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Old 11-17-2020, 05:55 PM   #1
soccercoach
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Default P0606 P0128 P1780 What is the real cause?

My L300 was on the freeway under moderate acceleration when "reduced power" and "check engine" appeared on the instrument panel. Limped to the next exit and turned off the car. After restarting, "check engine" stayed on and it seemed to drive fine. At home, the codes are:

P0606 ECM/PCM Confirmed
P0128 Temperature History
P0606 ECM/PCM History
P1780 Torque Reduction History MIL

I checked obvious things like ECM fuses for P0606.

For P0128, the car does warm up, the coolant level is fine, and the scan tool reads a warm engine in the high 170s, which looks about normal in the instrument panel gauge. This code is meant to signal a stuck thermostat, but the temperature does not act like the thermostat is stuck.

Do I need an ECM, or does anyone have other ideas?
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Old 11-17-2020, 07:41 PM   #2
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Default Re: P0606 P0128 P1780 What is the real cause?

Disconnect the temperature sensor (while engine is OFF) and then try to start the engine and report what happens.
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Old 11-17-2020, 08:38 PM   #3
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Default Re: P0606 P0128 P1780 What is the real cause?

P0128 is pointing to a faulty t-stat. Temps in the 170s is below normal. Our t-stats are rated for 195F. While it is a pita, replace it.

My L300 popped P0128 and I reset it several times during one winter until spring several years ago. I borrowed a reader that didn't display cooling temps. Several hours of work later and a personal reader displaying temps, the error code went away as temps run between 195F-200F, summer or winter in NYC (with or w/o ac use). The ecm monitors cooling temps from cold engine startup to expected operating temps to determine if a t-stat is operating correctly or not.

P0606 isn't saying much - ignore it for now as service manual descriptions explains internal ecm issues. P1780 is the TCM requesting the MIL (check engine light) to illuminate. Search within these forums for other L300 owners with this error code and any solutions for some background.
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Old 11-18-2020, 02:18 AM   #4
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Default Re: P0606 P0128 P1780 What is the real cause?

Quote:
Originally Posted by Rj 2000 LS2 View Post
Disconnect the temperature sensor (while engine is OFF) and then try to start the engine and report what happens.
Without the temperature sensor connected, the car throws code P0118, Engine Coolant Sensor Circuit High Voltage. Also, the electric cooling fan is on solid. The data stream of the code reader reports the temperature as -40F.

Measuring the resistance of the temperature sensor, it started ~ 450 ohms with a warm engine. Once the engine was running, it got as low as 390 ohms, and then crept back up pretty steady around 400 ohms.
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Old 11-18-2020, 02:54 AM   #5
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Default Re: P0606 P0128 P1780 What is the real cause?

Quote:
Originally Posted by fdryer View Post
P0128 is pointing to a faulty t-stat. Temps in the 170s is below normal. Our t-stats are rated for 195F. While it is a pita, replace it.

My L300 popped P0128 and I reset it several times during one winter until spring several years ago. I borrowed a reader that didn't display cooling temps. Several hours of work later and a personal reader displaying temps, the error code went away as temps run between 195F-200F, summer or winter in NYC (with or w/o ac use). The ecm monitors cooling temps from cold engine startup to expected operating temps to determine if a t-stat is operating correctly or not.

P0606 isn't saying much - ignore it for now as service manual descriptions explains internal ecm issues. P1780 is the TCM requesting the MIL (check engine light) to illuminate. Search within these forums for other L300 owners with this error code and any solutions for some background.
I inclined to suspect the thermostat. The car hasn't been driven much since the COVID excitement, so I am wondering if the thermostat is flat out broke, or might be stuck from disuse.

I cleared the codes and drove it on several trips this evening, and the codes have not come back yet. The ECT as monitored during driving peaked at 181 F, but mostly were in the low 170s, with an ambient temperature in the mid 50's.

I'll check it cold to see if the temperature sensor reads about the same as the outside temperature. Although it sure would be easier to change the temperature sensor than the thermostat, I suspect the temperature sensor is going to check out OK.

So assuming it is the thermostat, does anyone have a list of gaskets to replace during the process. I'd like to order the thermostat and all miscellaneous pieces at once.
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Old 11-18-2020, 08:47 AM   #6
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Default Re: P0606 P0128 P1780 What is the real cause?

T-stat may come with a new housing and polymer gasket but you'll need to order a pair of large O-rings to replace the ones on the extension coolant tube. This extension tube feeds hot coolant from t-stat to upper hose going to the radiator. I don't recall any other gaskets - my '03 uses polymer gaskets on the center intake and intake spacer. They were reusable and haven't leaked or shown signs of any vacuum leak. There's a small O-ring under the center intake manifold. The German clicker clamps are reusable too. Pry apart with a flat blade screwdriver, reattach ends with a plain slip joint plier. There's a thread or two with snapshots of t-stat replacement around here. You can find GM parts diagrams online to have an idea of what you're about to remove for t-stat replacement.

