Vehicle details: Audi Q7 e-tron
Engine code: CVZA (3.0 TDI V6)
Year: 2017
Symptom: Poor charging
Author: Steve Smith

Audi Q7 e-tron | Fails to charge via plug-in charger

While electric vehicles appear to be the future, plug-in hybrid vehicles seem to be the order of the day when it comes to the best of both worlds. Such vehicles can drive on pure electricity, meeting the current low emission regulations for cities, yet, with old faithful on-board – the internal combustion engine, we can take advantage of seamless mileage without suffering range anxiety. (What’s not to like?) Both power sources have their distinct pros and cons but somehow, when you combine the two, you have a formidable vehicle that ticks almost every box. The following case study focuses on the plug-in high-voltage (HV) charging system of a hybrid vehicle and the challenges encountered along the way. 

Customer’s description

The vehicle was failing to charge from the mains charger unit supplied with the vehicle. 
Note: Such charging methods are referred to as “Mode 2 charging” that utilise the domestic mains supply via a 3-pin plug. Their typical consumption is around 10 A with a power output of approximately 2.2 kW.

Technical description

Verifying the customer complaint is an essential step in the diagnostic process but it is quite often a time-consuming task with little success. On this occasion, the fault was apparent both when we used the owner's charger unit or “Mode 3 charging”, which refers to an AC wall-mounted charging pod with a consumption rate of 32 A and a power output of 7.2 kW.

You can find more information on charging modes here.

When we connected the Type 2 charging plug to the vehicle, we heard the charge inlet locking pin cycle three times before a red warning LED illuminated to inform us of a charging error. 

Look at the image below. Note that the vehicle instrumentation recognised the connection of the mains charger but that the charging would not commence.

Audi eTron dash display showing that the vehicle instrumentation recognized the connection of the mains charger but that charging would not commence.

The following video clarifies the reported symptoms. Listen for the cycling of the charge plug locking actuator.

Note: While the mains charging was inoperative, the high-voltage battery could be charged by running the engine until the battery had sufficient charge to drive the vehicle a short distance in electric mode alone. We can therefore surmise that the battery can accept charge and deliver sufficient current to power the electric drive motor. 


With the customer's complaint verified we confirmed the Vehicle’s ID and Specification. Confirmation of vehicle specification is of the utmost importance when it comes to diagnosis as there is often a temptation for customers to modify their car with fashionable accessories that lack the fundamental quality control and engineering that was intended for the vehicle. A brief evaluation of the vehicle did not reveal any concerns in the form of aftermarket accessories. However, we discovered that the vehicle had been purchased from an auction so it was impossible to find and question the original owner. 

The Customer Interview followed the four targeted questions below using the open question principle to establish facts from fiction:

1.    How long has the problem been evident? 
The vehicle was recently purchased from an auction with the fault already present.  

2.    When did you first notice the problem?
Upon purchase. 

3.    Has any work been carried out on the vehicle recently?
No history was supplied with the vehicle on purchase and we had no access to the previous owner. The new owner had installed a new charge inlet socket complete with cabling and locking pin since the purchase.

4.    When do you experience the problem?
When attempting to charge the HV battery via charging modes 2 or 3. 

Based on the summary above, we had very little to go on in terms of historical facts which often provide essential clues. Not to worry, let’s march on.

The Basic Inspection confirmed no fluid leakages, no visible signs of damage to hoses, connections or wiring harnesses or indeed my favourite discovery: accident repair.

A vehicle scan of all onboard control units revealed multiple fault codes listed below:

Image of fault codes revealed

Based on the fault code descriptions for each of the above, I have highlighted the code relevant to HV charging: “Charging Socket A; Charging Plug Lock P33E8 00 [008] - Mechanical Malfunction”. You can see the additional data surrounding this fault code below.

Audi eTron Charging Socket A; Charging Plug Lock P33E8 00 [008] - Mechanical Malfunction”. You can see the additional data surrounding this fault code

Let me summarize the data above:

“AX4” is the VAG code for On-Board Charger (OBC). It has reported a mechanical problem with the lock pin designed to lock the Type 2 charger plug into the charge inlet socket. In addition, several parameters have been stored at the time of fault code detection (i.e., Freeze frame data). So what do these parameters reveal?

