2003 Mazda MPV P0300 P0151 P0171 Diagnosis: Misfires, Lean Codes, and EGR Valve Issues

The 2003 Mazda MPV is known for ignition coil issues. For P0300 (multiple misfires), start with a thorough inspection of the fuel system — check fuel pressure and EGR valve operation. A lean condition like P0171 can often be caused by engine overcompensating for a misfire, meaning the MAF sensor isn't always the root cause. Prioritize proper diagnosis before replacing parts. Investing in accurate diagnostics typically yields better results than swapping components blindly. Best of luck with your troubleshooting, Mike.

Hi Jeff — thanks for the insight! I've now isolated the issue to the EGR valve. The code list has expanded to include P0103, P0131, P0151, P0171, and P0174. When stopped at a light in drive, the vehicle rocks and pops like a NASCAR — very noticeable. I’ve also noticed an odd brake pedal behavior: pressing the brake, releasing, then reapplying causes sudden stiffness and requires significant effort to stop. With my wife driving regularly, I haven’t tested it much myself. But with a trusted friend in the car and me under the hood, I can temporarily eliminate misfires and shuddering by gently tapping the EGR valve (not hitting it). The issue seems mechanical — I’m now waiting for the replacement part to arrive. Thanks so much for your input — this kind of detailed feedback is rare on forums. It’s truly appreciated! Mike

The rough brake pedal and engine shudder suggest low vacuum pressure, possibly affecting power braking. To test this: temporarily remove the EGR valve and install a block-off plate (e.g., from a tin can) to stop exhaust flow through it. If the vehicle runs smoothly afterward, that strongly indicates the EGR is stuck open and causing vacuum loss. This would confirm the root cause before replacing any components.

Update: I’ve completed repairs on my van. Tonight’s work included replacing the rear three spark plugs and ignition coils (after 189,000 km — they were likely worn). I also replaced the EGR valve. During disassembly, I found a severely degraded rubber elbow near the firewall at the back of the intake runners. It had a hole and was likely the main vacuum hose to the master cylinder. This component was probably being exposed to hot exhaust gases due to the EGR valve sticking open — leading to burning odors. Results: idle is now smooth, response and power have improved significantly, no new codes appear, and there are no unpleasant smells. I’m very satisfied with the outcome! Thanks again for your help — this was a great troubleshooting journey. Mike

Great to hear it’s working well now, Mike! A smooth ride and responsive brakes are definitely worth the effort. Given that the MPV shares platform components with Ford vehicles, vacuum hoses can degrade early — especially in older models. I also know from experience that many Mazda engines have recurring EGR issues. Always check the EGR valve before replacing a MAF sensor — it’s often the real culprit. Did you replace all six coils or just the rear three? I’d recommend focusing on the rear three, as they’re harder to access and more likely to be worn given their location and usage.

Hi Jeff — I replaced only the rear three spark plugs and coils (since I had the intake off). The front coils were last serviced about 30,000 km ago, so I didn’t prioritize them. Overall, the work went smoothly. Now I just need to locate and replace the worn PCV hose at the back of the manifold — that rubber elbow was clearly a weak point. Thanks again! Mike

FYI: There was a known recall on the PCV hose in 2004 for this model year — it’s prone to cracking and failure. This explains why that rubber elbow failed so early. Always check for such recalls when working on older vehicles.