Vehicle details: Peugeot 406
Engine code: RHY
Year: 2001
Symptom: MIL on,
Poor acceleration
Author: Dave Hill |

Peugeot 406 | Low Power P0191 P1138 MIL

Intermittent faults are the bane of our lives in the garage and often a fault can happen before your eyes, but might pass without a chance to spot the glitch on your scan-tool or to take a measurement with your multimeter. Even an oscilloscope can miss a glitch, so it is important to use a setup that is most likely to capture the moment a problem occurs.

Recently though we were fortunate to have the scope set up whilst a problematic Peugeot faltered before our eyes, thankfully the scope was gathering data. This Peugeot 406 2.0 HDI (code RHY Bosch system) was reported to be randomly losing power and bringing on the MIL (Malfunction Indicator Lamp). We initially started off by plugging the scan-tool in and noting the following fault codes:

P0191 Fuel rail pressure signal coherence.
P1138 Fuel pressure control solenoid circuit range/performance.

Hmm… where to start?

There is no simple answer here and your approach will probably depend on the ease of access to the associated components. A physical injector leak off test might be a first step, but on the other hand if the injectors are buried deep and the rail pressure sensor is easier to access then that might be the focus of your efforts. In our case as a “solenoid circuit issue” is mentioned, it was felt that that the high pressure fuel regulator control should be studied in greater detail.

It is worth taking a moment to consider the various ways that we can evaluate things with a modern oscilloscope and I use the word “modern” with good reason. Historically the technician has been restricted to some degree when using a scope, by the limitations of sample rate over longer time bases, which would mean that the finer detail needed for close scrutiny would be lost. In the past, the best way of capturing fast events, would be with a short collection time and this would have been considered an adequate method. These days though we are spoiled somewhat with the immense detail that Picoscope can capture, for the past few years we have had the opportunity to capture impressive detail, with massive amounts of screen time in comparison to older models and the majority of alternative automotive scopes.

Anyway, back to the patient. This vehicle would occasionally display the fault at idle and with no driver intervention. By studying the duty cycle and current flow of the pressure control solenoid at the same time, then maybe a clue to the fault might be uncovered. Following on from my previous comments about larger screen time bases, here is a capture that demonstrates the point very well. Using what James Dillon terms as a “Helicopter View” we are taking an overview of the crime scene. It gives us a greater perspective of what is happening and things can often take on a greater significance when looked at this way. Here we are looking at the fuel pressure control. With fifty seconds per screen, there is a greater chance of seeing any significant shifts in the current flow and sure enough we can see exactly that, just that toward the end of the capture (as the red trace rises).

Blue = Regulator Duty Cycle, Red = Regulator Current

In the animation above, you can begin to appreciate the benefits of using the “helicopter view”. In days gone by, we would be restricted to the view that we see when the waveform is zoomed to a very high level, as seen in the following animation:

Just imagine how quickly, an event would have flashed up on screen and then disappeared into the buffer (if you had that luxury) or possibly lost forever.

Many technicians will study either duty cycle or current flow, when looking at actuators, yet there is such value in looking with both duty and current traces combined. This capture from the pressure control solenoid highlights the point nicely. We are able to see both the command and demand relationships of the component, zooming into the detail we can see that whilst the current flow through the solenoid has increased dramatically, there has been no corresponding increase in command, witnessed in the duty cycle signal. This suggests that the PCM (Powertrain Control Module) has not initiated the change. The conclusion drawn from this waveform, is that the fuel pressure regulator solenoid, through a partial internal failure has demanded more current from the circuit. Replacing the component fix the issues with the vehicle.

Imagine the variety of conclusions that you could come to, if you had limited yourself to looking singularly at duty cycle or current and then being further limited to a small time capture. Don't restrict yourself with poor and outdated equipment and the practices that went hand in hand with it. More importantly if you have the equipment make sure that you use it to its full potential, don’t be blinkered into a single approach to scoping. Enjoy the experience of studying events that many technicians are unable to access.


17 comments | Add comment

September 02 2016

I do see some blue spikes there…Shouldnt that be considered an DUTY CICLE increase?

June 27 2014

just struggling with a ford kuga 2.0dtci 2008 non start. p0090 code. very helpful article

June 28 2012

Very interesting keep them coming ,some people may differ in their approach still i am impressed with the level of expertise shown in this case study, as a former Alfa Romeo workshop foreman with over twenty years in the trade it is great to see that the right approach to diagnostics is on the increase , sadly still many garages have not grasped this concept and still rely on out side help thank you for a very informative arcticle

Tony Ude
February 08 2011

I commend you guys contributions, highly appreciates Dave’s work and also the flexibility of PicoScope.And I wish more of this kind of solution to some critical problem be added always

colin white
February 05 2011

old age saying ...without a scope its difficult to cope… i can see what you have diagnosed excellent diagnostics with an oscilloscope a tool that tells no lies ...well done

Brian West
February 05 2011

Although I’m not involved with vehicle diagnostics I always find these articles interesting.
One point I would make is that it would be interesting to see a similar set of traces taken from the vehicle after the repair had been completed, to see if the disturbance on the command signal (that speedy refers to) is still there. This disturbance seems to occur regularly on the command signal positive line but looks to be slightly out of sync. with the command signal pulses so is probably being caused elsewhere. Also, isn’t the positive line voltage a bit high at just over 15V?
These comments are in no way meant to be criticisms of the article, which demonstrates well how the PicoScope provides the means to narrow in on such an event.

