P0305 Code on a Cadillac: Cylinder 5 Misfire Detected (Cadillac Guide)

P0305 on a Cadillac means "Cylinder 5 Misfire Detected." It can only set on a Cadillac with six or eight cylinders. The 2.0T, 2.5L and 2.7T four-cylinders in the ATS, CTS, CT4, CT5, CT6, XT4, XT5 and XT6 have no cylinder 5. If a scan tool shows P0305 on one of those, suspect the tool or the engine identification. On the Gen 2 3.6L and 3.0L twin-turbo V6, cylinder 5 is one of the two cylinders GM deactivates. GM's 2025 preliminary information PIP5824D gives a free first test: swap the two AFM oil control solenoids between the cam covers and see whether the misfire jumps banks. GM's original cold-start coolant bulletin even uses a cylinder 5 photo as its example of real block porosity. This guide covers where cylinder 5 is on every Cadillac engine that has one, and what GM says about it.

What Does P0305 Mean on a Cadillac?

The ECM sees the crankshaft fail to accelerate on cylinder 5's firing event often enough to set the code. P0300 usually sets alongside it. If more than one cylinder is counting, start at our P0300 Cadillac hub.

Which Cadillacs Can Set P0305

• Yes: every Cadillac V6 (3.6L LY7, LLT, LFX, LGX; 3.0L LF1/LFW; 2.8L turbo; 3.6L twin-turbo LF3/LF4; 3.0L twin-turbo LGW/LGY; and the 1997–2001 Catera 3.0L). Also every Cadillac V8: Northstar, Escalade, CTS-V, CT5-V and the CT6 4.2L.
• No: four-cylinder versions of the ATS, CTS, CT4, CT5, CT6, XT4, XT5 and XT6. They can only set P0301–P0304.
• No: Lyriq, Optiq and other Cadillac EVs.

Several Cadillac nameplates were sold with both a four and a V6, including the ATS, CTS, CT6, XT5 and XT6. Check the engine RPO code on the glovebox label before you start.

Where Cylinder 5 Is on Your Cadillac

Gen 2 V6: The AFM Solenoid Swap Test

GM's 21-NA-036 says cylinders 2 and 5 "are the active fuel management cylinders" on the LGX, LGW and LGY. It covers the 2016–2019 ATS and CTS, 2020–2022 CT5, 2016–2020 CT6, 2017–2022 XT5 and 2020–2022 XT6, and its DTC list is P0300, P0302 and P0305. Cylinder 5 is on bank 1 and cylinder 2 on bank 2, and each bank's cam cover carries its own AFM oil control solenoid. GM's 2021–2025 follow-up uses that layout for a simple test.

PIP5824D (2021–2025 CT5 3.0L, XT5 and XT6) describes misfires on cylinder 2 or 5 with a possible rough idle. They can happen all the time, after an AFM event, or on a cold start and then go away. GM's correction, in order:

1. Swap the AFM rocker oil control solenoid from side to side in the cam covers and re-evaluate.
2. If the misfire goes away or moves to the other bank (P0305 becoming P0302), replace the solenoid.
3. If it stays on cylinder 5, or comes back after an AFM event or a cold start, remove the bank 1 cam cover. Turn the engine by hand while watching cylinder 5's valves.
4. If the valves open and the cylinder is a dead miss all the time, continue normal misfire diagnosis. If an intake or exhaust valve doesn't open, replace all four rockers (SRFFs) and all four lash adjusters (SHLAs) on that cylinder.

GM lists the parts as switching valve rocker arm 12666601 (quantity 4) and dual-feed hydraulic lash adjuster 12690246 (quantity 4). Confirm the current numbers by VIN, because GM notes the PI will change when new parts come out. GM also says the PI will become a bulletin once engineering decides "there is no longer a parts concern with these rockers," which tells you the rockers themselves were the issue.

On these same engines, 21-NA-036 describes solenoids that break when removed, leaving the tip stuck in the cam cover bore. That piece must come out before a new solenoid will seat.

Cold-Start Coolant on Cylinder 5

Bulletin 18-NA-115 describes cold-start misfires with "medium to high misfire counts, always occurring on one or two cylinders" on ATS, CTS, CT5, CT6, SRX, XTS, XT5 and XT6 V6 engines. The cause is block porosity at the liner-to-deck face letting coolant in. The earlier version, PIP4959D (2012–2014 CTS and SRX, 2013–2014 ATS and XTS with the 3.0L LF1/LFW and 3.6L LFX), illustrated what true porosity looks like with a photo captioned "# 5 at the 10:00 o'clock position."

