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1996 Dodge Grand Caravan P0420: Catalytic Converter Failure & Fuel Trim Issue After Repair

Model: 1996 Dodge Grand Caravan Fault Code: P0420 Posted: 2006-12-29 21:30

I own a 1996 Dodge Grand Caravan with a 3.3L engine, automatic transmission, and approximately 135,000 miles. I recently encountered the P0420 diagnostic trouble code (DTC), indicating a catalytic converter efficiency failure. Using laptop-based scan software, I observed that the front upstream oxygen sensor was oscillating normally between 100 and 700 mV — within expected range. However, the rear downstream O2 sensor showed readings closely following the upstream sensor, which strongly suggested the catalytic converter had failed. Upon inspection, I found several spark plugs with erosion so severe that their gaps were nearly double the manufacturer's recommended value. I replaced all spark plugs and ignition wires (based on mileage), along with the front oxygen sensors as a precaution, and installed a new catalytic converter. My hypothesis was that the worn spark plugs caused intermittent misfires — even without any stored misfire codes — leading to excessive fuel delivery that overwhelmed the catalytic converter, effectively 'eating' it out. After installation, the scan software shows the front O2 sensor still cycling between 100–700 mV as expected, while the rear downstream O2 sensor is now stable around 700–750 mV. I have two specific questions: 1. Even though the downstream O2 sensor readings are now stable, shouldn't they typically fall in the 400–450 mV range when the catalytic converter is functioning properly? 2. Why is the Short Term Fuel Trim for Bank 1 Sensor 2 showing a constant value of 99.2% — an extreme upper limit? Is this reading accurate, or could it be a software artifact? Any insights on what else to investigate for these issues? Thanks in advance.

Related fault codes

P0420

Comments (5)

Anonymous 2006-12-30 07:25

I suspect either a software issue with the Powertrain Control Module (PCM) or that your vehicle is still running too rich. Has the P0420 code cleared and does the check engine light remain off? If so, don't overemphasize the 99% fuel trim reading at this stage.

Anonymous 2006-12-30 20:19

I was considering whether the Powertrain Control Module (PCM) might reset the P0420 code after completing a sufficient number of drive cycles. I’ve been hesitant to drive aggressively until I could confirm other underlying issues. You mentioned a software glitch — were you referring to the PCM, the scan tool, or both? For now, I’ll give the vehicle some real-world driving over the next week. If the P0420 code clears and the check engine light goes off, I’ll focus less on the fuel trim values (unless the catalytic converter turns red). It does appear the converter is working since the downstream O2 sensor readings are now stable. Thanks for your input — you're a regular on these forums!

Anonymous 2007-01-02 17:39

If the 99% fuel trim value were truly accurate, I’d expect to see additional related codes from the PCM. Since the P0420 code is gone and the check engine light is off, but the fuel trim remains at 99.2%, and both O2 sensors are new and functioning properly, my next step would be to verify these readings using a professional-grade scan tool. I’m not overly familiar with AutoTap or similar software due to limited daily use, so I’d recommend comparing the fuel trim data against a dealership-level scanner to confirm whether it’s truly out of range. If the vehicle is running smoothly, maintaining good mileage (e.g., 24–26 mpg for a 3.3L engine), and showing no drivability issues, then a 99% correction factor seems unlikely. Potential root causes like vacuum leaks, EGR valve problems, or low fuel pressure could still be present but might not be immediately noticeable. A 99% trim would typically indicate significant drivability concerns.

Anonymous 2007-01-04 21:49

After about two days of normal driving, the check engine light turned off on its own, and the vehicle is now running very smoothly with no stumbles or misfires across any RPM range. Using the scan tool (Autotap), I observed that after the engine warmed up, the downstream O2 sensor stabilized between 300–350 mV — a significant improvement from before. The upstream O2 sensor continues to cycle normally between 100 and 700 mV as expected. Fuel economy is now consistent with what I’d expect for highway driving (approximately 24–26 mpg on a 3.3L engine). While Autotap provides more detailed data than basic code reading, I’m concerned about the reliability of some of its reports — particularly inconsistent 'complete/incomplete' monitor statuses and the fuel trim values. As noted, when I selected Long-Term Fuel Trim (LTFT) for playback mode, no graph displayed even though I could see the value changing during recording. I’ve already sent an email to Autotap’s support team regarding this issue. Thanks so much for the helpful feedback — your insights provided a solid sanity check! And happy new year!

Anonymous 2007-01-05 11:00

It sounds like the 99% value is being used as a placeholder in Autotap software when it lacks access to essential data needed to plot the Long-Term Fuel Trim (LTFT) properly. This may explain why no graph appears during playback — the tool can’t retrieve or display the actual LTFT values due to missing sensor or system data.