I was able to use my reader to display the same P0128 error on a Nissan Sentra belonging to a relative. It showed 170F temps. I don't recall operating temps but after replacing the t-stat, operating temps rose to around 185F with the error code going away on its own. The Sentra t-stat was easier to replace.
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Old 11-18-2020, 08:56 AM   #7
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Default Re: P0606 P0128 P1780 What is the real cause?

You'll need a full intake set of gaskets and you might as well replace the valve cover gaskets while in that deep. I found a complete set of gaskets on amazon which included the head gaskets and everything above.

It's a real pain in the rss to do. The EGR is the most tricky part. It's like a catch22 because one part can't come off until the other part is off. Best way is to remove the EGR completely and disconnect the exhaust return pipe and then leave the part the EGR (sits on and connects to) on upper intake and remove it all as one part. Don't try to remove the tiny aluminum shield, just remove a bolt that anchors the EGR mount to the rear engine lift bracket. Once done, the upper intake with the EGR mount will come out together.

The front and rear engine lifting brackets are also fun because there is no room for wrenches and some bolts are hidden underneath and covered with grime.

The thermostat is definitely bad, likely stuck OPEN. The engine needs to get to 195 degrees otherwise you are constantly pumping too much fuel attempting to get to operating temperature. I'm not too sure about the temp sensor, that range seems too small. I'd change that sensor too.
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Old 11-18-2020, 03:31 PM   #8
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Default Re: P0606 P0128 P1780 What is the real cause?

I never removed valve covers for a t-stat replacement (not required). A reader displaying coolant temps on a cold engine compared to intake air temps should be within a few degrees of each other to verify good sensors. All temperature sensors use the same thermistor.
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Old 11-18-2020, 05:44 PM   #9
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Default Re: P0606 P0128 P1780 What is the real cause?

Thanks all.

I'll get the thermostat and gaskets and then carve out a weekend to go through it. Since the codes all came at once, I'll keep my fingers crossed that fixing the thermostat fixes all of the codes. If not, then I might be asking for more advice.
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Old 11-19-2020, 01:16 AM   #10
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Default Re: P0606 P0128 P1780 What is the real cause?

It you have an Ir gun... I would test the temperature of the hoses cold and hot. This will confirm the thermostat is bad. You might have already done this. You seem to be a pretty good diagnostician. Either way, report back and we'll be here to help.

I suppose it is not necessary to replace the valve cover gaskets, but if you find any oil in the spark plugs wells or oil on the exhaust manifolds... change the valve cover gaskets!
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Old 11-23-2020, 10:47 PM   #11
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Default Re: P0606 P0128 P1780 What is the real cause?

I got it done. Monitoring temperature while driving is 189 to 199 F, and settles to 198 F when idling. I'm confident P0128 is fixed, and none of the other codes reappeared yet, but the scanner shows some of the OBDII tests are still incomplete. I'll keep my fingers crossed.

I removed the front intake runner, but just the bolts on the rear intake runner for wiggle room. I did not remove the valve covers. I did not disconnect the fuel lines. There is one position that the intake manifold was able to lift up the back edge to get it out of the well and flip it back on top of the fuel runner.

I replaced the thermostat with housing and the upper/lower intake manifold gaskets.

The biggest headache was the hose clamp at the front of the engine going to the radiator. It is so buried that it is almost impossible to reach. If I could have gotten a cable style hose clamp tool right away, I would have bought one.
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Old 11-23-2020, 11:43 PM   #12
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Default Re: P0606 P0128 P1780 What is the real cause?

I hope you took photos of the party!

Was the temperature test with the iR gun before or after the thermostat was changed? If the temp was getting to normal run operation, why did you change the thermostat?

New parts are normally better than old, but if it ain't broken, why bother? Just Cause, is an appropriate answer!
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Old 11-24-2020, 03:42 AM   #13
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Default Re: P0606 P0128 P1780 What is the real cause?

I don't have an IR gun. I used the engine's temperature sensor via my code scanner and logged data during a drive cycle. I was pretty sure the engine sensor was working because it matched ambient temperature after being parked overnight, but only reached the low 170s while driving. This reading in the 170s instead of near to the thermostat's 198 degree specification is what tripped the P0128.

To be clear, engine temperature sensor was reading temperature peaking in the 170s, causing code P0128. After thermostat replacement, engine sensor reads about 198 degrees.

Incidentally, ambient temperature was in the mid 50s. Colder weather would likely show lower than 170 with a broken thermostat. My thermostat could have failed a while ago, but it was in the 80s the previous time I drove it.

Last edited by soccercoach; 11-24-2020 at 03:50 AM.
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