Intro - skip to TL;DR for the main part of the question...

I keep getting P0402 errors indicating EGR excessive flow on a VW Golf IV Variant, 1.6 l, 16V, engine code AZD. When warm, the engine has a bit of a rough idle. Being aware that EGR errors can get thrown for a number of reasons not limited to the EGR valve itself, I have cleaned the throttle, the IAT/MAP sensor and the EGR valve itself using carb cleaner spray. Together with changing the air intake filter, all of this helped the car run much better, especially when being driven at or accelerating out of low engine RPM. But - the P0402 error keeps coming back, and the engine tends to run a bit rough in idle, which could, next to the error code, be an additional indication of some exhaust gas being fed into the air when it shouldn't. This could be a hint that the error is not thrown because of a sensor being faulty or misread, but because of an actual problem with EGR excessive flow, just like the code's translation says.

TL;DR label - End intro ;-)

It appears that the cleaner spray worked well on the parts that don't get the very hot exhaust and had an oily carbon buildup (throttle, IAT/MAP sensor). It didn't work so well on the EGR valve itself, which had hard carbon deposits.

My particular EGR valve has the actual mechanism buried in an inaccessible place, so there is no good way of mechanically scraping off the deposits. I am looking for suggestions on how to chemically get rid of the deposits, or any other good tricks.

enter image description here

It appears that it should be no problem to immerse the lower part of the valve in any non-corrosive solution. Anything electrical is well separated and above the aluminum block where the valve sits.

What chemical agents might work besides carb cleaner? Maybe gasoline or purified alcohol? WD40 even? Any experience with letting the valve sit immersed for a day and then using carb cleaner?

Also, I might hook it up to a relay/solenoid driver fed by an oscillator. The idea is that I would have the valve click back and forth all the way for many, many cycles and this might knock away the hard carbon deposits and allow the valve to close all the way again. How are my chances that this will do the trick?

  • Brake cleaner would work well ... I find it to clean out carbon better than carb cleaner. Commented Aug 10, 2017 at 21:44
  • MAF cleaner works pretty good too.
    – Ben
    Commented Aug 10, 2017 at 22:52
  • Are you sure that it is impossible to disassemble the egr valve? Perhaps you want to post a photo of the mounting flange?
    – Martin
    Commented Aug 11, 2017 at 20:55

2 Answers 2


Letting the valve soak over night seems to be a common way. Just make sure you only flood the pipes and not the actuator.

There are specific EGR cleaning agents (sprays) sold at a lot of places; might work better than generic carb cleaner, or not.

Somebody reported success using oven cleaner from the kitchen, but I'd go for soaking with EGR cleaner first.

Acutating while soaking seems to be a good idea, or rather have the valve fully opened during the process to have the cleaner reach as much of the mechanism as possible.

  • Oven cleaner was the first thing I found on youtube when I searched for EGR valve issues. I'm a bit hesitant to use it - just like you are, though, because I figure it might be too aggressive on the metal parts. I prefer solvents based on oil, such as brake cleaner, carb cleaner, or EGR cleaner (which I wasn't aware of). Here's a breakdown of the ingredients in brake cleaner and carb cleaner: mechanics.stackexchange.com/a/19070/31544
    – zebonaut
    Commented Aug 11, 2017 at 9:05
  • re: fully opened - Some suggest the power dissipation might become too high when the valve is being left connected to 12 V. In the car, it is actuated with a pulse-width modulated signal, meaning it mostly gets only a percentage of the full voltage and is probably never fully open (100 % * 12 V) for a longer time. Typical EGR currents @ 12 V seem to be in the range of 0.6 A to 1.3 A, i.e. the power dissipation is 7 to 16 W. The overall size of the valve suggests 16 W are OK, but I don't know the details inside. What I don't want to have is a hot valve lying in flammable solvent over night ;-)
    – zebonaut
    Commented Aug 11, 2017 at 9:14
  • @zebonaut Good point :) So better play it safe. Or mechanically lock the valve in a (half-)open position?
    – JimmyB
    Commented Aug 11, 2017 at 9:15

After having cleaned the valve and having put ca. 400 km on the car without any further errors in the ECU's memory, here's a report about what may have done the trick, at least as a somewhat temporary fix. The least thing I can say is that things have became a lot better; before the repair, the error kept coming back almost immediately.

