Wednesday, 13 March 2013

Engine Management Light On? (How Its Really Fixed!)


If you came to this page because of fault/trouble codes you retrieved from a diagnostic tool you may find our iPhone/iPad or Android App useful.

Not only does it give you 'Fault Descriptions' but also gives you 
Possible Causes!! 

 Check it out on the App Store or Play Store!! 

I don't really know where it all went wrong! I guess I never will.
The diagnostic procedure below can be applied to many different electrical faults and to help put the engine management on vehicles out.

Let me start from the beginning. I'm deep into my first job of the week, a Peugeot 307 with the engine management light on and the cars LCD display warning of an antipollution fault. When I get a call from the office, its a customer with the engine management light on, and he wants to wait while its repaired.
I collect the keys and walk out to the car I fit a seat cover, place a floor mat in and fit a steering wheel cover. I start the car and its immediately apparent that the car is suffering from a miss fire. I pull the car into the workshop and decide to collect the diagnostic tool from its allocated position, I plug it into the cars diagnostic socket.

"Just clear the codes for me mate! It'll fix it.". I turn my head to find the customer standing there. "I will see what I can do?" I politely reply, knowing full well that is not going to fix this car nor will it fix most cars with the engine management light on.

Engine management light
Engine Management Light On
"My mate got the kit like yours from Ebay for twenty five quid. He reckons it just needs the codes clearing!" The customer responds. Needless to say the thought goes through my mind, the tool I'm using cost in excess of £5000 I highly doubt his kit is even remotely like mine. "I'm just pulling up the codes now mate" I  inform him.
Snap-on modis diagnostic tool

There is in fact two diagnostic trouble codes present. The first is code P0202 with the description of 'Injector 2 circuit malfunction' the second code is P0171 with the description 'System too lean Bank 1. I print the information and report my findings to the customer who requests again I just clear the codes. I explain to the customer that this is unlikely to fix the fault and that further diagnostics work will be required. "Well John told me it will fix it" was his unshaken response. I reluctantly clear the codes, ( Far be it from me to argue with the guy that's in effect paying my wages.)  I start the car and the miss fire is still as prominent as ever as is the glaring engine management light on the dash.

I don't have to to inform the customer of my findings this time as he is standing beside me. "So your kit says its injector two, how much is an injector?" The customer asks. I pause for a second to consider my answer to his question. "Well, my kit, tells me there is a fault somewhere in the circuit of the second injector. It also tells me the engine is running lean, probably caused by injector 2 not working. It could well be that the injector is faulty but it could just as likely be a broken wire or faulty PCM/ECU." Is my considered response. "So what does that mean?" He quite rightly asks. "Well it is probably best we find which part of the circuit is faulty, before we replace any parts." I reply. "How long and how much?" Is his next question. We head to the office and he is provided with an estimated cost diagnosing the fault and is offered a courtesy car while his is in for repair. He accepts and the vehicle is now left with myself to have the fault diagnosed.
While in one respect the customer is right. The diagnostic tool did tell me the general area that the fault is hiding. It did not tell me what the fault was. This is the reason I am employed. I now need to use the knowledge gained through experience to do the work the diagnostic tool cannot do.

I decide the logical first step is to test the electrical resistance of the injector. Mainly because its the easiest part of the circuit to test. I do this with my multi-meter and the relevant technical data for that vehicle which tells me the expected result of the the test. I compare my findings to this And conclude that injector appears to be OK circuit wise.

The next step is to test the supply voltage to the injector. I do this with an oscilloscope set up to show the signal from the PCM when it instructs that injector to fire. The result is that there is no signal being received by the injector. This could be due to either a broken wire between the PCM and the injector or the PCM itself.
Diagnostics Injector oscilloscope pattern
 I am faced with the choice of either stripping the wiring loom to look for a break in the wire that may not exist, or test the correct pin on the PCM for the signal I expected to see at the injector. I choose the second option. I again refer to my technical data and locate the pin to be tested. Again I test with my oscilloscope, this time I find the correct signal.

I now know that there is a break in the wire between the PCM and the injector. So I trace the wire back from the injector. I locate the break and repair it. I test again for the signal at the injector and to my delight the signal is now making its way all the way to the injector.

I can now clear the diagnostic fault codes with some confidence that the engine management lamp won't return. I road test the vehicle and the customer is informed his car is ready for collection.
Upon collecting his car the customer wishes to discuss the diagnostic process with me. I explain the various tests that lead to the eventual repair. He explained that he honestly thought the diagnostic tool just told us what part needed replacing. He left with the knowledge that its not that simple.

Know what does bother me about this is where do people get the idea that the diagnostic tool does my work for me. Or that clearing the codes magically fixes things. Who started this rumour and why. I would really like to meet that person because he or she has made my job look like its been made redundant by a computer.

I think not!!!

Has your car ever had its engine management light on, if so how stressful was it to get it fixed?

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  1. Very interesting story. I didn't find it boring to read. In fact, I really had a lot of fun reading your post. Thanks.


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  5. Excellent! I am exactly in the same situation and desperately need you! would you fly over to tokyo?

  6. I read your blog.I thought it was great.. Hope you have a great day. God bless.


  7. Daniel Clister3 May 2016 at 13:42

    Having a car really comes with lots of headache. Lots of things need to keep in mind! I cannot do all these stuffs. I think getting a mechanical support is better and no risk is there. Doing the stuffs by own means you're going to be responsible if any mistake happen. Honestly, this post is a good one to learn something but I prefer Volkswagen specialist Thousand Oaks for all my mechanical issues and these guys are really amazing. This post is going to help out many with some DIY tips. Lots of appreciation! Keep posting!

  8. Nice read..!

    I have had many issues with lights on my Peugeot 307 XSI. The last time for for a charge of £100+ they diagnosed and replaced the wheel sensor (the part was around £20, most of the cost in the diagnostic). About 9 Months later the light is back on, in fact I now have 3 lights and a service spanner - my local garage is going to have fun when I next return it to them... at least I have more of understanding of what the mechanics face when using their expensive diagnostic tool and the added steps/decisions that they may have to make ...cheers, Mark

  9. Very interesting article, lucky to be reading it. Thank You Posts

  10. Thank you for sharing this article. great information