P1138 code Need Help

[howardkunz]

My 02 cavalier runs terrible rough on CNG, and has check engine light on. Codes read P1138, P0134, and P0202. Runs good on unleaded. I changed Oxygen sensor but no difference.
Has anyone had anything like this happen? Any help is appreciated.

[siai47]

Re: P1138 code Need Help

First thing I would try with any misfire on a CNG Cavalier is to invest in new plugs and wires. Clear the codes and see what happens. A lot of times a misfire that is not noticable on gasoline is very pronounced on CNG because of the higher quality of spark required to ignite the mixture. Plugs and wires are cheap insurance and even if it doesn't correct the problem, the car most likely needed them anyway . As far as the codes you are seeing: The P0202 code shows a problem that should be seen on the gasoline side, not CNG. This is a failure of the number two cylinder gasoline fuel injector drive signal. If this is the case, there should be a misfire, or low power, when on gasoline. The other two codes relate to O2 sensor operation and catalyst operation. If you have had a misfire on CNG (or gasoline for that matter) the catalytic convertor can be damaged. All of the codes listed can be electrical in nature due to corrosion on various connectors or grounds. Pay special attention to the connectors on the AF ECU located behind the driver side headlamp. Also check all connectors going to the various sensors. If you think you have a damaged convertor, take it to the dealer for repair if you are under 80,000 miles. It has a 8 year/80,000 mile warranty.

[CH-4]

Re: P1138 code Need Help

My 02 cavalier runs terrible rough on CNG, and has check engine light on. Codes read P1138, P0134, and P0202. Runs good on unleaded. I changed Oxygen sensor but no difference.
Has anyone had anything like this happen? Any help is appreciated.

1st check with dealer, could be recall - Then buy a Cavalier CNG supplemental service manual.
P0202 Injector Circuit Malfunction - Cylinder 2
Check ground- test injector
P0134 O2 Sensor Circuit No Activity Detected (Bank 1 Sensor 1)
Check ground- check heater fuse
P1138 Lack of Downstream Heated Oxygen Sensor Switch - Sensor Indicates Rich - Bank No. 1- check ground
Sound like its running rich in CNG mode-Test MAF


NOTE: On O2 sensors:
Do not solder repairs under any circumstances as this could result in the air reference being obstructed.

If the heated oxygen sensor pigtail wiring, connector, or terminal is damaged the entire oxygen sensor assembly must be replaced. Do not attempt to repair the wiring, connector, or terminals. In order for the sensor to function properly it must have a clean air reference. This clean air reference is obtained by way of the oxygen sensor signal and heater wires. Any attempt to repair the wires, connectors or terminals could result in the obstruction of the air reference and degrade oxygen sensor performance.

The following guidelines should be used when servicing the heated oxygen sensor:

* Do not apply contact cleaner or other materials to the sensor or vehicle harness connectors. These materials may get into the sensor, causing poor performance. Also, the sensor pigtail and harness wires must not be damaged in such a way that the wires inside are exposed. This could provide a path for foreign materials to enter the sensor and cause performance problems.
* Neither the sensor nor vehicle lead wires should be bent sharply or kinked. Sharp bends, kinks, etc., could block the reference air path through the lead wire.
* Do not remove or defeat the oxygen sensor ground wire (where applicable). Vehicles that utilize the ground wire sensor may rely on this ground as the only ground contact to the sensor. Removal of the ground wire will also cause poor engine performance.
* To prevent damage due to water intrusion, be sure that the peripheral seal remains intact on the vehicle harness connector.

The engine harness may be repaired using the J 38125-8.

[notdeadyet]

Re: P1138 code Need Help

If the AFECM is located in the left front bumper I would check the connectors for corrosion. When doing this make sure to disconnect both battery terminals to ensure not frying the AFECM.

[howardkunz]

Re: P1138 code Need Help

Thanks a million for the suggestions I will try them soon.:

[howardkunz]

Re: P1138 code Need Help

How do you get at the connections under front bumper to test them ie do I remove the front bumper or go from the top?

[siai47]

Re: P1138 code Need Help

Remove the bumper cover and the drivers side headlight. It's really quite easy. This will get you access to the ECU and the four control relays mounted above it. A couple of screws in the fenders and some push clips under the front end and off it comes . Check the ECU housing for corrosion damage, release the two connectors and make sure there is no corrosion inside the connector body or the ECU. Also check the four relays to make sure none of the plastic housings are broken and the connectors are free of corrosion. Check that all the ground wires in this area are tight and clean.

[chinos]

Re: P1138 code Need Help

hi im similar codes problems and trying to solve this i have a 2002 cavalier w 1138 ,202 and 1202 i found to begin a lot so fuel in cylinders i replace injectors and fuel reg on the gasoline side the car stop backfiring and blowing my air filter ,toke the mass air flow sensor apart and clean it found a screen like device out of place , i think it helps so the sensor get the right reading after that the car ran better not the best ,so i took a test pipe in place of the cat ,it works beutifull now, i when drove the car all day no ligth no problem,next day went to start , the ligth came on, hard to start when it start had a misfire which clear off after running a minute i do have a new o2 sensor and i put a simulator at the second sensor for not to cause the ligth to came on i did found what a thing is a short in the starter i,m not sure if this will create this particular problem,so i wonder what you end up doing

[siai47]

Re: P1138 code Need Help

Like the earlier posts in this thread say, change the spark plug wires and plugs. Most likely your problems will go away. The backfires are common on these cars when starting due to the fuel system design. Backfires when driving, under load, are indicative of some other problem. Make sure you are starting the engine correctly. Foot off the accelerator pedal, crank the engine and hold the key to "start" position until you are sure the engine is running. Sometimes you get a secondary stall and when you try to start it again, the mixture is all messed up and you get the backfire. The screen you found out of place is a result of a backfire. Check the entire induction system for loose fittings or cracks from the aircleaner all the way to the throttle body on the back of the engine.

[chinos]

Re: P1138 code Need Help

the wires and plugs are new is there a recomended plug i hear if you used iridium plugs on natural gas can cause misfires ,you have anny sugestions on the codes?

[bruin]

Re: P1138 code Need Help

Make darn sure the posts you connect the spark plugs wires to on the ignition module are free of rust or other corrosion. My 1999 was backfiring like crazy and I found 2 of the 4 were rusted so clean them up and with new wires and plugs... I used a double-platinum something like delco professional or whatever they were and the thing ran like a dream until recently when the CNG shut-down entirely. I need to TS my system now for a bad fuse or mass gas sensor which are also prone to go out over time. I don't recommend iridium plugs for CNG systems >>> platinum and silver are both better conductors for electrical spark and you need everything you can get with 120 octane equivalent methane.