Does the ECM control the MPH gauge?

More than likely a broken wire, corroded connector or a chafed wire.

Question edited for clarity

Take it to a good local auto-electrician as advised.

Jeff Armer Oct 11, 2021
Have it checked for DTC'S - diagnostic trouble codes , before you start playing with thing's . All the gauge setting information comes to the instrument cluster over serial data network GM Class 2 from the PCM/ECM engine computer . Could have U codes stored, Do you know anything about serial data networks or U codes ? If not, do yourself a favor an have a qualified repair shop check it.

https://www.youtube.com/watch?v=I6RALaUk...

Fuel Gauge
The instrument panel cluster (IPC) displays the fuel level based on information from the engine control module (ECM). The engine control module (ECM) calculates the fuel level based on information from the fuel level sensor. The engine control module (ECM) sends the fuel level information to the body control module (BCM) via the serial data circuit. The instrument panel cluster (IPC) displays the fuel level as received from the BCM. The fuel gage defaults to E (empty) if:

• The ECM detects a malfunction in the fuel level sensor circuit.

• The IPC detects a loss of serial data communications with the BCM.

• The BCM detects a loss of serial data communications with the ECM.

When the fuel level is less than a pre-determined value, the Low Fuel indicator message illuminates.

Speedometer
The instrument panel cluster (IPC) displays the vehicle speed based on information from the engine control module (ECM). The engine control module (ECM) calculates the vehicle speed from the vehicle speed sensor (VSS) input. The ECM sends a serial data message to the BCM indicating the vehicle speed. The instrument panel cluster (IPC) displays the vehicle speed as received from the BCM via serial data circuit.

The speedometer defaults to 0 km/h (0 mph) when any of the following occur:

• The ECM detects a malfunction in the vehicle speed sensor.

• The BCM detects a loss of serial data communications with the ECM.

• The IPC detects a loss of serial data communications with the BCM.

An intermittent problem like this is usually caused by a wiring/connector problem, probably at the back of or near the instrument cluster (IC). The IC performance should not cause engine problems - it is mainly a information readout device, but does have a datalink connection to the ECM and if there is a wiring problem with the datalink, you will get the intermittent.

This may have started with the way your aftermarket sound system was wired in and the physical impact it may have on wiring nearby. You may have a damaged cluster, but the only thing I can suggest at this point is to carefully check your wiring in the dash, particularly how you have wired in the power feeds to the radio.

have the fault codes read and fix the fault that put the ECM in limp mode
reset the ECM on completion of repairs

The first part is magic, (EFI system and good engine)
but?????
gauges go off can be a bad fuse or loose fuse to cluster.
when any Gas engine stalls. the RPM drops to 0
the EFI sees that and goes to key on, not started self test.
this is covered in the operator guide.
key light comes on KEY LIGHT, HUMMMMMMM
Let me look at the waring lamps, there are many.
one light is engine overheat that is logo of a thermometer but looks
just like a key.
http://dashboardsymbols.com/the-symbols/
is this a engine stall post
or engine runs perfect all the time , but only my dash acts funny
you didnt say?

key lamp?
immoblilzer (big trouble there)

a stall is anytime engine RPM drops to zero. called a STALL.
if your cars engine stalls, post that way, the lamps are all normal
on at any stall, on any car.
at a stall , rpm goes to zero and MPH drops to zero as you coast to a stop , stalled. that is all normal for any stall.

if engine stalls.
that is complex.
here is list.

1. lost 12vdc power to EFI system. (battery died, fuse blown)
2. never gave your car the 60,000 mile tune up, see opr, guide.
3. bad fuel.
4. overheated the engine.
5. engine oil gone, never ever checked at each fill up.
6. other more complex EFI system parts failures, ASK.

The speed sensor might be bad. I used to own a 99 Mitsubishi Eclipse with the same symptons. The car will start, but will not pass the 1st or 2nd gear, the speedometer will be at 0. I replaced the speed sensor and all the problems went away.

Not sure what is causing it, but it sounds like you're running on 3 cyllinders. Check plugs, wires, timing . timing belt may have jumped, bad injector, low fuel pressure.

The problem you describe has NUMEROUS causes. Most common of these is a COIL is going bad. The RPM gauge gets its signal from the ignition coil/crank sensor circuit (could be either one OR could be the wiring between the engine control module (ECM) and the engine components. Does the Check Engine light come ON when this happens? Whenever the "Check Engine" light comes on, there is always a DTC (Diagnostic Trouble Code) stored in the ECM (Engine Control Module). You'll need an OBDII (On-Board-Diagnostics) scanner to retrieve any codes recorded/set in ECM. Once you do this and let us know what, if any codes are set, then we can go from there.

have them check the oil sending unit

It could be the electronic power steeirng module . With the gauges dropping out like that , the module could be bring down the data bus . Don't know how much you know about automotive electronic's an how the insturment cluster know's what speed to set speedo at an how full fuel gauge is an rpm on the tach .
Speedometer
The instrument panel cluster (IPC) displays the vehicle speed based on the information from the engine control module (ECM)/powertrain control module (PCM). The ECM/PCM calculates the vehicle speed based on the class 2 message indicating the wheel reference speed from the electronic brake control module (EBCM) if:
• The ECM/PCM detects a malfunction in the vehicle speed sensor circuit.
• The IPC detects a loss of class 2 communications with the ECM/PCM.
The speedometer defaults to 0 km/h (0 mph) if the IPC detects a loss of class 2 communications with both the ECM/PCM.

012 GM Class II Communications

Checking the vehicle modules for DTC'S - diagnostic trouble codes is the first place to start .
The Power Steering Control Module (PSCM) receives a class 2 vehicle speed message from the Engine Control Module (ECM) via a class 2 serial data circuit. The PSCM uses this vehicle speed message, and other inputs, to determine the desired amount of steering assist.
DTC C0000
DTC C0475
DTC C0476
DTC C0545
DTC C0847
DTC C0848
DTC C0896
DTC C0899
DTC C0900

your best bet ,take it to a qualified GM repair shop .