That doesn't mean cylinder 5 is the only one affected. But it shows what GM's technicians look for: a break in the carbon ring at the deck-to-liner edge, where coolant has washed the carbon clean. GM's caution matters here. Small pits on the deck are normal and aren't a reason to replace the engine. Fuel on a piston crown can be mistaken for coolant, which is why GM uses dye and a blacklight.

Cylinder 5 on Escalade and Other V8s

Cylinder 5 is not an AFM cylinder on 2007–2020 Escalades or LT4 cars, so AFM lifter collapse is not GM's documented explanation there. Two V8 documents stand out for cylinder 5:

• PIP5169 (2002–2013 Escalade, 2006–2013 CTS-V and other GM V8s): P0300 setting after engine repairs. GM says injector connectors can be crossed between cylinders 5 and 7 and between 6 and 8. The misfire "may not be detected and may happen during deceleration." If your P0305 started after a valve cover, intake or injector job, check the injector connector routing first.
• 23-NA-043: 2021 Escalade 6.2L L87 built September 1, 2020 through March 31, 2021, with collapsed lifters and P0300–P0308, including P0305.

On the CT6 4.2L, cylinder 5 is a deactivating cylinder (20-NA-038). GM's 19-NA-253 relative compression test tells you whether a misfire is mechanical.

Symptoms of P0305

• Check engine light; flashing during a heavy misfire
• Misfire after the engine leaves V4 mode, or a cold-start miss that fades (Gen 2 V6)
• Cold-start miss with medium to high counts that disappears once warm (porosity pattern)
• Misfire on deceleration after recent engine work (crossed injector connectors on V8s)
• Rough idle; a tick from the bank 1 cam cover

Causes and How to Confirm Each

How Is P0305 Diagnosed on a Cadillac?

1. Confirm the engine is a V6 or V8. A four-cylinder cannot set a genuine P0305.
2. Locate cylinder 5. On a transverse XT5 or XT6 it's on the rear bank at the transmission end, the least accessible spot.
3. Swap the coil and plug, then test the injector.
4. On a Gen 2 V6, swap the AFM solenoids between banks and see if the misfire jumps to cylinder 2.
5. Check the conditions. Cold-start only points to 18-NA-115's dye test. After-AFM-event points to the rockers.
6. Inspect valve movement on cylinder 5 with the bank 1 cover off.
7. On a V8 after recent repairs, verify injector connector routing for cylinders 5 and 7.
8. Verify under the original conditions and check your VIN for bulletins.

Parts and Cost

GM Documents That Name P0305 or Cylinder 5 on Cadillacs

Have a dealer check your VIN for open recalls and bulletins. The check is free, and it is more reliable than any article, including this one.

What Other P0305 Pages Get Wrong

• Listing P0305 for every Cadillac. Four-cylinder Cadillacs can't set it.
• "Swap the coil, and if it doesn't move, it's the injector." On a Gen 2 V6, GM's next step is the AFM solenoid swap, then valve movement.
• "Any pitting on the deck means a bad block." GM says small pits are normal. True porosity shows as a break in the carbon ring and dye under blacklight.
• Forgetting recent repairs. On V8s GM documents crossed 5–7 injector connectors after engine work.

Frequently Asked Questions

My XT6 has P0305 only on cold mornings. Rockers or coolant?

Both patterns exist. PIP5824D notes AFM-cylinder misfires on a cold start that then go away, and 18-NA-115 covers coolant entry on a cold start. The solenoid swap and the dye-and-borescope test tell them apart.

Why would a scanner show P0305 on my XT4?

The XT4 uses a four-cylinder engine, so a genuine P0305 isn't possible. Recheck the code with a different tool and confirm the vehicle's engine.

Where is cylinder 5 on a CTS 3.6L?

At the rear of the passenger-side bank, next to the transmission bellhousing.

Can I replace just the one broken rocker on cylinder 5?

GM's instruction is to replace all four rockers and all four lash adjusters on the affected cylinder, not a single rocker.

Can a broken AFM solenoid tip cause P0305?

Yes. GM's 21-NA-036 describes oil bleeding off past a damaged solenoid, which deactivates the cylinder when it shouldn't. A tip broken off in the cover bore must be removed first.

Is P0305 on an Escalade an AFM lifter?

Not on 2007–2020 AFM engines, where cylinder 5 doesn't deactivate. It's possible on the 2021+ L87, which GM's 23-NA-043 covers.