Common Check Engine code reference for Honda/Acura

P0301 / P0302 / P0303 / P0304

This is related to a misfire on the affected cylinder.  P0301 is cylinder number 1 and P0304 is cylinder number 4.  This is known to happen due to buildup on the plugs from VCM operation on Honda/Acura vehicles with the 3.0, 3.5 V6.  Over time this buildup of uncombusted fuel and oil in the cylinder will cause micro-abrasions on the cylinder walls and eventually lead to high oil consumption once the vehicle is in the 40-50k mile range leading to lower compression.  Vehicles with the VCM-2 system shutdown cylinders 1-4 in certain scenarios on 2008+ model and some may be eligible for the engine misfire settlement from honda.  

The 2005-2007 Honda 3.5L V6 is a VCM-1 engine and only does cylinder deactivation on cylinders 1-3 instead of 1-4 like the VCM-2 engine.  The 2005-2007 Honda 3.5L V6 shows misfire and oil consumption issues at higher mileages in most cases because only 1/2 of the cylinders can be used in these model years during cylinder deactivation.  Above the 100k mile range is usually when oil consumption and misfire issues begin on this version of engine from wear over time.

In 2008, vehicles with VCM-2 engines showed high oil consumption as early as 10k miles due to aggressive programming in the where up to 4 cylinders were in cylinder deactivation mode; leading to the engine misfire settlement in 2013/2014.

Many mechanics who have worked and rebuilt these engines speculate that during cylinder deactivation, the engine vacuum starves the cylinders which are shutdown of oil and lubrication while vcm deactivation is occuring.  Some vehicle manufacturers like chrysler who utilize a similar system called MDS  have a timer/counter on how long the cylinder deactivation can take place to minimize the damage from cylinder deactivation technology and do a better job at re-lubricating the cylinders with shorter cylinder deactivation intervals.

P3400 / P3497

• DTC P3400 (VPS stuck off bank 1)
• DTC P3497 (VPS stuck off bank 2)
Follow these steps for fast, easy troubleshooting:
1. Check the engine oil level. Low oil pressure can set these DTCs.
• If the oil level is low, adjust it or replace the oil if needed. Then, follow the procedure in
S/B 11-033, Engine Oil Consumption; MIL May Be On With DTC P3400 and/or P3497.
• If the oil level is OK, go to step 2.
2. Clear the DTC(s), and turn the ignition switch to ON (II) without starting the engine (key on, engine off).
• If the DTC(s) come right back, there’s likely a problem in the rocker arm oil pressure switch circuit. Go to
step 3.
• If the DTC(s) don’t come back, the rocker arm oil pressure switch circuit is probably OK.
Go to step 4.
3. Unplug the rear rocker arm oil pressure switch 1P connector for DTC P3400, or the front rocker arm oil
pressure switch 1P connector for DTC P3497. Go to the PGM-FI Data List, and check the ROCKER ARM OIL
• If the switch signal reads OFF, the wiring is OK. Replace the oil pressure switch. Then, plug in the connector
and make sure the DTC(s) don’t come back.
• If the switch signal reads ON, there’s a problem in the wiring or the PCM. Continue with the normal DTC
troubleshooting in ISIS.

Engine Oil Consumption; MIL May Be On With DTC P3400 and/or P3497
(Supersedes 11-033, dated June 11, 2011, to revise the information marked by the black bars)
• The PROBABLE CAUSE was changed.
• A note to the Service Advisor was added.
rate time was changed because the MVCI updates
control modules more efficiently.
The engine oil level is low on the dipstick, and the MIL
may be on with DTC P3400 and/or P3497 (VPS Stuck
Off Bank 1/Bank 2).
The VCM (variable cylinder management) function may
repeatedly switch on and off during certain light throttle
operation, at cruising speeds, on flat roads. This
frequent switching can contribute to increased oil
consumption. The updated PCM software improves
VCM timing during very light throttle operation which
may reduce oil consumption.
DTCs P3400 and P3497 occur when the oil level drops
to a point where line pressure



This code relates to the catalytic converter showing up as not functioning at normal efficiency.  This code usually means the following on a Honda:

1. The catalytic converter is full of debris buildup, usually from oil consumption as the oil consumption increases due to the vehicle age, ring wear or due to vcm operation.  

2. O2 sensors worn or aged will cause the air to fuel ratio of the vehicle to be off enough where it runs too lean or too rich.  Running too rich is known to be related to this code.  When running too rich the ECU may throw a code for this condition.  The useful life of an o2 sensor is between 5-10 years.  