February 04 2011

Fantastic demonstration of the flexibility of the modern pico scope and the techniques you use.
Cheers Dave.

Dave Hill
January 27 2011

Hi Phil J

That is a very nice summary of the topic. I don’t think I could have put it better myself.

Nuff said!

Cheers Phil.

phil j
January 24 2011

Dave’s just trying to illustrate how how current correlates with a command signal and helps with a better diagnosis of a problem, if another channel was used on the fuel rail pressure sensor it would be seen that a fuel pressure drop/increase wasn’t the cause of the glitch seen.
Another point is that a digitally switched signal is always going to react quicker than a coil being saturated with emf as i understand it

January 04 2011

Hi guys,
I’m siting on very similar problem on nissan 2.2 Cr system . Average current draw by fuel pressure regulator is little over spec on ignition on and idle. impossible to erase fault code P1225 ( fuel pressure to high). on pico scope I see unexpected current ups and downs during wot.the car has almost no power. my decision - new fuel pressure regulator.

December 25 2010

from the trace: a signal anomaly of sorts aka spike, glitch in the command signal exactly coincides with the instant the solenoid current jumps, this spike could permanently alter the dielectric integrity of the solenoid windings insulation. to correct - replacement of solenoid would suffice, however the problem is likely to reoccur down the road. a signal buffer or snubber arrangement would correct the signal anomaly. clearly the real culprit is the power train module output stage that is picking up interference and regenerating/transmitting it. Peugeot should have the unit upgraded or recalled

Dave Hill
December 24 2010

Hi Alex.

This is my understanding……

The current remains in the 1 amp region (in this instance) purely because the circuit is turned back on again, before it reduces further. Given a longer “off” period, we would see the current fall to zero. Think of a balloon placed above your mouth, that you apply short “puffs” of air to. A little draught of air will cause the balloon to rise & then, during a pause, whilst you draw your breath, the balloon falls slightly, but not entirely, due to your next lungful pushing the balloon up again. This is how a duty cycle (PWM or Pulse Width Modulated) controlled circuit, regulates the electrical energy provided to command the actuator with such precision.

Hi Speedy.

I am not sure that I entirely understand what you are saying, but I suspect that you are suggesting that the current might be affected by the physical load imposed on the solenoid. In a similar way that, if we were to study the current load on a windscreen wiper motor, whilst at the same time restricting the movement of the wiper arm, we would see the current increase too. This is certainly something to consider isn’t it.

However, it is my belief that if a shift in rail pressure was to occur, due some influence other than that of the pressure regulator, then an appropriate shift in duty cycle would have been witnessed. This would be the management systems normal response to pressure fluctuations, as the “closed loop” method of control “catches the fall” so to speak.

I hope that addresses your question Speedy. If I have been misleading in any way, then I would appreciate you spelling it out for me. I certainly don’t know it all & these exchanges can prove to be valuable learning experiences.

Thank you Dave Foye & Crackerclicker for your kind remarks.

The “Pico Gap” thing really is a non-issue. Yes there are gaps, but they are evident at very long time bases, where Pico is capturing seamless data, many times larger than other scopes are capable of doing.

Cheers all


Don Roberts
December 24 2010

I need fuel air and fire to have a running engine. How the engine runs is a matter of knowledge, understanding, tools and information. I find the scope to be an extension of my eyes, ears and touch. Not all trouble with the operation of a vehicle can be corrected confidently without consistent use of appropriate human extension devices like the scanner and the scope and the DVM. The problem is there are DVMs and there are DVMs the same is true of scanners and scopes. It is important to understand voltage, current and resistance and how the deep topic of power consumed in electrical and electronic equipment and components will affect your success in diagnostics and hopefully your repair. A tool of any kind will not make us better as a mechanic but insight, understanding and due diligence will improve our long term success. If you need a two bound hammer why would you buy a pall peen hammer?

Dave Foye
December 23 2010

Top stuff Dave, a spot on and professional diagnosis.  I know there are other scopes out there that are very capable, but again this kind of fault is so easily captured and manipulated with Pico.

Dave F

December 23 2010

is it that the regulator solenoid circuit behaved as if there was a sudden loss(dramatic decrease) of rail pressure: which should have initiated higher duty cycle? then at least two other reasonable diagnostic approaches arise, no doubt you can guess what they are - but the neat way that the scope trace was used to accurately zoom in to the correct diagnosis could be considered wishful, or at worst misleading

December 23 2010

I don’t get it, can you explain the 1amp current draw when the pulse control is off? Is there any measurements after the repairs?

December 22 2010

this is one example of why i don’t feel the so called “pico gap” issue has any merit.  thanks for the case study dave 😊.

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Case study: Low Power P0191 P1138 MIL