2026 Jeep Grand Cherokee P0300–P0304? Save Every Spark Plug Before Restarting

P0300–P0304? Save All 8 Plugs

If the MIL flashes or the engine knocks or shakes badly, stop and tow it; repeated running can compound catalyst or cylinder damage. A technician should first check the VIN for applicable service actions, then remove and label all four main-chamber and four pre-chamber plugs by location. Clean, fouled, cracked and missing-ceramic conditions lead to different branches. Missing main-chamber ceramic requires cylinder borescope photographs and STAR-guided engine-replacement handling; it is not a cue to install one plug and retry.

When a main-chamber plug is missing ceramic, the next useful capability is a small inspection camera that can preserve the matching cylinder view before anyone turns the engine. An MRCARTOOL N220 can support that documented look through a compatible opening. N220 can photograph compatible cylinders through an approved spark-plug opening after plug positions are labeled and the engine is safe to inspect; it cannot recover every ceramic fragment, prove ring or valve health from one image, substitute for compression or leakage tests, authorize engine replacement or perform the required wiTECH reset.

This engine gives the technician eight pieces of removable evidence, and their locations matter. Mixing the four main-chamber plugs with the four pre-chamber plugs can erase the branch that separates a conventional misfire repair from possible internal engine damage.

Stop if the misfire is severe or the MIL flashes

A flashing MIL, violent shake, sharp mechanical knock or sudden loss of power is a stop-and-tow condition. Reduce load, pull over and switch off. Do not repeatedly restart the engine to identify the cylinder by feel. A severe misfire can overheat a catalyst, and Stellantis warns that spark-plug damage can precede engine failure in this scoped case. If the MIL is steady and the engine runs smoothly, avoid hard driving and arrange prompt diagnosis. Tell the shop whether the event began immediately after refueling, a cold start, recent plug service or a high-load acceleration.

Save freeze-frame data and check service actions

Before clearing anything, save P0300 and any P0301–P0304 codes with freeze-frame, misfire counters and complete module scan. Enter the VIN in the current Stellantis system and search for campaign 66D, TSB 09-017-26 and other applicable actions. The STAR document is a record of field experience and does not authorize every repair by itself. Record fuel grade, last fill station and service history. A single cylinder code is a locator; it is not proof that a coil, injector or plug caused the event.

Label all eight plugs before comparing them

This 2.0-liter application has four main-chamber and four pre-chamber spark plugs. As each is removed under the service procedure, mark its exact location before setting it down. Photograph all eight in a consistent orientation and lighting. Do not clean, gap or swap them before the comparison. Protect the open wells from debris. The location map lets the shop connect a damaged plug with the correct cylinder and plug type, supports any claim, and prevents a missing ceramic tip from becoming an anonymous part on a bench.

Plug condition	STAR direction
All clean and complete	Continue normal published misfire diagnosis
Fouled	Replace all eight, reset ECU and still find the fouling cause
Ceramic damaged but present	Replace all four plugs of that plug type
Main-chamber ceramic missing	Borescope/photo cylinder and follow STAR engine branch
Pre-chamber ceramic missing	Follow the controlled debris-clear procedure and replace that four-plug set

Separate clean plugs from fouled plugs

If the plugs are clean and complete, the STAR path says to continue published cylinder-misfire diagnosis. That means ignition output, injector control, air, fuel quality, compression and mechanical timing remain candidates. If the plugs are fouled, the document directs replacement of all eight with the latest main- and pre-chamber part numbers and the required wiTECH ECU reset, while still requiring checks for an underlying reason. Fouling is evidence, not a root cause by itself; oil, fuel, temperature or ignition conditions can foul the replacements again.

Distinguish a cracked insulator from missing ceramic

A crack that leaves the ceramic present is not the same branch as a plug whose insulator is gone. Compare each plug with the service examples and photograph the fracture surface. If a plug resembles the damaged-but-present example, replace all four plugs in that corresponding set—main chamber or pre-chamber—through the current instruction. Do not pry at remaining ceramic or use the plug hole as a vacuum-cleaner experiment. If any material is missing, assume it may be inside until the specified inspection proves what can be seen.

Photograph the cylinder when main-plug ceramic is gone

When a main-chamber plug is missing its ceramic insulator, Stellantis directs the technician to borescope and photograph the affected cylinder. Keep the piston and valves stationary until the view is planned; cranking can move debris. Capture the crown, cylinder wall and valve area from multiple controlled angles and label every image with the cylinder location outside the public photo. The STAR branch calls for engine replacement and restricted parts handling, so contact STAR promptly. A clean-looking frame cannot guarantee that all debris or damage is absent.

Follow the pre-chamber debris procedure exactly

A missing pre-chamber ceramic uses a different procedure. Block the other cylinder openings, then use the specified soft-tipped air method through the main-chamber opening to clear debris from the affected pre-chamber opening. That is a trained workshop step: eye protection, air control, piston position and debris containment matter. Replace all four pre-chamber plugs. Do not transpose the main-chamber engine-replacement direction or improvise with a hard nozzle. The plug type and missing-material path are exactly why the eight-plug evidence board must stay intact.

Reset and verify only after the approved repair

After the approved plug or engine repair, wiTECH must perform the guided ‘Reset ECU’ routine with ignition on and engine off as described, then the technician completes the applicable verification. Clear codes only at the prescribed point, start while monitoring misfire counters and inspect for noise before load. Check related bulletins and use the required fuel handling if an engine was replaced. A road test belongs last. The customer record should show original codes, labeled plug photos, cylinder images if required, authorized branch, reset and final misfire result.

The owner needs a safe answer, and the technician needs the evidence to give one. Stop a severe or flashing-MIL misfire, preserve the scan and label all eight plugs. Then let clean, fouled, cracked or missing-ceramic condition—and whether it is the main or pre-chamber plug—select the branch. An inspection camera earns its place only when material is missing. It does not turn one clear frame into proof that the cylinder is unharmed.

Official references

• Stellantis STAR Case S2618000003 Rev. G—2026 Grand Cherokee Misfire
• NHTSA Recall Lookup