P0303 Misfire Detected in Cylinder 3 - 1999 GMC Suburban Specific Diagnosis and Fixes

Hi! Check the fuel injector — it might be clogged. Try cleaning it thoroughly.

Thanks for the reply. I confirmed that my 1999 GMC Suburban does not have a throttle body; instead, it has a CPI unit under the intake upper plenum. Since this is an individual injector system, should I clean the CPI unit or focus on other components?

Check cylinder compression to rule out mechanical issues such as worn piston rings or valves.

A 1999 GMC Suburban does not use a throttle body. It has a CPI (Cylinder Pressure Injector) unit located under the intake upper plenum. This system uses individual injectors inside the CPI, not port fuel injection. So, injector clogging or performance issues in that unit should be considered.

I just checked cylinder compression this morning — cylinder 3 reads 140 psi. However, I accidentally reinstalled the spark plug and coil wire after removing them during testing, and now both cylinders 3 and 5 show constant misfires. Oh no! What did I do wrong? Any advice on how to fix this?

Could it be that the ignition wires were incorrectly swapped or not properly seated after reassembly?

Turns out, the spark plug cap and rotor I purchased were defective. A mechanic performed a full diagnostic and replaced them with higher-quality components. After this fix, the P0303 misfire disappeared when driving uphill. This experience taught me to invest in quality parts — even if they cost more. Thanks for the helpful input on this forum!

That scenario isn't uncommon. I've heard it before — many mechanics recommend using dealer-supplied components, especially for older GM engines like this one, because these systems are sensitive to quality variations. Aftermarket parts often match the price of OEM parts, so there's little financial incentive to take risks with lower-quality alternatives.

Don't laugh until you try it! My boss drives a 1999 GMC Suburban that he bought new. For over 18,000 miles, it has had multiple misfires — mostly in cylinder 5. The truck seems to know where the local dealer is better than its own home! After years of fixes with no results, we found a solution online: carbon buildup on valves. Solution: Use Sea Foam fuel treatment. Add 1/3 of a bottle to the fuel tank, 1/3 to the engine oil, and 1/3 directly into the intake while running. This has worked wonders — he's now driven over 1,000 miles with no misfire codes or uphill issues. A 16 oz bottle is available at AutoZone, Advance Auto Parts, or O'Reilly Auto Parts. If you're experiencing similar P0303 misfires, this might be worth trying!