  • Fault status 00000001
    Fault was present 
  • Fault frequency 4
    Four detected events of the fault. 
  • Lock for high-voltage charging connector not locked.
    The OBC could detect that the charging plug was not locked. Should be “locked”. 
  • The status of the connect recognition was present.
    The OBC could detect the presence of the charging plug via the Proximity circuit (PP).
  • The maximum current of the charge connector was 0 A
    Charging was not taking place as there was no current flow. 
  • Charge sensor temperature sensor DC recorded -55.0 °C
    The default charge socket temperature value as DC charging was not applicable. 
  • Current: 15.7 A. 
    The charging current available from the connected charger (communicated to the OBC).
  • Frequency: 1000 Hz
    Frequency of Control Pilot (CP) PWM signal.
  • Control voltage: 9006 mV
    CP Line voltage at peak of +9 V, which suggested that the OBC has paired with the mains charger but was not charging (normally peak of + 6 V when charging).  
  • Duty cycle: 26.2 %
    Positive duty cycle of Control Pilot (CP) PWM signal (26.2 % approx. 16 A charge current available). 
  • Signal recognized: 00000001
    The OBC could detect the charger plug connection and interpret charge information relayed from the charger (via CP).  

From the above freeze frame data, we can conclude that the OBC is detecting the presence of a charging plug. CP / PP lines appear to be interpreted correctly, yet the position of the charge plug lock pin remains “not locked”. Could the failure of the charge plug lock pin result in the no charge?  

Before we move on: You can find more information on the Type 2 Control Pilot (CP) / Proximity Pilot (PP) lines and the essential roles they play in mains charging in our Guided tests in the PicoScope 7 Automotive software.

Screenshots from PicoScope 7 Automotive showing the relevant Guided tests

Before we dived in here, I wanted to take a step back and check for Technical Bulletins (recalls and campaigns etc.). None were relevant, so based on the vehicle history and symptoms we could move on to possible causes.

Possible causes

•    A wiring harness error between the Type 2 Charge inlet socket and the OBC.
•    Incorrect signals/values relayed between the Charge inlet socket and the OBC.
•    The charging is inhibited by other onboard systems (perhaps via CAN messaging).
•    A faulty OBC (AX4).

The action plan

The action plan was predominately governed by accessibility, probability and cost. Based on the acquired fault code and the description of the fault, we focused on the Type 2 lock pin circuit.  
•    Wiggle testing and visual inspection of the OBC wiring and its connectors.
•    Measurements of the locking pin actuator and position sensor.
•    “Pinout” testing of all low voltage inputs/outputs at the OBC.

To recap

The vehicle failed to charge from both 2 and 3 mode charge points.

•    DTC “Charging Socket A; Charging Plug Lock P33E8 00 [008] - Mechanical Malfunction”.
•    A new charge inlet socket complete with wiring and lock pin actuator had been installed.
•    The charge plug lock pin actuator cycles three times when the plug is connected to the charge inlet socket.
•    Running the engine will charge the HV battery.
•    The vehicle will drive in electric mode when the HV battery is sufficiently charged.

We removed a variety of trim panels and under shields for access to the rear of the charge inlet socket, OBC and associated wiring.

Since we were trying to establish the functionality of the Type 2 charger plug lock pin and associated circuits, an element of reverse engineering was required and this was where the eight channels of the PicoScope 4823 came into their own. The advantage was that we could see how each of our chosen circuits interact with one another in order to evaluate the sequence of events when attempting to charge the vehicle. The disadvantage was that we were applying an 8-channel scope in an HV environment, making it necessary to adhere to the appropriate precautions.

N.B. The necessary and relevant risk assessment, training, PPE, signage, measurement hardware and best practices are paramount both before one commences diagnosis and during the diagnosis to ensure the safety of all personnel involved.   

Evaluation of Type 2 charge inlet circuit: Day 1

In the screenshot below, you can see that we connected to connector B (THRL) at the rear of the charge inlet socket to remotely obtain an overview of the interaction between CP, PP, lock pin actuation and position. Note how the lock pin was “cycled” three times over the entire capture before it returned to the disengaged position.  

An overview of the interaction between CP, PP, lock pin actuation and position.

Let’s break down the capture above into useful data and refer to the capture below:

1.    Channel A displays PP voltage and captures the change of state at the point of the charger plug insertions.
2.    Channel B captured the CP line voltage and PWM signal from the mains charger unit to the OBC. Note: The peak positive voltage, frequency and duty of the CP line match the freeze frame data supplied with our DTC P33E8 00 [008].
3.    Channels C and F measured each side of the lock pin actuator with reference to chassis ground. Note how any differential between these channels resulted in current flow (lock pin actuation) which was captured on Channel G.
4.    Channel D is the ground return from the lock pin position sensor.  
5.    Channel E captured the lock pin position voltage. Note how the voltage rests at approximately 3 V with the lock pin disengaged and approximately 6.5 V when it is engaged. Interestingly, the lock pin was then cycled three times (please listen to the initial video above), before coming to rest in the disengaged position. At this point, the red warning LED adjacent to the charge inlet was illuminated to inform the owner of a charging error.