  1. The best option of cleaning the valve seems to be using a variety of agents. I have changed back and forth between gasoline, WD-40 and brake cleaner. It appears that WD-40 was a particularly important member in the trio, because it doesn't evaporate right away and is able to soak into and soften the very hard and very solid carbon deposits around the critical areas inside the valve. Once the WD-40 managed to make the carbon somewhat soft and sticky, the other solvents (gasoline, brake cleaner) had a chance to further dissolve the deposits.
  2. It appears that it does help to electrically actuate the valve while it is being soaked in any of the three substances at a rate of approximately 1 Hz. Care must be taken because even a tiny electrical spark in the presence of evaporating hydrocarbons poses a serious fire hazard. I do not recommend doing this inside a building or garage under any circumstances, and even outside, it is wise to be very careful. Avoid loose contacts. The freewheeling spike caused by an energized coil will create significant sparks at loose contacts, and the mechanical movements of the actuated valve will further help loosen any less-than-ideal connections. Do not store any cans or jars with cleaning solvents near your set-up in order to minimize the amount of flammable liquid near the possible source of ignition. Make sure your set-up is ventilated well to keep the concentration of inflammable vapors as low as possible. Besides sparks, consider heat generated by ohmic losses in the EGR valve's coil. Be prepared for any surprise that may happen. Use at own risk. This being said, here's what I have used, starting with a schematic. If anyone's interested in building something similar, details are added at the very bottom (*): Solenoid Tester, Schematic Pictures of the quick and dirty breadboarded implementation:enter image description here enter image description here ... and a close-up of the valve's pins: VW EGR Valve from 1.6 l 16V engine, code AZD Opening and closing the valve repeatedly appeared to have helped knock some of the dirt away.
  3. However, in addition to soaking and spraying with the above-mentioned solvents, I have also used a solid copper wire and some plastic (taken from a q-tip) to scratch away whatever deposits I could reach. My hope was that the copper, being a fairly soft type of metal, did not scratch the surfaces inside the valve.

It will be interesting to see if this fix will turn out to be long lasting, but it certainly was an improvement.

(*) Circuit Description, schematic repeated for better readability: enter image description here

An Oscillator around Q1 and Q3 has a frequency of approximately 1 Hz. Even though this basic two-transistor multivibrator circuit is often called a "square wave oscillator", this is, at best, a euphemism. The signal at Q3's collector has anything but sharp, square edges: enter image description here This is why a Schmitt Trigger circuit follows, built around Q4 and Q5. Q6 takes the Schmitt Trigger's output and prepares the signal... enter image description here ... to be fed into the final transistor, labeled Q8, switching the output from 0 V to 12 V and back again... enter image description here ... driving the solenoid: enter image description here In case anything should go wrong (read: shorted output), there's a current limiter built with R15 and Q7 allowing a maximum load current of approximately VBE, Q7 / R15 = 0.65 V / 0.22 Ω = 3 A - enough for most automotive solenoids or relays to be tested, but also low enough so Q8 has a chance to survive the mis-connected output leads.

All transistors except Q8 can be any small signal jellybean parts, e.g. BC546/BC556 (for Europeans), 2N2222/2N2907 (for Americans) or 2SC1815/2SA1015 (for Asians) - if you're old enough to remember the now somewhat obsolete tradition of designers from the respective areas to use EECA-, JEDEC- or JIS-labeled parts. The output transistor, Q8, should have a rating of well above 1 A and at least 40 V, so a TIP31 or 2SC1061 or pretty much any other comparable transistor in a TO-220 package with a small heat sink will do just fine. R12 needs to be somewhat bigger because it dissipated quite some power. The reason for its low value (150 Ω) and the resulting, comparatively high current is the low beta of the output transistor. We need sufficient base drive from Q6's collector through R12 into Q8's base.

The freewheeling diode, D1, should be fast and have a rating of at least 1...2 A and 60 V. My particular EGR valve does not have the diode integrated. If this would be the case, one would have to be careful about the polarity when connecting the valve's solenoid to the test circuit, of course.

Yes, there are of course many more options of building a circuit like this (555 timer!), but for the fun of it, I made up this design using single transistors only.

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