P0303 Code on a Cadillac: Cylinder 3 Misfire Detected (Cadillac Guide)

P0303 on a Cadillac is "Cylinder 3 Misfire Detected." Cylinder 3 is one of the few cylinders that GM has named in a Cadillac software bulletin. On 2014 ATS and CTS sedans with the 3.6L LFX, GM's PI1122B says a flashing check engine light on acceleration with P0300 and/or P0303 can be fixed with an ECM reprogram, once the normal misfire tests come up empty. Elsewhere in the lineup, cylinder 3 behaves differently depending on the engine. On the Escalade and the Gen 2 V6, cylinder 3 never deactivates. On the CT6's 4.2L twin-turbo V8, it does. On a four-cylinder, it's an inner cylinder. This guide covers where cylinder 3 is on each Cadillac engine, the GM documents behind each, and how to keep from replacing parts that were never bad.

What Does P0303 Mean on a Cadillac?

When the crankshaft fails to speed up as expected on cylinder 3's power stroke often enough, the ECM logs P0303. P0300 usually comes along with it. Every OBD-II gasoline Cadillac can set P0303. If several cylinders are counting misfires, start with our P0300 Cadillac hub.

Where Cylinder 3 Is on Your Cadillac

On a transverse V6 SUV, cylinder 3 is the hardest coil to reach. It's in the middle of the bank tucked against the firewall. Leaving the swap test until after you've ordered parts is a common, expensive mistake.

The 2014 ATS and CTS Reprogram: GM PI1122B

PI1122B (February 2014) covers the 2014 Cadillac ATS built before VIN breakpoint E0158537 and the 2014 CTS sedan built before E0145205, both with the 3.6L LFX. Customers reported the check engine light flashing when accelerating from a stop or after a steady cruise, with engine speed between 2,000 and 4,400 rpm. Some noticed no drivability problem, while others noticed a lot. The ECM may show P0300 and/or P0303 in history.

GM's instruction comes in two steps. First, perform all the published misfire diagnostics and fix any root cause you find. Only if nothing is found should the ECM be reprogrammed with the updated calibration GM released for this concern. In other words, a cylinder 3 misfire on one of these cars may not be a cylinder 3 problem at all. If your 2014 ATS or CTS fits the description, ask the dealer whether the latest calibration is installed. The PI points to the emission warranty labor operation, but whether any coverage still applies depends on your vehicle's age and mileage.

Coil Damage That Starts on a Different Cylinder

On 2008–2009 CTS and STS with the 3.6L LLT, GM's PIP5038B describes a heat-stressed or damaged ignition coil behind P0300–P0306 and coil circuit codes P0351–P0356. The cause is the ECM latching a coil's dwell line "on." GM states that dwell times over 50 milliseconds damage the coil. It also says "the cylinder with the damaged coil may not be the cylinder causing the issue." Electrical interference fed back from another cylinder's plug or coil can trigger it. That is why a P0303 coil can come back if you only replace the coil.

GM's list of what to check includes:

• Cracked, worn, carbon-fouled or wrongly gapped plugs
• Battery cable connections
• Coil ground and system ground condition — grounds G130/G131 on the CTS and G109/G0112 on the STS, torqued to 20 N·m (15 lb-ft)
• Underhood fuse block (UBEC) connector engagement, especially after timing chain or front cover work

For the 2012–2014 SRX and the 2013–2014 ATS and XTS with direct injection, GM's PIP5062C gives a dead-cylinder routine. Swap the coil and plug, read the plug wet or dry, and run an injector test. A wet plug suggests a leaking injector. If the plug is dry, add propane to the intake: if the misfire eases, GM suspects a weak injector driver in the ECM. Finish with a leakage test for valve sealing.

