QuestionI had the fuel pressure regulator replaced by the dealer at 82K miles on a 2002 Hyundai Santa Fe. The check engine light came on at 109K & produced a P1166 code. A repair shop wants to replace the upper & lower O2 sensors and the fuel pressure regulator. Could it be "bad" after 27000 miles?
AnswerIt could be bad, especially if you purchased an aftermarket regulator as opposed to a factory one. If you purchased a factory regulator, I'd say no, it's probably not bad. Not only do they not fail particularly often in the first place (most vehicles with this particular regulator do not need it replaced in the lifetime of the vehicle), it's very rare for it to fail in a manner which would cause the P1166 code.
Presuming the repairs you mention are all in relation to the P1166 code, I'm going to suspect that the repair shop is guessing and has done nothing more than read the trouble codes. I.e., I suspect no actual diagnostic work has been done. The trouble code P1166 indicates that the engine control module (ECM) has reached its fuel mixture compensation limit on the rear bank. As I read your question, it seems that the shop is telling you that you need four sensors and a fuel pressure regulator. Not only is it difficult to believe that all these things failed at the same time, it's also impossible for a problem with either lower oxygen sensor to cause the P1166 trouble code, as the readings from these sensors are used solely for evaluating the catalytic converters, not for assessing air/fuel mixture. For the fuel pressure regulator to cause this trouble code, you'd need to have excessive fuel pressure or it would need to be leaking fuel into the vacuum line attached to it. For the shop to have recommended the fuel pressure regulator, they should have observed one of those problems.
Your ECM continuously revises data figures called fuel trims. These values report how long the injectors are held open as compared to preprogrammed values in the ECM, and there are separate readings for each bank of the engine. Sometimes, a problem like this occurs and it's just bad enough to set a code on one bank, and even though it affects both banks, it's not quite bad enough on the other to set the code there. So, before considering possible causes, it's important to check the fuel trims and oxygen sensor readings on both banks. If the problem is occurring on both banks, then it's important to look for problems that would affect both banks:
-- improper fuel pressure
-- stuck open purge control valve
-- vacuum leak
-- defective air flow sensor
-- air bellows between air flow sensor and throttle body leaking or not properly secured.
If the problem affects only one bank, then it's important to look for problems that would only affect that bank:
-- defective upstream oxygen sensor on the bank in question
-- leak in gasket between intake manifold and engine.
