P0420 Fault on 1997 Toyota Camry LE 4-Cylinder: Exhaust Leak or Catalytic Converter Issue?

Check for an exhaust leak upstream of the oxygen sensors, such as at the manifold. This could explain both the unusual noise and the P0420 code.

Thanks for the tip! I'll check for exhaust leaks at my local MONRO shop and update here in a day or two. Andrei_

I usually get around 28–32 MPG on regular gas, but since the Christmas lights were installed, fuel efficiency has dropped to 24.5–25 MPG. This could be another indicator of an underlying issue — worth considering.

Possibly, though it's not a direct cause.

I recently checked for exhaust leaks at a MONRO shop — no leaks were detected. However, they noted an intermittent 'frrrr' noise coming from near the passenger front tire when driving between 20–40 MPH. My dad replaced the timing belt three months ago and suggested it might be improperly installed (even one tooth off), which could trigger a P0420 code and reduce fuel efficiency. Any thoughts? Thanks, A_

The noise could stem from a faulty wheel bearing or timing belt. While the P0420 code might be related to timing belt issues, I suspect additional codes would appear if that were the root cause.

Thanks for your reply, Jeff. I'll have my dad inspect the timing belt installation since he's done dozens of these jobs before. He mentioned the water pump was in good condition at the time of replacement. While I trust his judgment, I understand that errors can occur during installation. I’ll also check for signs of a failing wheel bearing. In the meantime, I’ve been advised to replace the spark plugs as a low-cost step — perhaps this could resolve the P0420 code before moving on to a full catalytic converter replacement (which would require replacing the entire exhaust system from manifold to muffler). I’ll update once I have more results. It seems multiple factors can trigger a P0420, and it's one of the most common codes that activates the Check Engine Light.

If the water pump is driven by the timing belt, it should be replaced every time the belt is serviced. A tight belt can overload the bearing, leading to wear — eventually causing leaks or noise. Every Honda technician I know follows this practice!

Jeff — thank you for clarifying! The car is a Toyota Camry, not a Honda. My dad mentioned both the water pump and original timing belt were in excellent condition at the time of replacement. The noise only occurs when driving between 20–40 MPH and disappears during idle. Could worn spark plugs be contributing to the P0420 code or the noise?

I realize it's a Toyota, not a Honda — I was using that as an example of common industry practice among import mechanics (especially on VW and Mazda engines). Regarding spark plugs: incorrect heat range could potentially cause a P0420 code, though it’s unlikely. At this point, I’d recommend having the car checked by a mechanic experienced with Toyota systems to determine whether the issue is sensor-related or converter-related. No offense taken — I fully agree that most backyard shops lack the proper tools for such diagnostics.

Jeff — thanks again. I recall some Toyota dealership technicians in MA saying if the water pump is fine, it can be left alone — but they acknowledged this comes with risk. I’ve agreed to keep the original water pump and take that chance. I’m considering replacing the spark plugs as my last inexpensive step before moving on to a full converter replacement. If that doesn’t work, I’ll bring it to a local shop for $100 comprehensive testing. However, for an additional $20 (link: http://cgi.ebay.com/ebaymotors/ws/eBayI ... AMEWA%3AIT), I might have my dad install a new exhaust pipe — considering I already replaced both O2 sensors three months ago. What do you think? No offense taken — you're absolutely right about the lack of proper tools in a home workshop.

Let me know how it turns out!