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Discussion Starter #1
Did some digging about this code. When I dumped the freeze frame data on my scan tool, it showed the "ECT 1" at 159 degrees F, engine coolant sensor 1 close to the thermostat. When I checked the service manual, it indicates that coolant temp must be above 158 degrees F. That 158 number appears in the service manual as minimum temp for the test to operate correctly. With engine warmed up, 20 mile drive and ambient temp about 65 F, I used an IR temp gauge and looked at the sensor and surrounding metal and thermostat housing, I only see about 140 degrees F. Temp gauge on dash runs below the half way point. Could I be seeing the symptoms of a marginally cool running engine? I would think that the base of ECT 1 should show something close to engine coolant temp. Thermostat should start to open at 169 to 176 degrees F and fully open at 194 degrees F. Thermostats are cheaper than CC.
 

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Of all the Hondas/Acuras i've owned. The engine coolant temp level always stayed right below mid-way. Maybe these engines do run cooler. Who knows

However, the code you're referencing above is for an 02 sensor error.
 

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The code P0420 is (Rear Bank Catalyst System Efficiency Below Threshold) Bank 1 back side of engine. No other codes set. O2 codes are P0137 to P0141 and P0157 to P0161 taken from the service manual. It is interesting that the manual mentions that por quality fuel can cause this DTC code.
 

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I'm in this same boat. My '11 is at 144k and has had P0420 on/off for 25k miles. I kept checking the fuel trims and monitoring the voltage of the O2/AF sensors and I just couldn't come to a solid solution on what was the issue (I am no mechanic and only have a $30 scan tool). My car had the step 1 of the ring blow-by/VCM issue performed at 115k (it first threw P0301 at 90k and fouled that plug, I just swapped them out and kept on driving). I bring this up to point out that as much as 55k miles ago, my cylinder #1 fouled the plug, therefore there is a good chance that the the engine has been dumping oil vapor into the the bank 1 cat for a long, long time.

Previously I could clear the code it would stay off for 500 or so miles, now as soon as i clear it, it comes right back on. Also, previously (~10k miles ago) I could use premium fuel, fuel treatment and fuel injector cleaner and the code would go away for a while. Now, nothing gets rid of it. I took the car to my local dealership for a diagnosis yesterday and they confirmed it needs a bank 1 cat to be replaced. They want $1500 for it. Since I am a rust-belt lifer, every exhaust bolt is rusted to the point that you can;t even make out threads! So i am not going to tackle this on my own. I will search around for better prices and maybe even buy the OEM cat and try to find a shop to throw it in for labor. I am also considering doing the plug defouler trick on the downstream AF sensor to see if that makes the code go away. This is not a great choices obviously, but...
 

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The code P0420 is (Rear Bank Catalyst System Efficiency Below Threshold) Bank 1 back side of engine. No other codes set. O2 codes are P0137 to P0141 and P0157 to P0161 taken from the service manual. It is interesting that the manual mentions that por quality fuel can cause this DTC code.
I can attest to the poor quality fuels. Buy a branded 87 octane fuel that has fuel additives. Unbranded (Loves, Murphy, Sams etc) usually equals raw gasoline untreated. Higher octane fuel does not improve the situation. It can actually be worse for your cats with more fuel vapors entering them that can clog.
 
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