2024 and Newer Mustang Check Engine Light Codes: The S650 Diagnostic Guide

Diagnosing an S650 is not the same job as diagnosing an S550, and the difference is not the engine.

The Gen 4 Coyote is a meaningful evolution of the Gen 3, but it’s recognizable. What actually changed is everything around it. The S650 runs an encrypted, fully networked electrical architecture with a locked powertrain control module and over-the-air software updates. Your car can change its own calibration overnight. It can develop new behavior you didn’t ask for. And in at least one documented case, Ford issued a recall specifically because the check engine light wasn’t coming on when it should have.

That last one tells you most of what you need to know about this generation. The diagnostic conversation has moved.

This guide covers 2024 through 2026 Mustangs: what these cars actually throw, what the recall picture looks like right now, and the software-related failure modes that no traditional code guide addresses.

New to codes? Start with the complete guide to Mustang check engine light codes for how the system works, scanner tiers, freeze frame, and readiness monitors.

What You’re Working With

Transmissions: the 10R80 ten-speed automatic continues across the range. The Dark Horse uses the TREMEC TR-3160 six-speed manual, the same family found in the Camaro ZL1 and CT5-V Blackwing, replacing the MT82 that gave the previous generation so much trouble.

What actually changed in the Gen 4 Coyote

Bore, stroke, displacement, and compression carry over from the Gen 3: 93mm by 92.7mm, 307 cubic inches, 12:1, with port and direct injection retained and plasma spray cylinder liners.

What’s new is airflow and hardware:

• Dual air intake box with dual 80mm throttle bodies. The strategy biases toward one throttle body at low RPM and load, then brings both in and synchronizes them approaching wide open throttle.
• New cylinder heads and revised camshafts on all four cams
• Revised 4-into-1 exhaust manifolds, where previous Coyotes used tri-Y designs
• Steel oil pan replacing the aluminum unit
• Revised cooling system

The Dark Horse adds forged connecting rods derived from the Predator, revised camshafts, and a different block. Notably, most of the Dark Horse’s power advantage over the GT comes from calibration rather than hardware. The hardware changes exist to survive how Dark Horse owners drive.

The diagnostic consequence of dual throttle bodies: you now have two electronic throttle control units, two throttle position sensor sets, and a synchronization strategy between them. That’s a failure surface that did not exist on any previous Mustang. Throttle-related codes on a Gen 4 need to be read with attention to which unit is being reported.

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Part One: Your Car Updates Itself, and That Matters

This is the single most important thing an S650 owner needs to understand about diagnosis, and no conventional code guide covers it.

The S650 receives over-the-air software updates. Ford can and does push powertrain control module calibration changes to your car without you doing anything.

If your car is tuned, an OTA update will overwrite the tune.

This is not theoretical. Tuners working on these cars have reported customers stranded after an overnight automatic update: the car updated itself, the aftermarket calibration was replaced with Ford’s, and the next morning a car running E85 was barely drivable with a check engine light nobody could explain. The fix was reflashing the tune and disabling automatic updates.

Practical guidance if you run any aftermarket calibration:

1. Disable automatic updates. Do this before you tune, not after.
2. If an update installs anyway, the first diagnostic step for any subsequent code is checking what calibration is currently loaded. Do not start testing hardware.
3. A code that appeared overnight with no change in how you drove is a software event until proven otherwise.

This inverts normal diagnostic logic. On a 2015 Mustang, a code that appears from nothing means something failed. On a 2025 Mustang, it might mean your car changed its own mind.

Part Two: Scanning an S650 Is Harder Than It Should Be

The PCM is locked

Ford built the S650 with a hardware and firmware-level encryption lockout, secure boot, and key-signed calibration authentication. Ford’s own engineers described the entire stack as encrypted, with the system designed to shut down components if it detects a break in authentication.

For diagnosis, this has a specific practical meaning: reading codes still works normally. The lockout is about writing calibration, not about standard diagnostic access. You can pull DTCs, read live data, and view freeze frame the way you would on any modern car.

Where you hit walls is configuration. FORScan on an S650 is limited to configuration-level access rather than calibration, and even that has proven inconsistent. Owners have documented specific FORScan version sensitivity, where a module reads correctly on one version and gets truncated on another, and where module block numbers changed between early and later production. If a module looks wrong or incomplete in FORScan, the software version is a real suspect before the module is.

The aftermarket tuning picture has been moving. Tier 1 partners with Ford calibration access have offered supported paths since launch, and independent tuning support for the platform has been developing since. If you’re researching this, verify current status rather than relying on anything written more than a few months ago, because it has changed more than once.

