Multiple DTCs at Once

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TresGatosUnDog
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Joined: Wed Jun 23, 2021 2:30 pm

Multiple DTCs at Once

Post by TresGatosUnDog »

Recently I was driving to work. It's a 4-lane 55 MPH highway. Of course everyone cruises about 65, and I was also. It's a nice smooth relatively flat road.

About 15 minutes after leaving the house I get a check engine light, and the AT Oil Temp light starts flashing at me. I pull over, check the transmission fluid, it's where it should be on the dipstick. No weird smells or burning smells. I wipe the fluid off with my bare fingers, it's not hot, definitely not hot enough for an oil temp light.

I have my OBD device with me so I hook it up. I get a P0327 - knock sensor circuit low; P1700 - tps malfunction; P0108 - MAP sensor high input.

I have recently had the engine out for a head gasket replacement. I replaced several items but didn't touch those sensors.

I've been doing some diagnostics and I've checked the MAP and knock sensor per the FSM. The MAP sensor checked ok. The knock sensor was intermittent in working so I'm going to replace it. The wiring harness seems fine, I tried and tried moving it around and shaking it to get a fault on anything. I watched the PIDs on my phone while messing with harness to see if anything glitched, I didn't see anything.

Does anyone have any ideas what may have happened? I'm guessing the knock sensor caused the timing to retard to a point where the car didn't want to run well. That caused the MAP sensor to show a fault, and that caused a TPS code and flashing transmission light. Pure speculation on my part.
TresGatosUnDog
Scoobytruck Lurker
Posts: 2
Joined: Wed Jun 23, 2021 2:30 pm

Re: Multiple DTCs at Once

Post by TresGatosUnDog »

I worked on the Baja this past weekend. I took the MAP sensor off and cleaned it.

I bought a new Subaru knock sensor and replaced the old one.

I checked over the wire harness again. This time from the sensors all the way to the ECU. I didn't see anything.

I've driven it all week and it's done ok. (Knock on Wood). I've kept my OBD2 plug on the car and recorded the MAP sensor data, engine load, and timing advance to see if any issues came up. I haven't observed anything yet. If there were a knock sensor issue is expect the timing to not advance as far, and get some weird load readings.

Maybe it was just a faulty knock sensor that set it all off. It takes a pretty sensitive digital ohm meter to see anything bad with the sensor. The old sensor sits in the 500k range, but can fluctuate by a few thousand ohms just sitting in a quiet space with no one touching it. On a running engine that few thousand (+/- 2500 ohms) could turn into ten or a hundred thousand ohms.

One thing that's neat is I can ground the plug and tap the sensor and see it generate a few tenths of volts, but I'm nerdy like that. I used to want to be an electrician and still have an interest in the subject.
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