# High Resolution Circuit Fault



## Donny O. (Feb 11, 2005)

anyone have any idea what this is about? Friend of mine is looking at buying a 95 GMC truck with the 6.5 diesel and it runs like crap for a few minutes then the CEL comes on and it runs fine and the code it is throwing is '17-High Resolution Circuit Fault'. I have been searching the net trying to figure out what it is talking about. trying to see if it is a big problem, or just a bad sensor since when the light comes on i'm assuming it stops taking readings from something and jsut uses some fuel/timing/etc tables it has stored....going into open loop. anyone know anthing about this??


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## B&B (Nov 4, 2006)

That code is related to the optical sensor located in the top of the injection pump which is a somewhat of a common issue on the '94 newer 6.5's. The optical sensor sends what is called a "high resolution signal" as well as a "pump cam signal" to the powertrain control module (PCM) . The PCM uses this signal to determine injection timing as well as for fuel control and is one of the most critical inputs to the PCM. A faulty reading by the PCM will definitely cause a rough initial start.

Two things to try before replacing the sensor ($$$)...

There's a noise filter located inline in the wiring harness going to the optical sensor thats supposed to filter out any possible RFI interference from the alternator..sometimes it's this filter itself that will cause an erroneous signal from the sensor to the PCM. As a test simply remove the filter, it's just connected inline with two opposing weather pack connectors..just disconnect the connectors on each side of the filter, remove the filter and then reconnect the plugs...if the issue clears up you've got it narrowed down.

One other thing to do is run a few treatments of *good* diesel additive through it. Many times this will clean it out and solve the issue without replacing the Optical sensor. Stanadyne's Performace Formula and Redline Diesel Catalyst are the two that I would suggest.


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## Donny O. (Feb 11, 2005)

thanks for the help. I did ask the guy if he used any additives and he said every so often he would put some in but nothing regular. I myself put power service(white bottle) in every tank in my diesel(VW Passat TDi). We were planning on putting some in as soon as we picked it up since who knows how long it has been. i'm sure I can follow the wires and find said filter, but i'm gonna ask anyway...can you give me a location of this filter your refering to?? if that doens't fix it do you think it is safe to drive it 300 miles home to work on it?? he has been daily driving it that way for a while. if it does end up being that optical sensor, or filter do you have any idea what they cost?? easy to replace? tuning have to be done after?


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## B&B (Nov 4, 2006)

The optical sensors are rather pricey..last one I bought was around $250. Which is why I mentioned trying the two other tips first before simply replacing the sensor.

The sensor is located in the top of the pump right under the intake...the inline filter is located right near it as the harness comes off the optical sensor.

They'll usually run with a defective sensor....they'll just run like crap. 

Here's a pic showing the inline wiring filter (rubber like square thing). And the round shaped black thing on the left side of the pic with the square electrical connector is the optical sensor itself....


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## Donny O. (Feb 11, 2005)

thanks again that is a big help. I have to say when you said expensive I thought you were going to say a grand....not that 250 isn't a lot of money I was jsut expecting more I guess. will definitly try the 2 ideas first and go from there. will try the bypass first then a tank or 2 on the way home with extra fuel additive is the plan!! when it gets home then it is fuel filter/diesel purge, all new fluids, etc, etc.


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## mcginner (Jan 30, 2008)

One thing to try before buying the truck is to put your hand on the tailpipe to block the exhaust to make sure the engine has enough power.


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## Donny O. (Feb 11, 2005)

truck was bought and drove all the way home to wisconsin....died a few times on the road and the tranny didn't want to shift a few times but it made it. been sitting ever since waiting on money to figure out what is wrong with it. room mate is trying to sell his explorer then should be getting it fixed....it is his truck.

I did remove that filter but it still did the same thing when first started....no power for a while then it sould throw a SES light and good to go til it was shut off again.. it is really hard starting so I assume it is all connected. it was 75 outside and took til the batteries were almost dead then poped right off.


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## Hudson0602 (Nov 19, 2008)

best advise I can give you is go join dieselplace.com they will know exactly whats wrong and save you some $$ and time


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## Donny O. (Feb 11, 2005)

well right now i'm more concerned with finding a job, and fixing my mailbox that his girlfriend ran over than fixing his truck for him....ya know I probably will go there and see what I can find out though. cant help myself plus I don't wnat it sitting parked in my driveway like his last truck did....not running of course.


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## RichG53 (Sep 16, 2008)

Then move...


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