LS427 first start issues

[nick729]

Been a while, but finally got round to starting the engine for the first time. I am running Blueprint supplied LS427+T56 including the GM ECU and supplied engine harness. The car starts, idles roughly for around a second and dies, evidently the ECU is killing the engine.

OBD scanner is showing 5 faults all of which are relating to the same problem - Throttle position (TP) or Accelerator pedal position (APP) sensor/switch problems. I will paste the faults at the end of this post.

I did some live readings from the OBD scanner and the results are:

-Throttle position - live data from scanner showing TP at 30.2% (same result regardless if the APP connector is plugged in to the pedal or not). Not sure what the correct value should be.
-No Throttle - live data from scanner showing D and E at 19% and 9% (shouldn't these show as 0%?)
-WOT Throttle - live data from scanner showing D and E at 82% and 41% (shouldn't these show as 100%?)
-Unplugged - live data from scanner with the APP unplugged showing D and E at 0% (which is what it should show i guess?)
-Connector to the pedal - I checked voltages on the pins and both RED and White/Black show 5V when grounding the negative probe on a chassis ground. Shouldn't there be 2x 5V and 2x variable V depending on how much the pedal is pressed? Or does the variable V only show up when the APP is connected to the pedal (i tested with the connector unplugged)?

I am running LS cold air intake - MAF is connected and oriented the right way.

Any ideas/suggestions on where to start and what to test? :)

OBD faults:
Car Scanner ELM OBD2
Version: 1.121.0/401210/GP
DTC report
Connection profile: Chevrolet OBD-II / EOBDDate: 10/23/2025 5:00:18 PM
VIN: 19258269LS376R8
============================
OBD-IIOBD-II
DTCs: 5
----------------------------
P2120 [0x2120]
Throttle position (TP) or Accelerator pedal position (APP) sensor/switch D - circuit malfunction
Status: Confirmed
----------------------------
P2122 [0x2122]
Throttle position (TP) or Accelerator pedal position (APP) sensor/switch D - low input
Status: Confirmed
----------------------------
P2125 [0x2125]
Throttle position (TP) or Accelerator pedal position (APP) sensor/switch E - circuit malfunction
Status: Confirmed
----------------------------
P2127 [0x2127]
Throttle position (TP) or Accelerator pedal position (APP) sensor/switch E - low input
Status: Confirmed
----------------------------
P2128 [0x2128]
Throttle position (TP) or Accelerator pedal position (APP) sensor/switch E - high input
Status: Confirmed
============================
Engine control unitEngine control unit
DTCs: 5
----------------------------
P2120 [0x2120]
Throttle position (TP) or Accelerator pedal position (APP) sensor/switch D - circuit malfunction
General Electrical Failures [0x00]
Status: DTC supported by calibration, Current DTC, Test failed since DTC cleared, History DTC, Current DTC since power up, Warning Indicator Requested
----------------------------
P2125 [0x2125]
Throttle position (TP) or Accelerator pedal position (APP) sensor/switch E - circuit malfunction
General Electrical Failures [0x00]
Status: DTC supported by calibration, Current DTC, Test failed since DTC cleared, History DTC, Current DTC since power up, Warning Indicator Requested
----------------------------
P2122 [0x2122]
Throttle position (TP) or Accelerator pedal position (APP) sensor/switch D - low input
General Electrical Failures [0x00]
Status: DTC supported by calibration, Test failed since DTC cleared, History DTC, Current DTC since power up, Warning Indicator Requested
----------------------------
P2127 [0x2127]
Throttle position (TP) or Accelerator pedal position (APP) sensor/switch E - low input
General Electrical Failures [0x00]
Status: DTC supported by calibration, Test failed since DTC cleared, History DTC, Current DTC since power up, Warning Indicator Requested
----------------------------
P2128 [0x2128]
Throttle position (TP) or Accelerator pedal position (APP) sensor/switch E - high input
General Electrical Failures [0x00]
Status: DTC supported by calibration, Test failed since DTC cleared, History DTC, Current DTC since power up, Warning Indicator Requested



Thanks, Nick

[PNWTim]

Not to state the obvious but there is definitely something going on between the TPS and the pedal input. The first thing I would do is ensure there are no bent pins or female pins out of position. It's probably tough to see on the pedal but definitely check on the the throttle body. I am guessing you are using an E38 ECU? If so, reseat the harness to ensure nothing is going on there. These are pretty fool proof.

I would post on the LS1tech forum as well. You will probably be able to get detailed answers to your specific OBD questions. Good luck.

[JimStone]

I don't have any experience with your codes, but I find ChatGPT to be very helpful for things like this. I copied and put in your codes and here was the response:








Overview of the Codes

Code Sub-System Meaning Likely Signal Issue

P2120 APP/TP Sensor “D” Circuit malfunction Open, short, or failed sensor
P2122 APP/TP Sensor “D” Low input voltage Signal < ~0.2 V
P2125 APP/TP Sensor “E” Circuit malfunction Open, short, or failed sensor
P2127 APP/TP Sensor “E” Low input voltage Signal < ~0.2 V
P2128 APP/TP Sensor “E” High input voltage Signal > ~4.8 V



---

⚙️ System Background

Modern electronic throttles use two separate potentiometers in the accelerator pedal assembly (often labeled APP sensors “D” and “E”).
The ECU cross-checks both signals continuously:

APP D and APP E each sweep roughly 0.5 V → 4.5 V across pedal travel.

They move together but on different voltage scales so the ECU can detect faults or shorts.

If one drops out, spikes, or disagrees by >10%, the ECU disables throttle control and sets “limp mode.”