Cylinder 3 on Other Cadillac Engines

Escalade and V-series

Cylinder 3 is not an AFM cylinder on the 2007–2020 Escalade or LT4 cars. GM's V8 AFM lifter bulletins therefore don't explain a P0303 on those engines. On a 2021 Escalade 6.2L L87 built September 1, 2020 through March 31, 2021, 23-NA-043 lists P0303 among the codes for collapsed lifters, because DFM uses switching lifters on every cylinder. For 2015–2019 Escalade L86 and 2016–2019 CTS-V LT4, PIP5159C says to check for a loose plug wire at the coil and a missing "C" clip on the wire end.

CT6 4.2L twin-turbo

Cylinder 3 does deactivate on the LTA (20-NA-038). GM's 19-NA-253 relative compression test is the CT6-specific way to tell a mechanical misfire from an electrical one.

Four-cylinders

On the 2.0T LTG (2013–2018 ATS, 2014–2018 CTS, 2016–2018 CT6), PI1178F describes low compression in one cylinder from a cracked piston. GM replaces all four pistons if the bore isn't damaged. On the CT4, 25-NA-037 lists P0303 with an exhaust roller follower tick.

Symptoms of P0303

• Flashing check engine light on acceleration between roughly 2,000 and 4,400 rpm (2014 ATS/CTS LFX pattern)
• Shudder on acceleration with coil circuit codes (2008–2009 CTS/STS LLT pattern)
• Rough idle or a steady miss
• Low power on one cylinder you can feel at idle on a four-cylinder
• Sometimes no symptom at all — the code is in history only

Causes and How to Confirm Each

How Is P0303 Diagnosed on a Cadillac?

1. Identify engine and build. For a 2014 ATS or CTS 3.6L, note the VIN against GM's breakpoints.
2. Record the conditions — rpm, load, cold or warm. "Flashing light on acceleration, 2,000–4,400 rpm" is the PI1122B signature.
3. Swap the coil and plug, then inspect the plug for wear, cracks and gap.
4. Check grounds and connectors on LLT engines before replacing a second coil.
5. Test the injector and use GM's wet/dry plug logic.
6. Run a cylinder leakage test if cylinder 3 makes no power.
7. Only then consider calibration. On qualifying 2014 ATS/CTS models, reprogramming the ECM is GM's fix when nothing else is found.
8. Verify by repeating the acceleration that set the code.

Parts and Cost

GM Documents That Name P0303 or Cylinder 3 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 P0303 Pages Get Wrong

• "A cylinder-specific code always means a cylinder-specific part." GM's PI1122B fix for 2014 ATS/CTS is software.
• "Replace the burned coil and you're done." GM's PIP5038B says the damaged coil may not be on the cylinder causing the problem.
• "Cylinder 3 is at the front." It's the middle of the bank on the V6 and second back on the V8.
• Skipping the plug check. GM lists worn, cracked or wrongly gapped plugs among the things that damage coils.

Frequently Asked Questions

My 2014 CTS flashes the check engine light when I accelerate. Is it serious?

A flashing light always means a misfire bad enough to stress the catalytic converter, so ease off and have it checked. On 2014 CTS sedans and ATS models with the LFX built before GM's breakpoints, PI1122B describes this exact complaint with P0303 and an ECM update.

How do I know if my ATS is before the breakpoint?

The last characters of the VIN are compared with GM's breakpoint (E0158537 for ATS, E0145205 for CTS sedan). A dealer can confirm it from the VIN in seconds.

Why did my new coil on cylinder 3 burn out again?

On LLT engines GM ties repeat coil damage to electrical interference from plugs, grounds, battery cables or connectors. Fix those, not just the coil.

Is cylinder 3 shut off by AFM on my Escalade?

No on 2007–2020 Escalades with AFM. GM lists cylinders 1, 4, 6 and 7. On the 2021+ L87, DFM can deactivate any cylinder.

Can P0303 set on a four-cylinder CT4 or XT4?

Yes. Cylinder 3 exists on every Cadillac engine. On the CT4, GM's 25-NA-037 lists P0303 with an exhaust roller follower tick.

Should I replace the injector if the plug is wet?

A wet plug suggests too much fuel, and GM's PIP5062C says to check for a leaking or stuck-open injector. Confirm it with a balance test before replacing it.