P0128 Code

[EdHale]

A couple of times over the past few weeks my truck sets a P0128 code. It has always happened if I idle it a few minutes longer than normal before driving off such as when I'm hooking up my 5th wheel RV. This code is always followed by P0621.

Yesterday when the outside temp was in the low 50's I started it and drove off immediately, but slowly. After approx 1/2 mile and while doing 30-35 mph the code set again.

Am I looking at a bad thermostat that isn't staying closed long enough for the engine to heat up as fast as the computer thinks it should? I will admit this thermostat is original with 172,000 miles and 10 1/2 years service.

Thanks.

[More Power]

Thanks for your message Ed. Code P0128 sets if the Engine Coolant Temperature (ECT) fails to reach a preset target temperature before the calculated amount of fuel is burned. Changing the thermostats is usually the cure for this trouble code.

Some trouble codes could persist even after resolving the problem. You may need to clear the codes using a scan tool or complete a prescribed number of ignition cycles before the ECM will clear them on its own – usually 40 ignition cycles.

Code P0621 should be unrelated to the above mentioned code. However, P0621 indicates that there is a problem in the generator lamp L control circuit. The vehicle computer monitors the generator turn-on signal voltage. P0621 will set when the ECM senses a turn-on signal voltage that it doesn’t expect.

The voltage regulator on the alternator contains a fault detection circuit. If the regulator detects a problem, the regulator will produce a signal ground that causes the ECM to set the P0621 code. In short, it may be a good time to replace the alternator.

If there is a direct relationship between the setting of these two codes, it may be temperature related. Hard to know for sure until more parts replacement has been completed. Good luck, and let us know what finally develops.

More info on your trouble codes can be found here:
http://www.thedieselpageforums.com/t...ad.php?t=30830

[EdHale]

Thanks for the info. The thermostats are original as I stated so I wouldn't be surprised if they weren't staying closed long enough.

The alternator issue seems to only happen when the first code is set and my truck has 2 alternators so which one would be the next question?

I'm thinking it is probably thermostats first and see what happens with the other code.

I do have the ability to clear the codes which I do every time it shows up which has been 3-4 times total.

When watching my coolant temp it is only getting up to 158 or so when at highway speed and not towing.

[EdHale]

[QUOTE=More Power;317061]Thanks for your message Ed. Code P0128 sets if the Engine Coolant Temperature (ECT) fails to reach a preset target temperature before the calculated amount of fuel is burned. Changing the thermostats is usually the cure for this trouble code.

Ed wrote:

I took the truck in to the local dealer and they confirmed the thermostat issue. They didn't have them in stock, but sent someone to get them. They should be installed by mid afternoon. Sometimes dealing with a very small dealer in a very small town isn't all bad and they do sell a lot of diesels so their diesel tech is up to date and current on training and equipment. And they loaned me a truck to drive back to the RV park where I am hosting.

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Code P0621 should be unrelated to the above mentioned code. However, P0621 indicates that there is a problem in the generator lamp L control circuit. The vehicle computer monitors the generator turn-on signal voltage. P0621 will set when the ECM senses a turn-on signal voltage that it doesn’t expect.

The voltage regulator on the alternator contains a fault detection circuit. If the regulator detects a problem, the regulator will produce a signal ground that causes the ECM to set the P0621 code. In short, it may be a good time to replace the alternator.

If there is a direct relationship between the setting of these two codes, it may be temperature related. Hard to know for sure until more parts replacement has been completed. Good luck, and let us know what finally develops.

Edd wrote:

The dealer scan didn't show the P0621 code so I'm hoping it is temperature related or lack of temperature in my case. Either way I will be in this same location until 3-31 so I have time to get the alternator issue resolved before I leave if it turns up again.

I will post updates as they occur. At least on the way back to the park this afternoon the heater should work sooner and better. It hasn't gotten too hot in a long time.

[EdHale]

Picked my truck up after the thermostat change and drove it 18-20 miles and the alternator code never came back. During this test run the engine temps were in the 185-190 range vs 158 as before the change. At least my heater will work better this morning.

[More Power]

Thanks for the update.

[EdHale]

Yesterday was the first long distance drive I have taken since the thermostats were replaced. Everything is working normally and the alternator code has not come back, so it must have been temperature related or some other cause. I drove a little over 200 miles yesterday so I'm good as far as I know. Fingers crossed.