EGR DTC P0401 INSUFFICIENT FLOW

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What caught my attention was when you said you have 12 volts until you reconnect a plug, then it goes to 0 volts. That is the classic symptom of a bad connection due to anything other than a total break in a wire or connector. As long as there's a tiny amount of corrosion or carbon-tracking in that break, enough current can get through for a digital voltmeter to pick it up and interpret that as a voltage. You can prove if this is happening by using a test light instead of a voltmeter. Test lights need lots of current to make them work, and that much current can't get through corrosion or carbon-tracking.

That carbon-tracking occurs commonly inside the insulation of a fuse link wire. I saw a very experienced mechanic get thrown by that in a radiator fan circuit. The new fan motor was good, he had 12 volts to the connector, but when he connected the motor, it wouldn't run. The 12 volts was there with his voltmeter but not with his test light.

This is identical to stepping on a garden hose and causing 99 percent blockage. Close the nozzle and you'll have full water pressure there. That's as long as no water is trying to flow, just like no current is trying to flow with a voltmeter in the circuit. Open the nozzle and try to do some work, and that water will just drip on your shoe. Trying to get water to flow past your foot is like trying to get enough current to run a test light to flow through a bad connection.

The goal now is to use a test light or plug in the item so current is trying to flow, then work your way back in the circuit with your voltmeter until you find the place where you have 12 volts again. When you find that 12 volts, you will have just passed over the break in the circuit.

Hi,
Thanks for getting back. I'll check the 12v with the connector attached to the EGR valve and let you know what that shows, but in the meantime, since it's too late to do anything until tomorrow, I was wondering what's your take on the issue with the pintle position at a momentary 'split second' zero out at 4v, then registering correctly at say 4.15v and slowly up to say 4.5v, which would be within proper range. Other than that 'glitch', the pintle position seems normal, as it returns with a slight 'glitch' at around 3.8v or so, then moves back down to say .6v, which is correct. Would that glitch cause the PCM to register an incorrect value causing a p0401dtc ? That dtc is an EGR low flow, so, what do you think ? Remember, I see this same glitch on the 'new' as well as the second 'new' EGR valves tested. The old valve had the same glitch, but the voltage varied too much, so I felt I should replace it.
I should also say that I had probably at least two or three other codes, but found a bad connector from the temp sensor to the PCM. Fixed that issue (fans would continue run on high, no a/c, no temp guage) and everything worked well until, then showed low coolant level and no a/c. I replaced the sensor and that seems now ok, but still no a/c. So the p0401 seems to be my main problem but a little confused why the a/c stopped. I was thinking that when the low coolant sensor was replaced, that might solve the a/c problem. What's wierd is the a/c went out after I repaired the connector. Maybe a relay? The temp guage is working accurately. Also I checked the MAS, Air Intake and MAP sensors. All are good. Crazy !
Thanks,
Larry

First let me tell you what I DO know. Any time you have a sensor that runs on 5.0 volts and its signal is a variable voltage, (as opposed to a square wave that goes between 0.0 and 5.0 volts), the acceptable range of signal voltage, for explanation purposes, is 0.5 to 4.5 volts. In actual practice you may find it going from 0.38 to 4.22 volts, for example, but it will never go all the way to 0.0 or 5.0 volts. Those are the conditions that trigger a fault code. If you understand electrical theory, you'll understand that to unplug a sensor would create an open circuit. If that's a two-wire sensor like for intake air temperature or coolant temperature, the 5.0 volt feed wire is the signal wire. Thanks to a dropping resistor inside the computer, with a complete circuit, meaning the sensor is plugged in, current flow will cause some of the 5.0 volts to be dropped inside the computer and you'll see what's left across the sensor. Regardless of the temperature, the signal voltage will never go outside of 0.5 to 4.5 volts. If you short the signal wire to ground, you'll set a fault code for "voltage too low" for that sensor. If you unplug it, you'll set a code for "voltage too high", meaning it went to 5.0 volts.

Things work a little differently for three-wire sensors. If we assume the 5.0 volt feed and ground wires are okay, it's just the signal wire we have to look at. (Also, to save time for our customers, if the signal voltage is okay, we know the 5.0 volt feed and ground wires have to be okay, so we don't waste time testing them). The throttle position sensor is easier to explain because it's a simple mechanical sensor. It works like the volume control on an older tv or radio. On one end the terminal is grounded. On the other end the terminal has 5.0 volts applied. The wiper, or movable contact inside, goes up and down to pick a point along that resistor depending on how far you open the throttle, however, there are mechanical stops inside it that prevent it from going beyond 0.5 or 4.5 volts. The only way you might be able to make it go too far is to remove it from the throttle body and turn it by hand beyond its normal range. On the car, if the 5.0 volt feed wire was missing, as in a broken wire, you'd see 0.0 volts on all three terminals, and that would set a fault code for voltage too low. If the ground wire were open, you'd have 5.0 volts all over and set a code for voltage too high.

The clinker is with the signal terminal. If that wiper inside the sensor makes intermittent contact, or if the sensor is unplugged, you would think you'd have 0.0 volts on that wire, and in fact, on a few car models you will, but on most cars you'll find 5.0 volts. That is because with an open circuit on that signal wire, we don't know what that voltage will go to. Because of being tied to all the other circuitry inside the computer, that signal voltage is going to "float" to some random value. If that random value is between 0.5 and 4.5 volts, the computer might accept it and try to make fuel metering calculations based on it. To prevent that, we use a "pull-up" resistor tied between the signal wire and 5.0 volts. That resistor is so big electrically that it has no effect if the circuit is working properly, but when there's that open circuit, it places 5.0 volts on the signal wire to force a fault code to set. That also tells the computer that signal voltage can't be trusted and it suspends any other self-tests that rely on the signal for comparisons.

