2005 Ford Thunderbird 3.9L PO420 Catalyst System Efficiency Issue - Misfire Without MIL Light
I own a 2005 Ford Thunderbird 3.9L 25th Anniversary Edition, purchased new and driven over 144,000 miles. It has been a reliable vehicle until about six months ago when intermittent misfiring began. Approximately two weeks ago, the car started misfiring occasionally without any check engine light (MIL) illumination. Over time, this progressed to frequent misfires at idle—still with no MIL light detected. After about two weeks of this behavior, the MIL finally came on. I scanned the vehicle and found a PO420 code: 'Catalyst System Efficiency Below Threshold (Bank 1)', which is not a direct cylinder misfire code like PO301–PO308, but confirms that the Powertrain Control Module (PCM) detects an issue with exhaust catalyst efficiency. Five days later, the MIL turned off again and has not returned. However, the misfiring persists—especially noticeable at idle. I replaced five of eight coil packs about three months ago due to multiple misfire codes. I also installed OEM Motorcraft iridium spark plugs at that time, which temporarily resolved the issue until recently. My question: Can the PCM fail to detect and log a misfire code even when actual misfiring is occurring? Given how noticeable the misfires are, I would expect the PCM to trigger a code. Is it possible that the problem lies elsewhere—such as fuel injectors or internal combustion issues? I’m looking for a diagnosis without replacing too many parts. Any insights or suggestions would be greatly appreciated. Thanks!
Using an OBD2 scanner, check the misfire counters in Mode 6—the actual misfire count for each cylinder may be high even if no PO30x code is set, meaning the misfire is below the threshold to trigger a code. You should also inspect fuel injector performance and consider using a fuel system cleaner like SeaFoam or Techron. A buildup of varnish in the injectors could cause intermittent misfires. With the scanner, you can analyze fuel trim values (FTs), check plug conditions (wet/dry, black/tan), and identify which cylinder is most affected. Suggestion: Add a bottle of SeaFoam or Techron to your fuel tank—this may resolve a minor varnish issue. Let me know the results. PS: OEM parts are generally recommended for best performance—especially in spark plug-related issues.