Screenshot from PicoScope 7 Automotive showing that the everything in the capture looks normal on the surface.

On the surface, everything in the capture above looks to be normal.

•    Our PP line voltage changed to indicate the charger plug insertion (This was also confirmed within freeze frame data)
•    Our CP line voltage changed upon charger plug insertion (peak + 9 V) and communication between mains charger / OBC commences with a 1 kHz PWM signal at 25% positive duty (again, this was confirmed in the freeze frame data)   
•    However, for charging to commence, our CP line voltage must change from + 9 V .. -12 V to + 6 V ..-12 V and this event does not occur! Please refer to our Electric Vehicle Guided Test > Charger-vehicle tests > Charger-vehicle communications (type 2) > “Further guidance” for a detailed explanation of CP line voltage during charging. 
•    Our lock pin position switch informed the OBC of engagement and disengagement.

N.B. Our lock pin was physically engaging with the charger plug as I could not remove the plug when the pin was momentarily engaged.
The question, therefore, is why does the vehicle not charge? At this stage, I did not know!

In the screenshot below, we have an identical test to those above, but over a longer time frame to demonstrate how the symptom did not change. I.e., the lock pin cycled three times after each insertion of the charger plug into the vehicle inlet.  

Screenshot from PicoScope 7 Automotive showing the same test as before but over a longer time frame.

Based on the fault code description “Charging Socket A; Charging Plug Lock P33E8 00 [008] - Mechanical Malfunction”, I could not see how such a description could apply to a lock pin that was working correctly and physically engaging with the lock plug. Why would the OBC report a mechanical error with a functioning lock pin, and more importantly, how did it know that the lock pin had failed? The answer had to be related to the lock pin position sensor and its associated circuit.

After searching various SSPs and technical data sites, I could not locate the relevant literature to take the diagnosis any further. In hindsight, I should have looked for a test plan to accompany fault code P33E8 00 [008]. However, using back-to-back data from a Mk7 e-Golf seemed like a quick route to establish real-time lock pin values.
Below, we have the lock pin position signal from a functioning Mk VII e-Golf when a Type 2 charger plug was inserted into the charge inlet.

Screenshot from PicoScope 7 Automotive showing the lock pin position signal from a functioning Mk7 e-golf Type 2 charger connected.

A breakthrough at last, as the lock pin position voltages varied between our e-Golf and Q7 Plug-In Hybrid. Both vehicles returned comparable lock pin position voltages when disengaged:

Q7 lock pin disengaged: Approximately 3 V (2.93 V)
e-Golf lock pin disengaged: Approximately 3 V (3.4 V)

However, with the lock pin engaged it was a different story:

Q7 lock pin engaged: Approximately 6 V (6.23 V)
e-Golf lock pin disengaged: Approximately 9 V (9.22 V)

If we calculate the voltage differentials between locked and unlocked for both vehicles, we can further analyse the discrepancy:

Q7 lock pin position differential voltage = 6.23 V – 2.93 V = 3.3 V (allowing for variables we can round this down to 3 V)
e-Golf lock pin position differential voltage = 9.22 V – 3.4 V = 5.82 V (allowing for variables we can round this up to 6 V)

A word to the wise here regarding back-to-back testing, “when it’s all you’ve got its all you’ve got” but keep in mind that I was not comparing like for like vehicles (even though they fall under the VAG umbrella).   
I could not find any data to support the expected values for lock pin position voltages and as a result, our only option was comparison testing of numerous similar vehicles (I guess you can call this approach “spot the difference”). 

Armed with the information from above, I was keen to return to the vehicle and evaluate the lock pin circuit further. However, the customer had made the decision to install a second-hand OBC! While this was a gamble and a “leap of faith”, the good news is that the fault cleared and the vehicle now charges! That doesn’t happen every day!

The onboard charger unit -OBC (VAG I.D AX4).

Photo of the on-board charger unit

The onboard charger unit -OBC (VAG I.D AX4).


Day 2 

To say I was keen to remeasure this vehicle and understand how our values had changed would be an understatement!  
I returned to the vehicle and repeated our initial measurements. This time we focussed solely on the lock pin position voltages. In the screenshot below, you can see how the lock pin position values are now closer aligned to those of the e-Golf donor car at 3.3 V disengaged and 9.4 V engaged (a differential of 6.1 V).