3. catalytic converter just worn or damaged.  The EPA rates most vehicle catalytic converters with a useful life of 100,000 miles or 10 years.  

Usually this code on a honda with VCM operation is due to buildup of uncombusted fuel/gas into the catalytic converter of the vehicle.  The precursor to this code is usually engine misfires on the VCM effected cylinders (e.g. P0301,P0302,P0303,P0304).  If the engine is misfiring occasionally or the spark plugs have severe buildup then combustion is not very efficient.


1. Before replacing any major components, a product such as cataclean ($25) can be used to assist in cleaning up the cat converter.   CAUTION: Sometimes this may resolve the problem or may end up further damaging the ceramics on the cat converter if this process is repeatedly used on a worn out cat to where they split and break.  Some have recommended sea-foam and it has briefly worked as reported by some vehicle owners.  Famous Mechanic Scotty Kilmer claims sea-foam can cause further damage to the cat converter and gum it up as it's oil based and this oil effectively burns on the catalyst, sometimes fixing some gumout and other times destroying the catalyst honeycomb after repeated use. ( ).  Some have also claimed the use of oil-based sea-foam caused  problems with carbon deposits lodging inside the o2 sensors, effectively damaging them.  All carbon deposits dislodged in the engine using a fuel treatment system end up going through the cat converter further exacerbating the problem.  Most fuel treatment additives work well on non-EFI engines without sensors or cat converters.  Your mileage may vary.

Recommend using the manufacturer's specifications on additives to maximize the effect, drive the vehicle somewhat rigorously at 3-4k RPM occasionally by putting the vehicle in a lower gear to assist in blowing out builtup deposits on both the catalytic converter and the o2 sensors.  

2. Check/Replace spark plugs - only use genuine Iridium NGK's in honda vehicles with VCM.  If black or soot covered, replace them.

3. Disable the VCM system with a product like the VCMTUNER or VCMTUNER II to keep the oil consumption and corresponding buildup on the spark plugs from occuring to maxmize the catalytic converter life and reduce further ring wear to cylinders 1-4.  Honda's own TSBs admit the VCM system is the culprit in oil consumption (see P3400/P3497 code above and corresponding links).. 

4. If the above is done and the code continues to crop up every few drive cycles assuming the oil consumption is not over 1qt per 1k miles, you may need a new catalytic converter.  Cost is between $800-$1500 depending on which state you live in (e.g. it's always more expensive in CA and NY).


This check engine code is related to the ECT1 sensor.  If the vehicle does not reach up to a 160F temperature during two consecutive drive cycles, this code will be thrown.   If not using a vcm defeating device, this code may be related to a faulty thermostat (e.g. stuck open).   

If using a vcm disabling device, this is common where too much resistance is added to the ECT1 circuit and it must be reduced as the reported temperature to the ECU is below 160F.  This will usually cause poor gas mileage as if the vehicle does not reach 158F (70c) the vehicle may be stuck in open loop mode (e.g. warmup mode).  Any temperature at 158F(70c) or above will cause the vehicle to reach normal operating conditions and go into closed loop mode where the o2 sensor and the air intake sensors are used to regulate the air-to-fuel ratio.

A product like the VCMTUNER II will not throw this code as it has dynamic adjustment and will report a temperature of 163-165F when warmed up to keep the vcm system disabled assuming the product is installed properly.


This code can occur if the ECT1 circuit is disconnected while the ECU is monitoring the circuit, or during the installation of a vcm disabling device.  It will usually clear up after a few drive cycles when the problem has been corrected. 



This code can occur if the 12v connection to the VCMTUNER II circuit is disconnected while the ECU is monitoring the circuit, or the unit in rare circumstances can be frozen due to a high ESD startup surge causing the temp gauge to not respond during the drive cycle.  Power cycling the unit (e.g. pulling the fuse, and insuring the 12v connection may correct it).  In some cases surges in excess of 500v may freeze the VCMTUNER II units on some vehicles - usually with a non-OEM starter.   You can reach out to us for support on this code if the unit does not respond under any circumstance to a warmup cycle (e.g. the unit is frozen at the last position or does not move past the cold mark).

It is also possible this code pops up if a bad or intermittent connection to the ECT sensor is present.


Other General Suggestions

Always use a full synthetic oil on a VCM capable engine.