What this means for tool selection

• A basic Bluetooth reader will pull only generic powertrain codes. On a car with this many modules, that’s a small fraction of the picture.
• A mid-tier enhanced scanner is the practical minimum, giving you Ford-enhanced codes, transmission module access, freeze frame, and Mode $06.
• FORScan still offers the broadest module coverage available to owners, with the version-sensitivity caveat above.
• Dealer FDRS is required for anything involving reprogramming or calibration.

Part Three: The Recall Picture Is the Story

Across NHTSA data, the 2024 through 2026 S650 model years carry the highest concentration of current recall actions of any Mustang generation. Several of these present exactly as a check engine light or a wrench light, which means the single most valuable diagnostic step on an S650 is a VIN lookup, before you touch anything.

Open and recent actions affecting S650 Mustangs

Two things to take from this table.

First, several of these are engine-internal or fuel system items with real consequences, and they are free. The piston circlip and cylinder head recalls result in engine or cylinder head replacement at no cost. Nobody should be paying to diagnose a problem Ford has already agreed to fix.

Second, recall status changes constantly. This list is a starting point for your VIN lookup, not a substitute for it. Check NHTSA.gov and Ford’s owner portal separately, since Customer Satisfaction Programs and Field Service Actions never appear in the NHTSA database.

The 26E04 inversion, and why it’s worth understanding

Recall 26E04 deserves its own paragraph because it’s conceptually backwards from every other item on this list.

The defect is not that the car sets a code. The defect is that the malfunction indicator lamp may fail to illuminate for certain Intake Air Temperature sensor faults, meaning the vehicle does not conform to federal onboard emissions monitoring requirements. The owner would have no way to know the IAT sensor had failed.

If you own an affected car, this means a specific thing: a clean dash is not proof of a clean car. An IAT fault skews air density calculation, which skews fuel delivery and timing, which affects how the engine runs. You could have been driving with that condition and no indication at all.

Two practical notes. One, the remedy is a PCM reflash, which for tuned cars means the same overwrite problem discussed above. Two, if you’re on an affected VIN and have been chasing a vague drivability complaint with no codes, this is worth ruling in or out with a scan of live IAT data rather than trusting the dash.

Part Four: Gen 4 Coyote Codes

The Gen 4 shares enough architecture with the Gen 3 that most of the diagnostic logic carries over. If you’re seeing a code that isn’t S650-specific, the 2018-2023 guide covers the Coyote fundamentals in more depth.

What’s genuinely different:

Throttle and airflow codes

P0121 range, P2135, P2101, P2110 and related electronic throttle control codes.

The dual throttle body setup means these codes need to be read carefully. The two units operate on a coordinated strategy, and a correlation fault between them is a different problem than a single failed unit. Live data showing both throttle positions is far more useful than the code number alone.

MAF and intake air codes deserve extra attention on this platform given the 26E04 situation. If you’re seeing intake air temperature codes, check recall status before assuming hardware.

Oil pressure

S650 owners have raised oil pressure questions repeatedly since launch, and the recurring answer from the community is that Coyote oil pressure behavior looks alarming to people coming from other engines. Typical reported figures are a pegged gauge on cold start, roughly 20 psi at warm idle, and pressure climbing with RPM from there.

That said, an actual low oil pressure code is not something to normalize. The distinction is between a gauge reading that looks low and a PCM that has decided pressure is out of spec. The first is usually normal. The second is not.

Given that this generation has an active recall for cylinder head ball plugs causing oil leaks and potential engine seizure, oil pressure concerns on a 2026 car specifically warrant a VIN check first.

Cam timing and misfire

The VCT logic is unchanged in principle from the Gen 3, which means the diagnostic order is the same: oil level and condition first, then the VCT solenoid on the affected bank, and the camshaft position sensor last.

Misfire codes follow standard practice: freeze frame, then swap coils between cylinders to see whether the misfire follows. On a tuned S650, add one step at the front: verify what calibration is currently loaded.

Part Five: 2.3L EcoBoost

The four-cylinder carries into the S650 with the same fundamental architecture and the same code families covered in the 2018-2023 guide: P0299 underboost, boost control solenoid and wastegate faults, EVAP purge codes, and the usual misfire causes made harder by boost.

The order of suspicion for P0299 does not change: charge pipe connections first, then boost leaks generally, then wastegate and actuator, then boost control solenoid, then the diverter valve, and the turbocharger last.

One S650-specific note: forum discussion around the 25SD3 cylinder head recall indicates it affects four-cylinder models. If you have a 2026 EcoBoost and are dealing with an oil leak or unexplained oil loss, check that VIN before anything else.

