 50-80 MPH at least six times in a row. If the cat. is not badly damaged, you might just be able to unclog it.
    – Carguy
    Sep 5, 2017 at 7:46

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