---

📊 What the Pattern Indicates

Because all five codes are present and confirmed, here’s what’s most likely:

1. Common Power or Ground Issue
Both D and E circuits are affected → strongly points to a shared 5 V reference or ground fault, not two simultaneous sensor failures.

Broken ground wire at pedal harness.

Corrosion in pedal connector.

5 V reference shorted to battery or ground (wiring chafe, water ingress).



2. Faulty Accelerator Pedal Assembly (APP sensor pack)
Less common but possible if internal potentiometers fail together.
Typical GM part houses both sensors in one molded unit.


3. ECU Connector or Internal Fault
Rare, but if 5 V ref and signal return measure fine at the pedal, the fault may be inside the ECM.




---

🔧 Diagnostic Path

1. Visual inspection

Unplug APP connector; inspect for corrosion, bent pins, moisture.

Check harness strain—often chafes near pedal pivot or firewall.



2. Voltage tests (key ON, engine OFF)

Pin Expected Notes

5 V Ref ~5.0 V If low → short to ground somewhere
Signal D ~0.5 V (idle) → ~4.5 V (full press) Smooth sweep
Signal E ~0.8 V (idle) → ~3.8 V (full press) Smooth sweep
Ground < 0.1 V drop If high → ground fault


Use a multimeter or scope; wiggle harness while monitoring to catch intermittent drops/spikes.


3. Clear codes → test drive
If all return instantly → circuit problem.
If they stay gone → intermittent harness connection.




---

🚫 Driveability Symptoms

Check-engine and/or “Reduced Engine Power” light

Throttle stuck at idle or limited to ~20%

No response to pedal input

Cruise control disabled



---

🧭 Recommended Fix Order

1. Clean/re-seat APP and ECU connectors (apply dielectric grease).


2. Verify 5 V reference continuity from ECU to APP.


3. Repair any damaged wiring.


4. If voltages test bad only at the pedal → replace pedal assembly.


5. Only if voltages test bad at ECU too → suspect ECU internal short or 5 V bus fault.




---

⚠️ Summary

All codes stem from the Accelerator Pedal Position sensor circuit, not the throttle body itself.
The simultaneous “D” and “E” faults (low + high inputs) nearly always indicate a wiring/reference/ground issue or a failed pedal module, not an ECM logic error.

[nick729]

thanks for the advice, last night went in with a clear mind, checked all the connections, grounds and what not and as per @PNWTim advice reseated the ECU connectors. Cleared all faults from the ECU, hooked it back up - no more faults and i can hear the TB opening and closing when i press the pedal! I am guessing reseating the ECU connectors was the solution as weird as that sounds, even though they lock into place and there is no way to connect them wrong or not fully - go figure :)

Anyways, now that the engine has no faults, I still can't get it to start. It cranks fine, but it is backfiring and spitting flames every time i try to get it started. Is this a sign of fuel flooded engine (due to many failed starts and never actually starting the engine)? Fuel pressure is 58psi so more or less ok, no other faults.

Pull out a plug and see if it is wet? If so, disconnect fuel pump, floor the gas pedal and crank the starter for 10s a few times? Or should i remove the sparks plugs, which is not what i want to do, as the driver side spark plugs can only be removed with the headers removed..

Any thoughts?

[nick729]

Update: spark plugs look ok.

220707220708

[Its Bruce]

With key on, engine off - does the gas pedal control the throttle body blade?

[nick729]

With key on, engine off - does the gas pedal control the throttle body blade?

Yes, the throttle body works just fine.

I think the issue i have is with the fuel system. I am running Holley 12-131 fuel pump with a return style fuel regulator just before the fuel rail. Fuel pressure gauge on the fuel regulator seems like it might be faulty, so I am waiting for a new one. I also ordered a fuel pressure testing kit that i can connect straight to the fuel rail to check if the pressure there is correct.

I tried to get the engine started yesterday with a little help from starter fluid, but with no success. There is a lot of backfiring and occasional flames from the exhaust, but it seems like the engine tries to start, but fails to fully start..

So i am waiting for the fuel gauge and test kit to make sure there are no problems with the fuel delivery. There are no engine fault codes, the fuel pump engages as it should on ignition, so until the parts arrive I can't do much...

[lewma]

Nick, I'd contact blueprint directly to ask for assistance. They are very good at replying and should be able to help you debug what's going on.

[lewma]

Nick, just noticed you're using a Holley 12-131 returnless fuel pump but have it hooked up with a return style regulator ? Not sure how that's going to work ? Could the regulator be flooding the fuel rails and that's why you're getting the backfiring and flames ?


For Reference, I'm using the Holley 12-132 return style.

mark

[nick729]

@lewma I am in contact with Blueprint, but it takes at least a day or more to get a response, so asking here for a second opinion and hoping for faster responses :)

Apologies, i mixed up the Fuel pump model, i have the same fuel pump as you do 12-132.

I have removed 2 easy to access spark plugs and they are now a bit wet after playing around and guessing the fuel pressure with the seemingly faulty regulator gauge. So i am waiting for new fuel regulator gauge and test kit for the fuel pressure at the rail. After seeing the pressures i will adjust as needed and hope for the engine to start.

In the meantime, i have discovered that 2 vacuum ports are open on the engine - power brake and dirty air return to intake. I had them covered with electric tape when testing and just realized that the vacuum ripped holes in the tape and they are now completely open. This could be the reason for the engine failing to start. I am waiting for silicone caps.

Anyways, i am going on a 2 week holiday, once i return i will have all the parts needed and will try my luck then :)