Why does my 2004–2006 Chevy Colorado 3.5 misfire at idle?

Short answer: On 2004–2006 Colorados and Canyons with the 3.5L five-cylinder (and the 2.8L four), a misfire at idle with a check engine light can be worn intake valve seats letting compression leak out. GM covered it under special coverage 07123 for 7 years or 100,000 miles, but that has long expired. Rule out plugs and coils first, then do a compression or leak-down test. If a cylinder is low, the cylinder head needs a valve job or replacement.

Symptom: A rough or shaky idle, a check engine light with a misfire code like P0300 (random) or P0301–P0305 (a specific cylinder), and in bad cases a cylinder with very low compression.

What to check first

1. Read the codes. A misfire code that names the same cylinder every time is a clue.
2. Swap the coil and plug with another cylinder. If the misfire moves, it's the coil or plug, not the valves.
3. Do a compression test or a cylinder leak-down test (pumping air into the cylinder and listening for where it escapes). GM's coverage required a confirmed leak-down test.
4. If compression is very low, air coming out the oil fill points to rings; out the intake points to an intake valve.

Likely causes

1. Worn intake valve seats. GM's special coverage says intake valve seats on these engines may wear and let combustion leak, causing a partial misfire and the Service Engine Soon light.
2. Worn valve guides / burned valve. A long-time 355Nation member said the real weak point was sub-standard valve guides that GM upgraded during 2008 production. Another owner found a burned exhaust valve.
3. Coils, plugs, or a dirty throttle body. Owners say to rule these out first; a rough idle without a misfire code is often something simpler.

What it usually costs

GM covered the repair under special coverage 07123 for 7 years or 100,000 miles, which has now ended. One owner had his own head rebuilt by a machine shop (new guides, seals and a valve job) for $315, plus $60 to fix broken exhaust bolts; another member quoted about $350 for that work. A GM reconditioned head was described as over $1,000, and a buyer worried about a $3,000 dealer bill he'd read about. Labor to remove the head is extra.

Is it safe to keep driving?

You can usually drive a short distance, but a steady misfire can overheat the catalytic converter and get worse. If the check engine light flashes, slow down and get it checked. Verify with a mechanic.

DIY or shop?

Compression and leak-down tests are DIY with tools. Pulling the head is a big job: owners note the cam timing procedure is easy to get wrong, and doing the timing set means dropping the front differential on 4x4s.

Recalls, bulletins and complaints in plain words

• No safety recall covers this.
• GM special coverage 07123 (a warranty extension, not a recall): 2004–2005 Colorado and Canyon with the 2.8L or 3.5L built in listed VIN ranges were covered for intake valve seat wear for 7 years or 100,000 miles. Owners on 355Nation cite revision 07123A, which they say added some 2006 trucks. The coverage has expired. GM's repair procedure is in bulletin 07-06-01-011.
• NHTSA complaints: 39 keyword-matched complaints for Chevrolet Colorado 2004–2006 (of 953 unique complaints queried, NHTSA complaints API by model year, queried 2026-10-18; keywords: misfire, valve, rough idle, compression, cylinder head, p030) GMC Canyon owners report the same problem.

What owners report

• In Service Bulletin #07123A engine intake valve seat/misfire on 355Nation, members shared GM's owner letter and described DIY checks: a long idle with accessories on, then a compression or leak-down test. Later posters with 2004–2005 trucks at high mileage learned the coverage had ended.
• In 3.5L I5 2006 Colorado (possible valve issue), an owner at 180,000 miles had a P0303 misfire. A new plug and coil didn't fix it, and cylinder 3 had 0 psi compression. With the head off he found a burned exhaust valve. A member who did his own valve job explained the guide problem and costs.
• In Worn valve seats, a buyer of a later 3.7L asked about warning signs. Members said the 2004–2006 3.5L was the main concern, though one 2007 3.7L owner said his head was replaced at 42,000 miles.

My take

My advice as a former lot tech: when a first-generation Colorado came in with a rough idle, I always did the cheap stuff first (plugs, coil swap, throttle body). But if a compression test shows one weak cylinder, stop throwing parts at it. It's the head.

Vehicle focus: 2004–2006 Chevrolet Colorado and GMC Canyon with the 3.5L five-cylinder (the 2.8L four-cylinder was included in GM's coverage too).

Safety / mechanic note

A steady misfire can overheat the catalytic converter. Verify with a mechanic.