Part Six: Mustang GTD

The GTD is limited production with a supercharged 5.2, and the owner population is small enough that a public code pattern hasn’t formed. Two things are worth noting.

It appears in the 25C43 body control module water intrusion recall alongside standard 2024-2025 Mustangs.

More practically: a car at this price and complexity level is not a candidate for owner-level guesswork. Scan it, document what you find, and take it to Ford. The diagnostic value you can add over the dealer is close to zero, and the downside risk is not.

Part Seven: Transmission

10R80

The ten-speed continues, and S650 owners report shift quality complaints similar in character to the S550 era, if generally improved.

The item to know is recall 25S19 (NHTSA 25V164) covering 2025 Mustangs with an incorrectly machined transmission main control valve body, which can cause reverse gear failure or unexpected forward movement in Reverse or Neutral. Ford noted that drivers may see both the MIL and the wrench light. This one is small in volume but severe in consequence, and Ford issued a delivery hold on affected inventory.

If you have a 2025 with any reverse or engagement complaint, VIN check before diagnosis.

For general 10R80 code families and the extensive TSB history on this transmission, see the 2018-2023 guide, which covers roughly thirty related DTCs and the bulletins addressing them.

TR-3160

The TREMEC six-speed in the Dark Horse replaced the MT82 and comes from a family used behind considerably more powerful engines. It does not carry the shift fork failure reputation that followed the MT82, and it is not a significant source of codes.

Part Eight: If Your S650 Is Modified

Everything in the modifications guide applies, plus three things specific to this platform.

One: the update problem is the modification problem. Covered above, and it’s the single biggest difference from any previous Mustang. Disable automatic updates.

Two: the aftermarket landscape is younger. Calibration support for this platform has developed in stages rather than arriving at launch, and parts that work on a Gen 3 do not necessarily work on a Gen 4. The dual-throttle-body intake, in particular, means that intake modifications on this engine are a different proposition than bolting a tube onto a Gen 3.

Three: catalyst codes behave the same. P0420 and P0430 after high-flow cats are the monitor reporting accurately, not a failure. The same caveats apply: masking the code hides a real problem if the converter is actually damaged, and working around an emissions monitor has legal implications that vary by where you live.

Quick Reference: Where to Start

Frequently Asked Questions

Can I read codes on an S650 with a normal scanner? Yes. The PCM lockout applies to writing calibration, not to standard diagnostic access. Reading codes, live data, and freeze frame works normally. What you cannot do is flash the PCM with aftermarket software the way you could on an S550.

Will a recall software update wipe my tune? A PCM reflash replaces the calibration in the module, so yes, expect to reload your tune afterward. Plan for it rather than being surprised by it.

My dash shows no warning lights. Does that mean nothing is wrong? Not necessarily, and recall 26E04 is the proof. It exists specifically because the MIL may fail to illuminate for certain IAT sensor faults. A scan tells you more than the dash does.

Is the Gen 4 Coyote reliable? It’s early, and the honest answer is that the data set is thin compared to the Gen 3. What we can say is that the current recall activity on this generation is concentrated in manufacturing defects, piston circlips, cylinder head ball plugs, injector O-rings, rather than design problems in the engine architecture. Those are different categories with different implications, and manufacturing issues tend to be batch-limited.

Should I be worried about the recall volume on this generation? Recall counts are a poor proxy for reliability, and modern cars generate more of them partly because more of the car is software and more faults are detectable. What matters is whether yours has an open one. Check the VIN.

Is FORScan worth it on an S650? For full module scanning, yes, with the understanding that its capabilities on this platform are narrower than on an S550 and that version compatibility has been genuinely finicky. Check current platform support before buying an adapter.

Related Guides

• Mustang check engine light codes: the complete owner’s guide — how the system works, scanner tiers, freeze frame, readiness monitors
• 2018-2023 Mustang trouble codes — the Gen 3 Coyote, 2.3 EcoBoost, Voodoo, Predator, and the 10R80 TSBs
• The 20 most common Mustang check engine light codes — with full diagnostic paths
• Full module scanning and hidden codes — the FORScan deep dive
• Check engine light codes after modifications — headers, tunes, boost, and E85

This guide is a diagnostic reference, not a repair manual. The S650 is a current platform, and its recall, TSB, and software picture changes frequently. Every item here should be verified against your specific VIN through NHTSA and Ford before you make a repair decision. Consult a qualified technician for anything beyond your tooling or experience, and for anything involving a vehicle still under warranty, let Ford perform the work and keep the documentation.