Screenshots from PicoScope 7 Automotive showing the Voltage signals from the Audi's lock pin position.

The CP voltage above had switched to +6 V... -12 V confirming that our OBC responded to the correct lock pin position and allowed charging to commence.
Channel D (lock pin position signal ground reference) had an increased noise level thanks to the commencement of charging.
Also, note how the lock pin is no longer cycled. The pin was engaged upon connection of the charge plug to the charge inlet and this state does not change until the remote key “unlock” button is pressed at approximately 31.77 s where the lock pin disengages and the charging was halted.
The positive duty cycle (between the time rulers) is measured at 28.57% suggesting an available charge current of approximately 16 A.  

Confirmation of repair 

The proof, as they say, is in the pudding. The fact that replacing the OBC resulted in differing lock pin position voltages, the removal of fault code P33E8 00 [008] and the vehicle finally charging again, confirms that our faulty component was indeed the OBC.

The video below demonstrates the characteristics of the charging systems after the fix.

List All Parts Fitted

Onboard charger OBC (VAG I.D. AX4)

Additional comments

Reading back through this case study you would be forgiven for thinking the scope did not play a part in the diagnosis of the OBC. Indirectly, that would be correct, but let’s take a look at the bigger picture and what we have learned as a direct result of the scope application.
The scope revealed the interaction between PP, CP and the lock pin position relative to “time”. 
Not only is the lock pin a desirable security feature, but also an essential input required to both initiate and terminate the charging process. 
Until the OBC receives the correct lock pin position voltage (confirming pin engagement) charging will not commence. In the case of the VAG models tested, the lock pin differential voltage between disengaged and engaged proved to be approximately 6 V. (Please note this may not be the same for all vehicles.) 
Below we reveal the sequence of events as they occurred in time, from the charge plug insertion to the end of charging when the remote key activated the unlock command.

Screenshot from PicoScope 7 Automotive showing the sequence of events as they occurred in time, from the charge plug insertion to the end of charging.

Charging system response to charger connection:  
1.    Activate remote key unlock button 
2.    Charger plug inserted
3.    The PP line voltage changes from pulsed signal to a fixed value dependent upon the current rating of the charge plug 
4.    The CP line voltage changes from 0 V to +9 V peak
5.    The lock pin position is indicated as fully engaged 
6.    The CP line voltage changes to +6 V peak and charging can commence 

Charging system response to charger disconnection: 
1.    Activate remote key unlock button 
2.    The lock pin actuator is activated 
3.    The CP line voltage simultaneously changes to + 9 V peak (charging is halted) 
4.    The lock pin position is indicated as disengaged 
5.    The charger plug is removed 
6.    The CP line voltage returns to 0 V
I now refer to the charger plug lock pin, as I was intrigued to discover how the locking mechanism functioned and, more importantly, how the lock pin position was relayed from the lock pin actuator to the OBC.
Since a new Type 2 charge inlet socket had been installed the temptation to dismantle the original unit (complete with lock pin actuator) was too much to resist.
Below you can see the lock pin actuator mounted directly above the Type 2 charge inlet socket.    

Photo of the lock pin actuator and its position

Inside the lock pin actuator, we have a drive motor, a gear assembly and a microswitch linked to the locking pin. Note also the two resistors and their relationship to the microswitch (by-pass).  

Photo of the inside of the lock pin actuator - microswitch by-pass

Photo of the inside of the lock pin actuator - micro switch and lock pin.

As the lock pin travels through the actuator body, a sector gear linked to a cam acts on the microswitch. With the lock pin disengaged, the total circuit resistance is equal to 11 kΩ as both the 1 kΩ and 10 kΩ resistors are in series. With the lock pin engaged, the rotation of the sector gear and cam close the microswitch, bypassing the 10 kΩ resistor. (Total circuit resistance 1 kΩ.) 
The change in the resistance of the lock pin position circuit results in our 3.3 V (disengaged) and 9.4 V (engaged) values which the OBC utilizes to qualify the locking of the charger plug.
The burning question is, therefore, what has failed inside the OBC to change the lock pin position signal from 6.23 V to 9.4 V when fully engaged? The answer is “I don’t know”, but I will find out when we have permission to dismantle to original OBC.  
Many thanks to Steve Winn Autocare and Pete Melville at HEVRA for their invaluable support and technical input.


1 comment | Add comment

Gordon McFarlane
November 01 2021

Absolutely thougherly enjoyed reading this fascinating Can we have more please

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Case study: Fails to charge via plug-in charger