Help Needed - 5th Gen won't start

Hello All!

Am at my wits end and about to tow my 2016 4Runner to the dealer for help. Maybe someone here can help before I do that. My son reported that he tried to start the truck and heard a click but it didn't start, tried again, another click, no start, tried a 3rd time and there was NO clicking and nothing after that. I assume the clicks he heard were the solenoid on the starter pulling in, but not sure.

Battery was found at 11.6V, so I replaced it as well as both the positive and negative battery post terminal connectors. I have also checked every fuse in the main fuse box in the engine compartment as well as every fuse in the compartment in the driver side interior....All good. I also replaced the ignition key switch. I checked what I think is the starting relay and it was good. I did not have positive 12V on the open contact for the starter though which I don't think is normal. I have not changed the starter because I don't think the relay is operating when the key is turned. When the ignition key is turned to accessory, the dash lights up and has many alarms lit. When turning the key to start, NOTHING happens. No relay, no nothing. The headlights work and the interior dome lights work. Many other accessories do not work. The radio, sunroof and many others do not work. When I connect my OBD reader, it does not power up from the plug. This plug may not have power either or there just might be an incompatibility on the pinouts but I doubt it.

Is the ECU toast?

Any help would be much appreciated. I don't have any electrical diagrams so it is very difficult to troubleshoot.

 

I asked the all mighty ChatGPT and this is what is spit out:



Here’s the good news: it’s almost certainly not the ECU.
On a 5th-gen 4Runner, ECU failures are extremely rare, but the symptoms you’re describing exactly match a blown AM1/AM2 circuit or a failed main fusible link, not a dead ECU.

Your clues are very strong:

• Battery was low (11.6V), then suddenly dead.
  
  
• After replacement, many interior circuits dead (radio, sunroof).
  
  
• OBD port has no power → critical clue.
  
  
• Dash lights come on but no crank, no starter relay click.
  
  
• Key switch replaced, still nothing.
  
  
• You don’t see 12V at the starter relay control side.
  

This is the exact symptom set for a blown fusible link or open AM1/AM2 40A/50A fuse in the under-hood fuse block.

The Most Likely Culprits (Ranked)
1. Blown Fusible Link Block (100A ALT, 30/40A AM1, 30/40A AM2)
This sits in the under-hood fuse/relay box and feeds the entire ignition circuit and OBD2 port.

When the battery was low and someone repeatedly tried cranking, plus you replacing terminals, it is extremely common for the fusible link to blow or crack internally.

Symptoms of blown fusible link on a 5th gen 4Runner:

• No power to OBD port
  
  
• No accessories like radio/sunroof
  
  
• No power to ignition-start circuit
  
  
• No crank, no relay click
  
  
• Headlights still work (they bypass the link)
  

2. AM1 or AM2 Fuse specifically
These two fuses supply:

• Ignition switch power
  
  
• OBD-II power
  
  
• Interior electronics
  
  
• Starter relay control side
  

If either one is blown → the truck will NOT crank and the OBD port is dead.

Check these carefully:

• AM1 (40A)
  
  
• AM2 (30A)
  

Sometimes they look good visually but are open when tested with a multimeter.

3. A loose or missing ground (critical)
When replacing terminals, the engine-to-chassis ground strap or body grounds can loosen.

If the main ground isn’t solid:

• Dash may light up
  
  
• But ignition/start circuits don’t complete
  
  
• OBD port won’t power
  
  
• Starter relay won’t trigger
  

Specifically check:

• Passenger side fender apron ground
  
  
• Engine block ground (near alternator bracket)
  

4. IG1 or IG2 Relay failure
These control power from the ignition switch out to key circuits.
If either relay is dead → OBD port, starter relay, and many accessories get no power.

Try swapping with a known-good identical relay in the fuse box.

❌ What it is NOT (most likely)
Not the ECU
If your ECU were dead:

• The dash would not light up correctly
  
  
• The check engine light would not illuminate
  
  
• The immobilizer light would likely flash erratically
  
  
• The truck might still crank
  
  
• Your OBD port would still have power
  

Your symptoms point straight to a primary power feed issue, not an ECU failure.

Quick Tests You Can Do Right Now (No diagrams needed)
Test A — Does the OBD port have 12V at Pin 16?
Pin 16 = constant 12V from AM1 circuit.
If it’s dead, AM1 or the fusible link is blown.

Test B — Check the ALT 140A / 100A fusible link
Use a multimeter — don’t trust visual inspection.

Test C — Check AM1 and AM2 with continuity
They often look fine but aren’t.

Test D — Check voltage at the ignition switch feed wire
If there’s no 12V arriving → fusible link or AM1 is open.

Why this probably happened
Low battery → multiple crank attempts → voltage sag → surge when connecting new terminals → fusible link popped.
This is very common on Toyotas.

 

For a 9 year old vehicle the integration relay bar may be worn out. Relays are the hardest thing to test because it makes contact doesn't mean it's allowing enough current thru, and you can't test the Amperage with regular multimeter. It's the first thing you'll come across in the engine bay fuse box. You'll need a long slender flat bit screwdriver to push in two tabs and lift the whole thing out (be careful of the attached 4 connectors). The relay bar contains 10 individual relays. As far as I know it contains the relays for ACC #1, Fuel pump, and Air/Fuel heater, and most critical electrical paths. Check your starter relay as well (purple one). At 100k miles or 10 years is when I start looking to replace all the relays.

Part number 82641-60140. (This is for '23 and '24 models); verify the part# for your particular model year.

 

Welcome

 

Welcome

 

welcome

 

Did you try putting it in neutral (N) and starting it?

 

This is SWEET. Nice Job @Borracho Loco

 

Yes, I tried a start with it in neutral. No go. Also, there is NO AM2 fuse at all and the AM1 fuse is a 7.5A in the interior box. I replaced it just to try and no go. The issue definitely seems to be a fuse or open circuit somewhere as many accessories are not working, but some are. I did replace the 30Amp starter fusible link and saw 12V at the starter relay contact. I have not found any fuses blown at all...

 

List all the dead accessories and that should help lead back to a common point. A schematic is really helpful at this point.

 

Radio, windows, door locks, AC did not work at a minimum. Headlights, dash lights, dome lights and horn worked. Almost seems like the 140 amp alternator fuse was blown given all of the loads that didn't work, but it was good as far as I could see. I ended up running out of time and shamefully towing it to the dealer for help. I will let everyone know what they find. I did not have exact schematics of the electrical system so it was challenging to go much further.

 

FYI. It was a blown 140A alternator fuse. I could not check this one out but visually it looked good. It is bolted inside of the fuse box. MAJOR pain to get to and I just ran out of time diagnosing it myself. Thank you to everyone who tried to help!

 

Looks like Chat GPT & @Borracho Loco nailed it (at least covered it) in his post:

 

What are the chances the tech at the dealer used this exact same query?