1999 Mazda 626 V6 Missfire Diagnosis: Codes P0400, P1131, P0174 - EGR, Intake Gasket, or Injector Issues?

Start with the EGR valve and intake gasket—these are common causes in 1999 Mazda 626 V6s. A MAF sensor will not cause a misfire on only two cylinders; vacuum leaks or EGR flow issues can, but not a faulty MAF. Similarly, a crankshaft position sensor issue is unlikely if the diagnostic tool identifies specific cylinders as misfiring. If your valve cover gaskets are leaking oil onto spark plugs and wires, replacement is necessary regardless of other causes. The coil is also highly unlikely to be the root cause—vacuum leaks from intake gaskets or EGR problems are very common on Mazdas. Mechanics who don’t understand these fundamentals should not be diagnosing drivability issues. If multiple mechanics give conflicting advice, consider visiting a Mazda dealer technician—they often have deeper experience with this model. Also, if a mechanic removed an ignition wire to inspect a plug and then the engine misfires during drive, that suggests they may have damaged or improperly handled components—only replace original wires after confirming necessity. Pull over, find a trusted mechanic, and get a second opinion. Four different estimates don’t add up to a solid diagnosis—waste time and money on unnecessary parts.

Jeff, thanks for the response. I’ve already replaced the coil and it didn’t fix the issue. So I’m planning to inspect the EGR valve and intake gasket. The spark plugs and wires are around 15,000 miles old. I’d like to know: what’s the best way to test the EGR valve? And yes—I’ve been told to spray carb cleaner around the intake gasket to check for leaks—does that work?

Any additional input on this situation? Specifically, where is the EGR valve located on a 1999 Mazda 626 V6?

To test the EGR valve: use a vacuum pump. Apply vacuum to the EGR port—when you do, the engine should stall or stumble if the valve is stuck open.

I wouldn’t recommend replacing all those parts on a list without first diagnosing the root cause. Since P0174 shows Bank 2 is running lean and cylinders #2 and #6 are both in that bank, possible causes include clogged injectors or damaged wiring harnesses. First check for visible damage in the injector harness. If nothing stands out, perform a fuel injector drop test to determine if any injectors are plugged or failing. A lean condition caused by faulty injectors would explain both the misfire and P0174 code. While a bad MAF sensor won’t cause a two-cylinder miss, an EGR valve stuck open could—still, I’d prioritize injector testing before moving to EGR. Don’t hate me, COMP.

I disagree with the priority of injector testing first. For most DIYers, it’s more practical to start with cheaper, simpler fixes—like vacuum leaks—that affect all cylinders before replacing expensive components like injectors. I’m amazed how many DIYers don’t understand basic concepts such as vacuum leaks. I’ve seen Mazda 626s come in with both EGR issues and intake gasket leaks—so multiple problems are possible. But after four different mechanics giving conflicting advice, it’s clear that at least one of them lacks proper training or experience. If a mechanic removes an ignition wire to check a plug, writes an estimate, and then the engine misfires during drive, that suggests improper handling—don’t replace original wires without knowing they’re being replaced. Pull over, find a trusted technician, and go see them. Four estimates don’t add up to a solution—they just waste paper.

I approach problems from a 'what if the car were in front of me' perspective rather than what’s easiest or cheapest for a DIYer. That's why different mechanics give different advice—each has their own method and experience level.

Thanks for both replies. I should clarify—I’ve replaced air compressor, dryer, and orifice tube in my wife’s 2002 Camry last Thursday night, and I’ve also rebuilt the EGR valve and intake manifold on my 1996 Chevy truck. I believe I can handle this. However, after spending an hour under the hood and with the car on a rack searching for the EGR valve, I couldn’t find it. When I sprayed carb cleaner around the intake while running, there was no change in engine behavior. So if anyone could point me to where the EGR valve is located on this model, I’d really appreciate it. Also, I’ll check the injector harness and injectors as soon as possible. The issue appeared suddenly—so I didn’t suspect injectors failing simultaneously. I can confirm: you all earn your pay working on these vehicles—the V6 engine has a very tight fit. Thanks for the help.

Do you have a vacuum diagram or schematic under the hood? On some Mazda models, the EGR valve is located near the intake runners. The dealer parts department can confirm this—otherwise, consult a manual or an AllDataDiag subscription.

Thanks Jeff. No diagram under the hood—I’ll bet you’re right about it being under the intake. Does anyone else know the exact location?

Found it! The EGR valve is in a tight spot—when I disconnected the top vacuum line, anti-freeze poured out. Is this normal for a 1999 Mazda 626 V6?