New to the Forum -- glad you folks are here. As an observer here everyone has really helped me keep the Ranger running sweet for 20+ years -- thanks. Question -- 1999 Ford Ranger XLT 4.0 V6. Run well for the last 20 plus years. Check Engine Light came on -- pulled codes. Lean both banks. Cleaned MAS with MAS cleaner. CEL stayed off for a few weeks. CEL came on and got new MAS (not Ford OEM). Cheap Ebay (foreign made? maybe). Cleared codes, etc. CEL off for a few weeks. Check codes (same) Lean both banks. Try to pass CO emissions and all PASS except NOx GPM is slightly over 1.5000. (IIRC 1.51 or 1.52) Has to be below 1.5000. Clean (very well) MAS and reinstall when dry. Drive for two days -- 2 five mile trips NO Issues with CEL -- computer is happy and tells Auto Trans exactly when to shift (improvement). I get bold/brave and can have a recheck done within 10 days at no charge from emissions station. Watch them rev the heck out of Ranger and lady hands me form and says "You Passed". I check the report and I am at 1.4400 and I pass all of the emissions tests and I am of course thrilled. Now I can get plates (a few months late). Here's the punch line. I get in my "newly passed" Ranger to drive out of the Emissions Station and the Check Eng Lite is on. Wasn't on when they drove it in. Now the CEL is glowing at me. Remove the battery for a few minutes and the CEL goes off and stays off for my first 5 mile trip. What gives?? Bad MAS? Should I buy a REAL Ford MAS to replace it? Is it something else. (Just when I thought it was safe to go back in the water) What would you do?.

 

Welcome to the foum

You need a Code Reader
Bluetooth OBD2 code reader is under $20, ELM327 or similar is fine
APPs are FREE

You can then read and clear codes of course, but you can also see LIVE DATA, you can see what computer sees from all the sensors while engine is running
And works on ANY Vehicle 1996 or later that was sold in the US or Canada, its "the Law", so a good tool to have

CEL being on after an emission test can be because they took the gas cap off
Unhooking the battery causes another issue, the P1000 code and CEL on for that

O2 sensors are the ONLY sensors that wear out, 100k miles or 12 years and they are out of the chemical they need to detect oxygen in the exhaust
1999 vehicle should just be getting its 3rd set of O2s at 23 years old

O2 sensor can only "see" oxygen not fuel
O2s generate their own voltage
0.1volt is high oxygen
0.9volt is low oxygen

Lean is high oxygen, so low volts, and as O2s get older the voltage drops, so....................false Lean, and engine runs RICHER than it should so HIGHER emissions that it should
If you don't know when the O2s were last changed, change them
You only need to change the 2 front O2s, but changing them all would be best