P0128 Issues
 

Hey all, Haven't been around the site recently (College has been a handful) anyways, as i was on my way to get m car inspected wouldn't you know my check engine light pops on. I scanned the code and it turns out to be p0128 coolant temperature below thermostat regulating temperature. I talked to a few papers and came to the conclusion that the thermostat must have been stuck open. Bought one, installed it, changed the coolant and cleared the code. Less than five miles down the road it came back on again same code. Any ideas? I tried using the search to no avail. Any help is really appreciated i need to get this thing inspected.

So your dash temperature gauge is reading in the correct range?

Yes the gauge is reading normal and I havent noticed any gas mileage change either. I checked my fans and they run at the approriate times as well. Im stumped.

DTC P0128
Circuit Description

An engine coolant temperature (ECT) sensor monitors the temperature of the coolant. This input is used by the powertrain control module (PCM) for engine control and as an enabling criteria for some diagnostics.

The air flow coming into the engine is accumulated and used to determine if the engine has been driven within conditions that would allow the engine coolant to heat up normally to the thermostat regulating temperature. If the coolant temperature does not increase normally, or does not reach the regulating temperature of the thermostat, the diagnostics that use ECT as enabling criteria may not run when expected.

This DTC will only run once per ignition cycle within the enabling condition. If the PCM detects the calibrated amount of air flow and engine run time have been met, and the ECT has not met the minimum thermostat regulating temperature, DTC P0128 sets.

Conditions for Running the DTC

• The engine is running.

• DTCs P0112, P0113, P0117, P0118 are not set.

• The ECT is more than −40°C (−40°F).

• The ECT is less than 78°C (172°F).

• The intake air temperature (IAT) is more than −7°C (+19°F).

• The engine has been running for more than 2 minutes, but less than 30 minutes.

• The vehicle speed is more than 24 km/h (15 mph) for more than 1.609 km (1 mile).

• The mass air flow (MAF) average reading is more than 15 g/s.

Conditions for Setting the DTC

The PCM detects the following:

• The calibrated amount of engine run time has been met.

• The calibrated amount of engine air flow has been met.

• The calibrated ECT of 80°C (176°F) has not been met.

Action Taken When the DTC Sets

• The control module illuminates the malfunction indicator lamp (MIL) on the second consecutive ignition cycle that the diagnostic runs and fails.

• The control module records the operating conditions at the time the diagnostic fails. The first time the diagnostic fails, the control module stores this information in the Failure Records. If the diagnostic reports a failure on the second consecutive ignition cycle, the control module records the operating conditions at the time of the failure. The control module writes the operating conditions to the Freeze Frame and updates the Failure Records.

Conditions for Clearing the MIL/DTC

• The control module turns OFF the malfunction indicator lamp (MIL) after 3 consecutive ignition cycles that the diagnostic runs and does not fail.

• A current DTC, Last Test Failed, clears when the diagnostic runs and passes.

• A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other emission related diagnostic.

• Clear the MIL and the DTC with a scan tool.

How does the PCM know the engine has been running for more than 2 minutes? (or less than 30 minutes?)


The fans are not coming on too soon, are they? It should take a pretty long time for them to activate this time of year.

I haven't looked at the schematic, but some circuits have a 5 volt reference line which can cause false codes to set if it goes slightly low..... for example, a faulty TPS can cause the gas gauge to malfunction for that reason.

How does positraction rear end work? it just does. :posi:

I guess its base on the RPMs, that how my 84 works

But my friend had that code on his 99GA and his fans would go to high speed after two mins. Also his trans would shift real bad. Turns out it was the coolant temperature switch. Ohm out the switch.

Listened to it run today and realized the fans are going to high speed after a few minutes and occasionally it will shift hard. I'll definitely check the switch and let you guys know how it turns out. Thanks for all the input.

the ecm/bcm w.e one it is keeps track of how long the engines been running. when you look at it with a tech 2 you can see how long the engines been running before each dtc is set. kinda handy.

but...

we like to call it FM signals...

Fcking Magic.

gm has this chart for the diagnoses and ohming the sensor value. i tried to make it look nicer but the forum would cooperate.

°C °F OHMS Temperature vs Resistance Values (Approximate)

150 302 47
140 284 60
130 266 77
120 248 100
110 230 132
100 212 177
90 194 241
80 176 332
70 158 467
60 140 667
50 122 973
45 113 1188
40 104 1459
35 95 1802
30 86 2238
25 77 2796
20 68 3520
15 59 4450
10 50 5670
5 41 7280
0 32 9420
−5 23 12300
−10 14 16180
−15 5 21450
−20 −4 28680
−30 −22 52700
−40 −40 100700

Yeah, I know it DOES it, HP Tuners also runs a timeline chart on the bottom of the main window. I asked HOW, as in what sensors are involved. Or is there an internal clock?

But since old-school cars measure RPM with only basic electrical /mechanical / rotational diameter means, I'm sure the explanation is very simple :lol:

I'm just too lazy / old / forgetful to Google it.....:p