If my sad story is correct, with your sensor unplugged, you should find 5.0 volts on the supply terminal and on the signal terminal, but only with a digital voltmeter. Those draw such little current from the circuit that the pull-up resistor won't drop any voltage.

That brings me to your EGR sensor. The first thing to consider is you aren't getting a fault code for that sensor. Under normal operation, that pintle valve might only open part way. Just like with taking the throttle position sensor off and turning it manually, you may be pushing it too far and beyond its normal range. A better test would be to monitor the signal voltage while the sensor is plugged in, then run the engine and apply vacuum to the EGR valve. You should see the voltage stay within the acceptable limits.

Now I'm going to tell you what I don't know. The fault code you got was for insufficient flow which means the computer was operating the EGR valve and not seeing the expected results. The sensor on the valve just reports that it opened, not how much exhaust gas was flowing, so that means the sensor was working. What I'm not sure about is how the insufficient flow is detected. That could be by watching the change in readings from the front oxygen sensors.

You said you checked for blocked passages too, but you might try opening the valve manually with a vacuum hose while the engine is idling. That might cause the engine to stall, but for sure it will run rough and slow down for a few seconds.

Hi,
Hope someone sees this. I have a very similar EGR issue as Laire's problem.
I have a 2001 Buick Park Ave. with continuing egr codes ( usually P0401 ).
I have went through the diagnostics twice. everything checks out. What I notice is I have 11+ volts at the power wire "E" on the harness when unplugged from egr. when I plug it in, voltage drops to .1 - .2v I have unplugged the harness from the ecm and traced the lead, it ohmed out to 0 from the ecm to the egr plug, so that tells me the wire should be good.
I did try a test light like "cardiodoc" mentioned above. It does NOT light up, but I have continuity so I am at a loss at this point. Plz Help!

The continuity test is what's fooling you. All you need is one tiny strand of wire left intact, or some moist corrosion around a corroded splice, and you'll have what appears to be normal continuity. The problem is you can't get enough current to flow through that restriction to run a test light. The same would happen if you stand on a garden hose and pinch it 99 percent closed. With the nozzle turned off, you'll have full pressure, (voltage), at the end of the hose, but open the nozzle and try to get some flow, and all you'll see is a tiny dribble. Pressure will have dropped to nothing.

Other than a corroded connector terminal, a common cause of this is someone pierced the wire's insulation a while back to take a measurement, and the copper has corroded. The break is filled in with a carbon track that does conduct enough current for the voltmeter to see 12 volts, but only when nothing is plugged in at the other end. As soon as the circuit needs to pass significant current, it can't get through the carbon track, so the voltage at the end of the wire drops to 0 volts.

The bottom line is forget about the resistance tests. For this type of problem, rely on the voltage measurements, and this is where the inexpensive test light is much more accurate than the voltmeter.

OK, so I just checked voltage from the 12v lead (for the EGR) directly behind the ECM at the harness and at the pin on the ECM.
11.38v with EGR unplugged and .05v with the EGR connected (measuring directly behind ECM terminal block). I am guessing the egr should have close to 12v supply when connected (checking with the ignition in "on" position...
Does this indicate problems with the ECM?

Are you on the gray or the white wire? The white wire, (terminal 4 at the Engine Computer), appears to be the 12 volt feed. The gray wire, terminal 32, is listed as the control wire on the ground side. You should see 12 volts on both wires when the valve is turned off. The diagram doesn't specify when that voltage should be there. The engine might have to be running.

Regardless, I think you're in the wrong area. A problem with that circuit would be detected as an electrical problem, and the appropriate fault code would be set, (P0403). Code 401 is for a flow problem, meaning a mechanical problem. The 12 volts you see when the valve is unplugged could be a voltage put there by the computer to test the circuit. If it gets drawn down to 0 volts, which could only happen when the valve is connected, that may be what is telling the computer the circuitry is okay.

If you have a scanner, you should be able to go to "actuator test mode", or some similar title, and find a menu selection that lets you command the computer to cycle the EGR valve open and closed. If you hear it clicking, you're done with the electrical part of the system. The wiring has to be okay. Code 401 refers to a flow problem. Perhaps the valve is stuck closed due to carbon build-up, or a passage is plugged with carbon.

Thx for the reply CARADIODOC, I didn't mention before, but I have been through all the diagnostic procedures. Everything checks out! No carbon, valve operates properly (new oem) the Only thing I could find is the voltage issue on the power lead (white wire). sometimes it will set 403 & 405 codes, but usually it's 401. Anyway, I have been through this twice (to be sure), everything tests out good. So I have a ghost in the machine

The information that I have shows wire E (white wire) as 12v feed. and you are correct, it is term.#4 at the ECM. I believe I tested it with ignition on and engine running. So, I'm still at a loss. anyway, I've ordered a reman ecm with warranty that will be "plug & play".
I'll let you know if this exorcises my ghost...

Please let us know what happens, we are interested to see what it is.

Cheers, Ken